Biology Notes For O-level

Cell Structure and Organization

A cell is the smallest unit that can carry on all the processes of life.
All organisms are made of cells, organisms are made of several organ systems, each organ system contains several organs, each organ contains several tissues; each tissue is made of cells. Cells are very tiny they could be seen only through a microscope. We have two types of cells:

1. Planet Cells
2. Animal Cells

As you can see from the diagram, there are some features found in plant cells but not in animal cells.

Features found in both plant and animal cells:

• Cell surface membrane: This is a partially permeable membrane separating the cell from the environment its made of lipid and protein, it controls movement of substances in and out, its strong but flexible.
• Cytoplasm: This is a jelly like substance, its made of mostly water and protein. Metabolic reactions occur in it.
• Nucleus: This determines how the cell behaves and it contains chromosomes made of strings of DNA which also determines which proteins the cell should make etc.

Features found in only plant cells:

• Cell Wall: This is a rigid layer surrounding the cell made of cellulose, it gives the plant its shape and prevents it from bursting.
• Chloroplasts: They are sacs which contain chlorophyll which is a green pigment that traps sunlight for photosynthesis.
• Vacuole: This is a large room in the center of the cell, it stores sugars and salts and controls movement of water in and out of the cell.

Animal cells store sugars in glycogen form but plant cells store it as starch. Animal cells have an irregular shape but plant cells have a regular shape.

Both types of cells contain Mitochondria these are structures that convert chemical energy in foods to energy that could be used in moving, dividing, etc., it is evidence that the cell is an Active Cell.

Specialised Cells:

Red Blood Cells:

Red blood cells are found in the blood of animals, its function is to transport oxygen from the lungs to all the body cells, and carbon dioxide from the body cells to the lungs.

They are adapted by four ways:

• They have a biconcave disc shape that gives it a large surface area to carry more oxygen.
• They contain a chemical called hemoglobin that combines with oxygen and carbon dioxide.
• They have no nuclease to carry more oxygen and CO2
• They are tiny enough to squeeze through capillaries.

Muscle Cells:

They are cells found in muscles in animals, they contract and relax together to move the organisms. Their function is to contract to support and move the body.

They are adapted by two ways, First, Is that they are made of contractile filament to help in contraction. Second is it contains lots of mitochondria to supply the cell with energy.

Ciliated Cells:

Ciliated cells are present in the trachea and bronchi of out respiratory system.

Their function is to use their cilia to move the mucus up the trachea to the throat. The mucus traps bacteria and dust particles. When it reaches the throat, mucus is swallowed to the stomach where the acid kills the bacteria.

They are adapted by the tiny hair like projections called cilia which sweeps the contaminated mucus upwards.

The mucus is secreted by goblet cells which are next to ciliated cells.

Root Hair Cells:

These are cells situated in the roots of plants. They contain no chloroplasts. Their function is to absorb water and minerals from the soil. And to anchor the plant in the soil. They are adapted by 3 ways. One, they have an extension that increases the surface area for more water intake. Two, they have a large number of mitochondria for respiration to become more active. Three a concentrated vacuole to help absorbing water by osmosis.

Xylem Vessels:

These are dead lignified cells that exist in the stem of a plant.

Their function is to transport water and minerals from the roots to the leaves and the rest of the plant through the stem. And to support the plant.

They are adapted by 2 ways. Firstly, they are hollow to allow water and minerals to pass through them with no resistance. Secondly they are strong and lignified to support the plant.

The Division Of Labour: the specialization of cells to carry out particular functions in an organism.

Movement In And Out Of Cells

Substance move in and out of cells by three ways:

• Diffusion: The net movement of particles from a region of their higher concentration to a region of their lower concentration down a concentration gradient, as a result of random movement.
• Osmosis: The diffusion of water molecules from a region of their higher concentration (dilute solution) to a region of their lower concentration (concentrated solution) through a partially permeable membrane.
• Active Transport: The movement of ions, in and out of a cell, through a cell membrane, from a region of their lower concentration to a region of their higher concentration, against the concentration gradient, using the energy released by respiration.

Diffusion:

Diffusion is the process by which oxygen enters the blood from the lungs, and by which carbon dioxide enters the leaf from the atmosphere. There are many more examples of diffusion in biology.

Diffusion always takes place down a concentration gradient, that means that the particles that diffuse try to spread evenly in all spaces, so it moves from where it’s very concentrated to where it’s not concentrated.

There are some factors affecting the rate of diffusion, like the steepness of the concentration gradient. The steeper the gradient the faster the particles diffuse.

The surface area of the exchange membrane also affects the rate of diffusion. The larger the surface area of the exchange membrane the faster particles diffuse.

Thickness of exchange membrane too determines the diffusion rate, the thinner it is, the easier it will be for particles to go through it, the faster the diffusion rate.

Temperature is another factor affecting the diffusion rate, increasing the temperature will give particles more kinetic energy, making them move faster, thus increasing the rate of diffusion.

Osmosis:

Osmosis is the diffusion of water molecules. When we speak about osmosis, we don’t say water concentration; instead we use the term water potential. A dilute solution means it has lots of water molecules, and a high water potential. A concentrated solution has few water molecules and low water potential. Osmosis has to take place through a partially permeable membrane (or Semipermeable) this means that the Water molecules move from a place of their  high concentration to a place of their low concentration through a membrane with pores in it that lets some molecules through but not others.

The diagram shows two solutions, one dilute and one concentrated, separated by a semipermeable membrane. The solution on the right is diluted while the concentration on the left is concentrated. The water molecules will move from the right handside solution where they are very concentrated to the left handside solution where they are of a very low concentration, osmosis took place.

Osmosis happens all the time in cells. If you place an animal cell in distilled water. Osmosis will result in the water molecules moving from the distilled water where they are very concentrated to the Cell Where they are of low concentration Through the cell surface membrane. The cell becomes fat. As more Water molecules enter the cell, the cell will eventually burst and die.

If we do the opposite, and place a red blood cell in a concentrated salt solution, the water in the cell has a higher water potential that the concentrated salt solution. Water molecules will move from the cell to the salt solution causing the cell to become shrunken and shrivel as in the diagram.

Hypotonic Solution	Isotonic Solution	Hypertonic Solution
			Animal Cell
			Plant Cell

In plant cells, if a plant cell is placed in distilled water, water molecules will move from the distilled water to the cell, the cell swells up and becomes turgid but it will never burst because plant cells are surrounded by cell walls, which are made of cellulose and is elastic, it will stretch but never break, the cell becomes turgid.

If we place a plant cell in a concentrated salt solution whith low water potential, water will move from the cell to the solution causing the cell to become plasmolysed as in the diagram.

Active Transport:

Active transport occurs in cells, it is basically the movement of molecules or ions from a region of their low concentration to a region of their high concentration (against the concentration gradient) using energy of respiration. Active transport occurs in living, active cells only because it needs energy, these cells usually have a structure called mitochondria which respires producing energy to be used in active transport.

Active transport happens in roots to absorb mineral salts from the soil. It also occurs in the digestive system of mammals.

If oxygen is absent, respiration won’t take place, active transport will stop. Molecules are taken into the cell by protein carriers within the cell membrane.

Enzymes

What are enzymes? Enzymes are proteins that function as a biological catalyst. They are proteins in nature. A catalyst is a substance that speeds up a chemical reaction but isn’t changed by the reaction. Hydrogen peroxide (H2O2) is a substance that decomposes into Water (H2O) and Oxygen (O2) if it is left in room temperature for a period of time. This reaction could a long time, but it could be sped up if we add a catalyst. Each catalyst can catalyse a specific substance and nothing but it. The catalyst for Hydrogen peroxide is called Manganese4 oxide. If it is added we will get water and oxygen gas in a very short time, and the manganese4 oxide could be obtained again as it was, it remains unchanged.

How Do Enzymes Work?

Enzymes work the same way as catalysts do, they can work with only one substrate and they can be used more than once.

Enzymes have a structure that is called active site. Only one substance can fit into the active site to be digested, and it is the only substrate that this particular enzyme works with.

The figure above shows the function of enzymes:

• The substrate enters the active site of the enzyme.
• The reaction takes place.
• The substrate exits the enzyme as two simpler products.

You can also think of the way enzymes work as a key and a lock, the key is the substrate and the lock is the enzyme. The key should be exactly the right shape to fit in the lock, so does the substrate to fit in the active site of the enzyme. The key could only open only one lock, and the lock could be unlocked by only that key.

Enzymes are two types, Builders and Breakers. Builder enzymes do the opposite of breaker enzymes. Breakers break large molecules into smaller simpler ones, builders combine smaller ones to make large molecules.

Breaker enzymes are used in the digestive system to break down large insoluble molecules into simpler soluble ones to be used by the body. They are also present in cells that respire to break down sugars and oxygen into carbon dioxide, water and energy. Builder enzymes are present in plants to be used in photosynthesis, the opposite of respiration, in photosynthesis, oxygen and water are combined together to form carbon dioxide and sugars.

Naming enzymes depends on the substrate they work on. For example:
The sucrase enzyme works on sucrose.
The maltase enzyme works on maltose.

Enzymes are reusable and are only affected by the change in temperature and pH.

Affect of temperature on the enzyme’s activity:

Each enzyme has an optimum temperature, this is the temperature at which the enzyme is most active, below this temperature the activity of the enzyme decreases until it becomes inactive at low temperatures, above this optimum temperature the enzyme becomes denatured and can no longer work.

At low temperatures the enzyme is and the substrate are moving very slowly and collide weakly, the enzyme is said to be inactive and doesn’t work. As the temperature increases, the enzyme and substrate gain more kinetic energy and move faster colliding more, the enzyme becomes more active and the reaction takes place. When the enzyme reaches it’s optimum temperature, it is in its most active mood, if the temperature crosses the optimum the enzyme begins to die and become denatured. The enzymes become denatured when the shape of their active site changes as a result of high temperature, thus the substrate cannot fit into the active site and the enzyme is useless.

Each enzyme has its own optimum temperature, enzymes in humans have optimum temperatures of around 40 degrees. Plants have enzymes with optimum temperature of about 25 degrees.

The Effect of pH on the enzyme’s activity:

As in temperature, enzymes have an optimum pH. The pH is a scale  measuring the acidity or alkalinity of a substance or solution. The scale runs from 1 to 14. pH 7 is neutral, below that it is acidic and above that it is alkaline.

Each enzyme has an optimum pH, if this pH changes, the shape of the active site of the enzyme is changed, thus the substrate will not be able to fit in it, and the enzyme becomes useless.

Uses Of Enzymes In Seeds Germination:

Seeds grow into plants by germinating. Seed germination involves enzymes breaking the materials stored in the seed down to be used in growth, energy and building cells. The seed contains stored substances such as:

• Starch: Starch is broken down by amylase enzyme into maltose, maltose is then broken down by maltase enzyme into glucose which is used in respiration.
• Proteins: Proteins are broken down into amino acids by Protease enzyme, amino acids are used in building up cells and growth.
• Fats: Fats are broken down into fatty acids by lipase enzyme, they are used in making cell membranes.

In order for a seed to germinate, some conditions must be present:

• Water: To activate the enzymes.
• Oxygen: To be used for respiration.
• Warm Temperature: For providing the best conditions for enzymes to work and optimum temperature.

Uses Of Enzymes In Biological Washing Powders:

Washing powders contain detergents that help in cleaning clothes by dissolving stains in water. Some stains are made of insoluble substance, these cannot be removed by normal washing powders, instead, a biological washing powder is used. Biological washing powders contain enzymes that break down the insoluble stain into smaller soluble substances, which are then dissolved in the water. For example, if your shirt gets stained by egg yolk or blood, there is an enzyme called protease in the washing powder that will break down the insoluble protein into amino acids, which are dissolved in the water and sucked away. Thus the shirt becomes clean.

The best removal of stains is maintained by providing the optimum temperature for enzymes, presoaking to leave time for the enzymes to digest, putting the suitable amount of the powder.

Use Of Enzymes In Food Industry:

Enzymes are often used in the manufacturing of different foods.

Baking – Brewing – Cheese Making: In baking, both yeast and sugar are used. Yeast cells contain enzymes that ferment sugar by anaerobic respiration producing carbon dioxide bubbles which causes the dough to rise as in the photo. Brewing is the process of making wine or beer. In this process fermentation is Involved producing alcohol which and carbon dioxide that gives wine and beer its sparkle. In making cheese, an enzyme called rennin extracted in enzymes, helps by clotting milk.

Making Juices: In fruits such as apples or oranges, a substance called pectin holds the cells together making it  hard to squeeze them. An enzyme called pectinase digests pectin making it much easier to squeeze the fruit and to make the juice more clear than cloudy.

Making Baby Foods:

It is hard for new born babies to digest food such as high protein foods. That is why foods like that are treated with proteases to break down protein to amino acids, making it easier for newborns to absorb and assimilate them.

Making Sugar:

Sugar producing companies get sugar from starch by using the amylase enzyme to digest starch into maltose. For dieters a sugar called fructose is very useful because it provides a sweater taste than other sugars from a less quantity. Fructose can be obtained by using the isomerase enzyme to convert glucose to fructose.

Meat And Leather Production:

Proteases are used to make meat less tough and acceptable for consumers by treating cuts of meat.

In leather industries hairs are removed from animal skin by digesting them using protease enzymes.

Enzymes Extraction:

The Enzymes used in the industries are taken from either fungi or bacteria. This takes place in a Fermenter, this is a large sterilized container with a stirrer, a pipe to add feedstock and air pipes. 
The following steps take place:

• The micro-organisms and the feedstock are added and the liquid is maintained at 26 degrees and pH of 5-6.
• The micro-organisms produce two types of enzymes, either extra-cellular or intra-cellular.
• Extra-cellular enzymes are extracted from the feedstock by filtering.
• Intra-cellular enzymes are extracted by filtering the micro-organisms from the feedstock, crushing them, wash them with water then extracting them from the solution.

Enzymes And Antibiotics:

Antibiotics are powerful medicines that fight bacterial infections. Micro-organisms are used for the production of antibiotics.

Some Antibiotics, like bactericides, fight bacteria by damaging its cell walls causing them to burst and die. Other antibiotics interfere with the protein synthesis and stop the bacteria growing.

Antibiotics have no effect on human cells because human cells have no cell walls and the structures involved in protein production are different than that of bacteria.

Antibiotics are obtained from sources like:

• Bacteria (Actinobacterium Streptomyces): this bacterium produces the antibiotic strepmycin.
• Fungi (Penecillum fungus): penicillin, the first antibiotic discovered is produced by thing fungus.

Different types of penicillin are produced by different species of the fungus. They are chemically altered in lab to make them more effective and make them able to work with different diseases.

Steps of production:

1. The Fermenting tank in filled with nutrient solution of sugar (lactose) or corn liquor which contain sugars and amino acids,
2. Minerals are added,
3. pH is adjusted around 5 or 6,
4. Temperature is adjusted about 26 degrees,
5. The liquid is stirred and air is blown through it,
6. The micro-organisms are added and allowed to grow for a day or two in sterile conditions,
7. When the nutrient supply is decreased, micro-organisms secrete their antibiotics,
8. The fluid containing the antibiotic is filtered off and the antibiotic is extracted.

Nutrition

Nutrition is taking in nutrients which are organic substances and mineral ions, containing raw materials and energy for growth and tissue repair, absorbing and assimilating them. Nutrition is one of the characteristics of living organisms. All organisms do it, they do it to obtain energy for vital activities and raw materials needed for growth and repair.

Every Individual needs to take in a certain amount of each nutrient daily, depending on their age, size, sex and activity. There are 7 Types of nutrients, these are: Carbohydrates Proteins Fats Vitamins Minerals Roughages Water

Carbohydrates, proteins, fats and vitamins are all organic substances. This means that they are made by living organisms (plants) and contain carbon atoms in their structures. Plants make organic substances from inorganic materials like carbon dioxide, water and inorganic minerals. Animals are unable to do this.

Carbohydrates:

This nutrient is an organic compound composed of carbon, hydrogen and oxygen.

Function:

It is used as an energy resource, essential in respiration to release energy.
It is used in creating the cellulose, the substance forming cell walls of plant cells.

Carbohydrates are 3 types:

	Monosaccharides: The smallest and simplest form Water soluble Chemical formula C6H12O6 Examples: Glucose-Fructose-Galactose Sources: Fruits-Honey
	Disaccharides: Each molecule consists of two Monosaccharide joined together Water soluble Examples: Lactose-Sucrose-Maltose Sources: Table sugar- Milk
	Polysaccharides: Each molecule has many joined monosaccharide forming a long chain. Insoluble in water Examples: Starch-Glycogen-Cellulose Sources: Bread-Potatoes-Pasta, Cellulose in plant cells and Glycogen in livers.

Monosaccharide and Disaccharides are sugars, they are reducing for Benedict’s reagent, except for the disaccharide sucrose, it is non-reducing.

Polysaccharides are not considered as sugars and don’t have a sweet taste. Excess polysaccharides are stored in the liver and muscles.

Lipids (Fats):

These are composed of carbon, hydrogen and oxygen. But their ratios are different than that of carbohydrates. One fat molecule is made of a glycerol unit and three molecules of fatty acids.

Fats are essential in a diet because they are needed to:

• Release high amounts of energy
• Make cell membranes
• Store them under the skin to insulate heat.
• Forming a layer of fats around organs to protect them from damage
• Storing energy (better than glycogen)

When fats are respired, they produce about twice as much energy as carbohydrates.

 

Proteins:

These are also organic compounds; they contain the elements Carbon, Hydrogen, Oxygen, Nitrogen and sometimes Phosphorus or Sulfur.
A molecule of protein is a long chain of simpler units called amino acids. 

These amino acids are linked together by what's called “peptide bond”.

Types of protein:

• Animal Protein: It contains the most biological value because it contains all essential amino acids (Meat, Milk, Fish, Eggs etc).
• Plant Protein: It contains a lower biological value to humans because it contains fewer essential amino acids (Cereals, Peas, Beans etc).

Needs of proteins:

• Making and new body cells
• Growth and repair
• Making enzymes (they are proteins in nature)
• Build up hormones
• Making antibodies

Although proteins are needed in high amounts, the body will only absorb as much as needed, so excess protein is delaminated in the liver and excreted as urea.

Vitamins: These are organic, soluble substances that should be present in small amount in our diets, they are very important though. Most of the amount of vitamins in our bodies was taken in as nutrients, the body its self can only make few Vitamins, so we have to have to get them from organisms that make them, such as plants. Each type of Vitamin helps in chemical reactions that take place in our cells. Types Of Vitamins: Vitamin C: This is present in most fruits and vegetables specially citrus fruits like lemon and oranges, however, it is damaged by heating so it these foods have no value of Vitamin

C if they are eaten cooked. Vitamin C is essential for the formation of Collagen, a protein that functions as cementing layer between cells, Vitamin C also increases immunity. Vitamin D: This is present in fish oils, egg yolk, milk and liver. Unlike Vitamin C, Vitamin D is made by animals as well as plants, this occurs when the skin is exposed to the Ultra Violet Rays of the sun. Vitamin D plays a big role in absorbing Calcium from the small intestine and depositing it in bones. So it is responsible for having healthy bones.

 

Minerals (Inorganic Ions):

These are a lot of types, each needed in small quantities. Iron and Calcium are the most important minerals, and they are needed in higher amounts.

Types Of Minerals:

• Calcium: This mineral is needed for the formation of bones and teeth as they are made of calcium salts, it also helps in blood clotting and transmission of nerve impulses. Good sources of the mineral Calcium are milk, dairy products and hard water.
• Iron: This mineral is needed for the formation of the red pigment haemoglobin which is essential for the transport of oxygen around the body in red blood cells. Good sources of Iron include red meat specially liver and green leafy vegetables.

 

Roughages (Fibre):

Although roughages are not even absorbed by the body, they are a very important nutrient in our diet. Roughages are mostly cellulose, which is the substance that makes up the cell walls of plants we eat. We humans, have no enzyme that could digest cellulose, that means that roughages enter the body from the mouth, go through the digestive system, and out through the anus unchanged. But as it goes through the digestive system, roughages take space in the gut to give the gut muscles something to push against, this process of pushing the food through the gut is called peristalsis, without roughages peristalsis is very slow and weak. Quick and strong peristalsis means that food stays in the alimentary canal for a shorter period, this prevents harmful chemicals of certain foods from changing the DNA of cells of the alimentary canal causing cancer, so roughages also helps stay away from cancer. Roughages are found in leafy vegetables.

 

Water:

About 70% of your weight is water. Water is perhaps a very essential nutrient we should take in. The functions of water include: As a solvent which reactants of metabolic reactions are dissolved in. It makes up most of the blood plasma which red blood cells, nutrients, hormones and other materials are carried in. It helps in lowering the body temperature in hot conditions by secreting it as sweat on the skin, the sweat evaporates using heat energy from the body, thus lowering the temperature.

Balanced Diet: A perfect diet contains all of the nutrients in reasonable proportions, not too much and not too little. The perfect diet should also contain energy as much as the total energy used by the individual. Unbalanced Diet (Malnutrition): Malnutrition is eating inadequate proportions of food. In other words, an unbalanced diet means it is rich in a nutrient and low in another, or even lacking of a substance. There are lots of effects of malnutrition, such as starvation, obesity or deficiency diseases.

 

Starvation:

Starvationis a severe reduction in vitamin, nutrient and energy intake. It is the most extreme form of malnutrition. In humans, prolonged starvation can cause permanent organ damage and eventually, death. The term inanition refers to the symptoms and effects of starvation. In case of starvation the body tends to feed on its own self. When the glucose level is decreased in the body, the liver breaks down fats to respire for energy, when the body is out of fats, it starts respiring proteins from the muscles to release energy.

 

Obesity:

is the opposite of starvation. It is eating too much of every nutrient, especially carbohydrates and fats. Obesity doesn’t strike alone, it brings with it several other diseases such as high blood pressure, cardiac diseases, diabetes, stress on joints and bones as well as other psychological issues like low self esteem and lack of confidence. To prevent obesity, you have to control your carbohydrates and fats intake and exercise regularly.

Another consequence of malnutrition is deficiency diseases.
These are results of a certain nutrient in the diet:

• Scurvy is the deficiency disease of vitamin C. Its symptoms include bleeding gums.
• Rickets is the deficiency disease of both Vitamin D and Calcium. Bones are made of calcium which Vitamin D helps in depositing in the bones, if any of both is lacking in the diet, rickets is developed.
• Anemia is the deficiency disease of iron. The amount of haemoglobin decreases causes short breath and tiredness.
• Kwashiorkor affects children whose diets are lacking in protein. It causes weakness and tiredness.

 

Special Needs:

There are certain types of people whose diets need to be different to normal ones.
Such as pregnant women, breast-feeding women or children going through puberty.

Pregnant Women:

The diet of a pregnant woman needs to be very rich of certain nutrients because she is not only feeding her self, she is feeding her baby as well. In order for the fetus to develop well, it needs extra Protein, Iron, Calcium and Vitamin D. Proteins are to develop the tissues of the fetus, Iron is to make haemoglobin and to store in the liver, while Calcium and Vitamin D are to develop the baby’s bones.

Breast-Feeding Women (Lactation):

Lactation means the production of breast milk. After pregnancy, the mother breast-feeds the baby for about 6 months or more. Breast milk needs to be high in Proteins, Calcium, and Vitamins to guarantee a healthy growth for the infant.

Growing Children (Passing Puberty):

At some point, each child gets a growing spurt. This is a very high growth rate that increases the child’s size and mass in a short period of time. A growing child’s diet needs extra Proteins to develop cells and enzymes because their metabolic rate is higher, Calcium and Vitamin D to develop bones and Iron to make hemoglobin.

Food Additives:

These are chemical compounded added to foods by the manufacturer because they have some benefits such as increasing the lifespan, prevent rotting etc.
Most food additives are good, such as ones that add colors or flavors to foods. But there are others which have been proven hazardous to humans.

Good food additives include flavorings and colorings which are used to make the food more appealing, antioxidants which prevent foods from combining with oxygen and rot, and stabilizers which stops foods like ice-cream from separating into water and fatty components.

Food preservatives though, are a widely used food additives which increase the lifespan of foods, making it cheaper to store and transport. However, scientists claim that some preservatives contain nitrites which combine with chemicals making a substance (nitrosamines) that causes cancer in animals.

Food Additives
Advantages Prevents rotting Improve color Improve flavor Keeps texture Increases lifespan Prevents poisoning	Disadvantages Allergic reactions Cause hyperactivity Damages liver/kidney Carcinogenic Makes bad food look good

 

Microorganisms And Food Industry:

Production of Single Celled Protein (Mycoprotein):

Mycoprotein is a protein made from microscopic fungus. Humans need large amounts of proteins in their diets, in some poor areas, sources of proteins like meat are unaffordable, mycoprotein is used.

The process takes place in a sterilised container called fermenter. The micro-organisms are grown in the fermenter and supplied with air which contains oxygen for aerobic respiration, ammonia as a source of nitrogen to be used by the micro-organisms to make proteins, and methanol which contains carbon for the formation of carbohydrates.

Advantages of mycoproteins are that it is cheaper than any source of protein but equal in value, and that it contains much less fats and more roughages and carbohydrates

Production Of Yoghurt:

• Milk is sterilised by boiling
• Certain types Bacteria are added to the milk
• The milk is kept warm to provide best conditions for bacteria growing
• Bacteria respire producing lactic acid, thickening the milk and giving it the pleasant flavour
• Yoghurt is cooled and flavours or fruits could be added.

 

Food Tests:

Starch Test:

• Put sample in a test tube
• Add water to make it a solution
• Add iodine solution
• Is starch is present the solution changes colour from yellowish brown to Blue Black.
• If starch is not present the solution remains yellowish brown.

Reducing sugars (carbohydrates) test:

Note: This test is only applicable on all sugars (monosaccharide and disaccharide) EXCEPT FOR SUCROSE.

• Add sample to a test tube
• Add Benedict’s Reagent
• Put test tube in water bath for heating
• If reducing sugars are present the solution turns from  blue to yellow,orange,red (fire colours)
• If reducing sugars are not present the solution remains blue.

Proteins Test:

• Put sample in a test tube
• Add water to make a solution
• Add Buiret Reagent
• If proteins are present in the solution turns Purple
• If proteins are not present the solution remains blue.

Note: Biuret Reagent is blue in colour and made of copper sulphate and a small amount of sodium hydroxide.

Fats Test:

• Add sample to a test tube
• Add ethanol
• Add water and shake well
• If fats are present the solution becomes unclear
• If fats are not present the solution remains clear

General Table:

Nutrient	Test	Colour	Positive	Negative
Starch	Iodine sol.	Yellow / Brown	Blue / Black	Yellow / Brown
Carbs	Benedict’s	Blue	Red (fire)	Blue
Proteins	Biuret reagent	Blue	Purple	Blue
Fats	Ethanol/water	-	Cloudy	Clear

Animal Nutrition

Animals eat to grow, repair etc. They simply eat to live. In this unit we will study how animals make use of what they eat. The journey of the food from the mouth to the anus through the alimentary canal includes 5 steps:

1. Ingestion: Taking in pieces of food into the mouth
2. Digestion: The break down of large, insoluble food molecules into smaller more soluble ones by chemical and mechanical means.
3. Absorption: Taking the digested food molecules into the cells
4. Assimilation: Making use of the digested food molecules for example to release energy or grow etc.
5. Egestion: The elimination of undigested food materials through the anus

*Don’t confuse egestion with excretion, excretion is to get rid of waste products of metabolism.

The alimentary canal (gut or digestive tract) is made up of several organs working together to perform all the processes mentioned above. Starting with the mouth and ending with the anus.

The Mouth:

The mouth performs several functions:

Mechanical Digestion: The action of the teeth biting a small piece of food from a large one is considered mechanical digestion, the teeth also tears and grinds the food into a bolus to give it larger surface area for faster chemical digestion.

Chemical Digestion: beneath the tongue lies a salivary gland which secrets saliva into the mouth, this saliva contains water and mucus to lubricate the food bolus and amylase enzyme that breaks down starch in the food into maltose.

After this the tongue pushes the food bolus into the oesophagus.

The Oesophagus: This is a tube that transports the food from the mouth deep into the body to the stomach. The food is pushed downwards by the muscles in the walls of the oesophagus, this process is called Peristalsis. Muscles contract and relax creating a wavy motion to push the food down.

The Stomach:

Here the food stays for a while. The stomach is a flexible bag that performs both mechanical and chemical digestion.

Mechanical Digestion: The walls of the stomach contain muscles that contract and relax together mixing the food with the content of the stomach and turning it into liquid chyme, this process is called churning.

Chemical Digestion: The walls of the stomach also secretes a liquid called “Gastric Juice” which contains Hydrochloric acid, Mucus, and pepsin enzyme. The pepsin enzyme digests proteins into simpler polypeptides, while the hydrochloric acid is to provide optimum pH for the enzyme and the mucus is to lubricate the food and protect the walls of the stomach from the acid.

After a few hours, the sphincter which is a muscular valve opens allowing the food into the small intestine.

The Small Intestine:

The small intestine is where most digestion and absorption takes place. It is divided into two sections, duodenum and ileum. The walls of the small intestine contain several types of liquids that help in providing suitable conditions and digest the food. These liquids are:

Bile Juice: it comes from the liver, stored in the gall bladder. It is squirted along the bile duct in the duodenum. The bile works on fats only, fats are very difficult to digest because they are very insoluble, the bile contains bile salts that breaks fats into tiny droplets that float in the content of the small intestine, making it easier for the lipase to digest fats into fatty acids and glycerol, this process is called emulsification.

Pancreatic Juice: it comes from the pancreas and secreted along the pancreatic duct. It contains enzymes and sodium hydrogen carbonate, which neutralises the hydrochloric acid that was added to the food in the stomach, creating better conditions for the enzymes to work. The pancreatic juice contains the following enzymes:

• Amylase to digest starch into Maltose
• Trypsin to digest proteins to polypeptides
• Lipase to digest fats into fatty acids and glycerol

Small intestine liquid: The small intestine itself also secrets a liquid that consists of lots of enzymes to make sure carbohydrates, fats and proteins are digested to their simplest form, these enzymes are:

For carbohydrates:

• Maltase to digest maltose into glucose + glucose
• Sucrase to digest sucrose into glucose + fructose
• Lactase to digest lactose into glucose + galactose

For Fats:

• Lipase to digest fats into fatty acids and glycerol

For proteins:

• Protease for further digestion of polypeptides to amino acids.

Absorption in small intestine:

Absorption in the small intestine takes place in the second section, the ileum. The walls of the ileum are fully adapted for absorption. The interior walls of the ileum is covered with a layer of villi, each villus is covered with another layer of micro villi.

Each villi has a branch of blood capillaries in it as well as a lacteal which is a lymph vessel, the lacteal absorbs fats and lipids with vitamins dissolved in them into The lymphatic system.

Villi and microvilli are adapted to absorption by: They give a very large surface area for faster diffusion of food molecules Each villus contains a large network of blood capillaries transporting more blood, thus faster diffusion Each villis is one cell thick, reducing the diffusion distance and making it faster Each villi contains a lacteal which absorbs fats

The Large Intestine:

By the time the food reaches the large intestine, there is not much left of it, only some water, minerals, and fibers. The water and the minerals are absorbed into the blood, while the fibers and dead cells of the alimentary canal are stored in the rectum then excreted through the anus (egestion).

Assimilation Of The Absorbed Food Molecules:

After the food molecules are absorbed from the alimentary canal, it is transported to the liver by a special blood vessel called The Hepatic Portal Vein. The liver is an organ that is considered a gland too. It carries out several jobs to “sort out” the food molecules it receives. Each type of nutrient has its own fate in the liver.

Glucose: when the absorbed glucose reaches the liver, the liver allows as much as needed by the body to pass to the circulatory system to by used for respiration or other processes. The excess glucose is converted to glycogen and stored in the liver cells, when the blood is short in glucose, glycogen will be converted back into glucose and secreted to the blood. Some glucose will also be converted to fats as an energy reserve. These functions are controlled by the Insulin and Glucagon hormones which are made in the pancreas.

Amino Acids: some amino acids will be used by the liver cells to make proteins, the rest will be allowed into the blood stream to be absorbed by the body cells which also convert it to proteins. If the body contains enough amino acids, the excess will undergo a process called Deamination, this involves the break down of amino acids into carbohydrates and amino group, which is then converted to ammonia then converted into urea, which is part of the waste product of the body, urine.

A part from sorting out food molecules, the liver performs the following jobs too:

• Dealing with old red blood cells:
  The liver changes dead red blood cells to iron and bile. Iron is stored in the liver, large amounts of iron give it the red colour and used to build up new red blood cells. The bile is stored in the gall bladder to be used in digesting food again.
• Detoxification:
  The liver breaks down toxic materials such as alcohol which damages cells to fats. Alcoholics are known to have liver diseases.
• Helps in generating heat:
  The liver contains a very large number of cells, which means a lot of metabolic reactions take place in it producing lots of energy to warm the blood.
• Making fibrinogen:
  This is a plasma protein which helps in blood clotting when the skin is cut.

Teeth: Teeth are small, calcified, whitish structures found in the jaws (or mouths) of many vertebrates that are used to break down food. Types of mammalian teeth: Incisors: They are 4 in front of each jaw. They act like a blade to cut food(eg. To cut a bite of a sandwich) they have a (chisel-like surface). Canines: They are two in each jaw. They are very pointed, in humans they are used for the same purpose as incisors. However in carnivores they are longer and sharper and used to kill the prey. Premolars: 4 on the sides of each jaw They are used to cut and grind food. Molars: They are 6 at the back of  Each jaw, 2 of them are wisdom teeth. They have the same use as Premolars.

Note: remember that we have two jaws, so 4 incisors in each jaw means that we have a total of 8 incisors in our mouth. We have 16 teeth in each jaw, 32 in the whole mouth.

The tooth is divided into two parts, the crown and the root. Parts of the tooth: Enamel: Made of calcium salts, it is very strong. Dentine: It is covered by the enamel and surrounds the pulp cavity. The pulp cavity: It contains the nerves and blood vessels. The part of the tooth above the gum is called the crown, the part buried in the jawbone is called the root. The enamel covers the crown, the root is covered by cement. And the tooth is held in place by fibres. Tooth Decay: when we eat, some food particles stay in our mouth. Bacteria that lives in our mouth feed on these food particles, they respire anaerobically producing lactic acid. Like any acid, lactic acid reacts with the enamel and dissolves it away reaching the dentine, here we feel the toothache.

Methods Of preventing Tooth Decay:

• Reduce sugar intake to prevent bacteria respiring
• Brush teeth to remove the plaque layer of bacteria and saliva on our teeth and nuetralise mouth
• Use toothpaste or water containing fluoride because it is absorbed by the teeth and helps stopping the attack by acid
• Pay regular visits to the dentist.

Adding Fluoride To Water
Advantages Suitable amounts prevent tooth decay It is a cheaper method of teeth caring	Disadvantages Too much causes teeth molting, illness and abdominal pain It is expensive

Transport In Humans

The human transport system is a system of tubes with a pump and valves to ensure one way blood flow. We need a transport system to deliver oxygen, nutrients and other substances to all our body cells, and take away waste products from them. The oxygenated blood (high in oxygen, red in color) comes to the heart from the lungs in the pulmonary vein; the heart pumps it to the aorta (an artery) to the rest of the body. The deoxygenated blood returns to the heart from the body in the vena cava (a vein), the heart pumps is to the lungs to get rid of the carbon dioxide. Oxygenated Blood: Red color, high oxygen low Carbon dioxide. Deoxygenated Blood: Blue color, low oxygen high Carbon dioxide. Did you notice that during one circulation, the blood went through the heart twice, this is why we call it double circulation. When the blood is flowing away from the heart, it has a very high pressure, when it is flowing towards the heart it has a lower pressure.

The Blood:

The blood is a fluid consisting of several types of cells floating in a liquid called plasma.

Red Blood Cells: These are one of the smallest cells in your body, they are round with a dent in the middle, we call this shape a Biconcave disc. The function of the red blood cells is to transport oxygen from the lungs to the body cells. A red protein called Haemoglobin, when the blood reaches the lungs, oxygen diffuses from the alveoli to the red blood cells and combines with haemoglobin forming an unstable compound called oxyhaemoglobin. When the blood reaches the body cells, the oxyhaemoglobin is easily split into oxygen and haemoglobin again, the oxygen diffuses through the blood plasma to the cells.

Red blood cells are fully adapted to their function by the following characteristics:

• Biconcave disc shape gives it large surface area to carry more oxygen
• Haemoglobin to combine with oxygen
• No nucleus that takes up space.

White Blood Cells:

White blood cells are one of the substances floating in the blood plasma. They are completely different in function than red blood cells. White blood cells are part of the Immune System, they play a big role in protecting the body by killing bacteria which cause disease, also known as pathogens. White blood cells can be distinguished from red blood cells easily because they are much bigger, with a nucleus, and present in fewer amounts.

Types Of White Blood Cells:

Phagocytes: They kill bacteria by engulfing them, taking them in the cell then kill them by digesting them using enzymes, this process is called phagocytosis.  Most white blood cells are the phagocyte type.
Lymphocytes: Unlike phagocytes, lymphocytes have a large nucleus. They are produced in the lymph nodes (in the lymphatic system). Lymphocytes kill bacteria by secreting antibodies and antitoxins which kill the pathogens directly or make them easier to kill. Each pathogen could be killed by a certain type of antibody

The Platelets:

Platelets are tiny cell fragments that prevent bleeding when the skin is cut, and stops bacteria from entering our systems through the wound. This works by blood clotting, when the skin is cut, some reactions take place that results in platelets producing a protein, this protein will change the fibrinogen (another soluble protein in the plasma) to insoluble fibrin. The fibrin forms long fibres that clot together blocking the cut, thus preventing any bleeding, this is called blood clotting.

Blood Plasma:

This makes up most of the blood. It is mostly water with some substances dissolved in it, these include carbon dioxide, hormones, food nutrients, urea and other waste products. The blood plasma transports substances from one place to another.

Functions of the blood:

• Transportation of R.B.C’s, W.B.C’s, oxygen, food nutrients, hormones, and waste products.
• Defence against disease, by white blood cells phagocytosis and production of antibodies.
• Supplying cells with glucose to respire and keep a constant temperature.

Blood Vessels (Vascular System):

This is a number tubes carrying blood away from and to the heart and other organs. The main types are Arteries, Veins and Capillaries.

Arteries: Their function is to transport blood away from the heart to the lungs or other body organs. The blood in the arteries always has a high pressure. The heart pumps the blood quickly into the arteries, resulting in the pressure, each time the ventricle of the heart contracts, the pressure in arteries increase, when the ventricle relaxes, the pressure falls. The lumen of arteries is also very narrow, adding to the pressure.

The structure is simple, beside the narrow lumen, the arteries have a strong thick wall to withstand the pressure. Their walls are also elastic and stretchable.

Brief description of characteristics of arteries:

• Transporting blood away from the heart
• Always in a high pressure
• Strong but stretchable walls
• Narrow lumen.

Veins:

Their function is to transport blood to the heart from the body.

The veins always always have a low blood pressure because by the time the blood with high pressure reaches the veins, it loses most of the pressure. This means that blood flows very slowly in veins, to help this, veins lie between muscles so that the blood is squeezed when the muscles contract.

They have a simple structure. Because they have a low pressure, they don’t need strong, thick walls like the artery, instead they have thin less elastic walls. Their lumen is much wider too. Veins have a unique feature, that is valves. Because blood in veins flows slowly with a low pressure, there is a risk of a backflow, specially in veins that move blood upwards against gravity, like the ones in the leg. The valves ensure that the blood is always flowing in the direction of the heart. When the muscles squeeze the blood, the valves are open the let blood through, when muscles relax, valves close to prevent a backflow.

Brief description of characteristics of veins:

• They carry blood to the heart
• Always in a low pressure
• Thin less elastic walls
• Wide lumen
• Valves present.

Blood Capillaries:

Blood capillaries are very we

Blood capillaries are the smallest blood vessels in our systems. Their function is to get blood from the arteries as close as possible to the tissues in order to exchange materials with the cells, and to link arteries with veins. When arteries come near and organ or a tissue, it divides into arterioles, these arterioles divide more into several blood capillaries that go through the tissue, this is when the exchange of oxygen and food nutrients with carbon dioxide and waste products such as urea take place by diffusion.

ll adapted to their jobs. They are one cell thick to reduce the diffusion distance of materials for faster diffusion. They also have pores in their walls between the cells, to allow the plasma to get out of the blood and become tissue fluid.

The Heart:

The heart is a pumping organ that is responsible for the movement of blood around the body. The function of the heart is to give the blood a push, keeping it flowing around the body all the time. That is why the heart is constantly working, if it stops for a minute, the other organs will not receive any oxygen or nutrients, thus the body fails and the person dies. The heart is located in the chest, the thoriac cavity between both lungs.

Structure: The heart is hollow, it has 4 chambers. Two of them are atria and two are ventricles. One of each of these on each side. When looking at the diagram of a heart, notice that your right is the left side of the heart, and your left is the heart’s right, as if you are looking at your own heart on a mirror. The sides of the heart are separated by a wall called septum. Each side contains an atrium (at the top) and a ventricle (at the bottom), there is a valve between the atrium and the ventricle in each side, it is called bicuspid valve in the left side and tricuspid valve in the right side. There are several blood vessels associated with the heart, these are:

• The Pulmonary vein, it transports oxygenated blood from the lungs to the right atrium.
• The Aorta, which is the biggest artery in the body, it transports oxygenated blood from the heart to the rest of the body.
• The Vena Cava, the biggest vein in the body, it transports deoxygenated blood from the whole body to the heart.
• The pulmonary artery, it transports deoxygenated blood from the heart to the lungs.

Note that blood vessels entering the heart are veins, and the ones leaving the heart are arteries. The left side of the heart always contains oxygenated blood because it receives blood fresh from the lungs and pumps it to the body, the right side always contains deoxygenated blood because it receives is from the body. You can memorise this by the word LORD:

Left Oxygenated – Right Deoxygenated

The heart receives blood from the lungs at the left atrium and pumps it to the body  from the left ventricle, then it receives it again from the body at the right atrium and pumps it to the lungs from the right ventricle. The red shows oxygenated blood and the blue shows deoxygenated blood. Notice that the walls around the left ventricle are much thicker than the ones in the right ventricle. The reason for this is that because the left ventricle pumps blood to the whole body, so blood will travel a long distance, so it needs lots of muscles to contract and pump the blood more strongly. However, the right ventricle pumps blood the lungs which are very close to heart, the blood does not need to be pumped very strongly.

Mechanism of the heart:

When the heart is being filled with blood (whether from the body or the lungs), this is called the diastole. When the heart is pumping the blood out of it (whether to the body or to the lungs), it is called the systole.

During diastole, the heart is getting filled with blood, the blood enters the atria first, the atria contract to force blood into the ventricles, both tricuspid and bicuspid valves are open to allow blood into the ventricles and the semilunar valves are shut. Once the ventricles get filled with blood, it is systole, the bicuspid and tricuspid valves get shut and semilunar valves are open, the ventricles contract strongly forcing the blood into the Aorta or pulmonary artery.

During diastole the semilunar valves are shut to keep the blood out of the arteries. During systole the tricuspid and bicuspid valves are closed, to prevent blood from flowing back into the atria when it is pumped. The tricuspid and bicuspid valves are kept fixed by fibres called tendons, they prevent the valves from opening in the opposite direction, allowing backflow.

The tendons also control the opening and closing of the cuspid valves, when the tendons are loose, the valves are open. When the tendons are tightened the valves close.

	Diastole	Systole
Ventricles:	Relax	Contract
Atria:	Contract	Relax
Cuspid Valves:	Open	Close
Tendons:	Loose	Tightened
Semilunar Valves:	Close	Open

If you listen to your heartbeat, you will hear two sounds, one low and one high. These are results of the systole and diastole. They are the sounds of the cardiac valves opening and shutting.

Coronary Heart Disease (CHD):

The heart, like any other organ, needs a supply of blood containing oxygen and  nutrients. In fact, the heart needs a higher amount of blood supply than any other organ because it is working all the time, and contains a lot of muscles. The coronary arteries are those which supply the heart tissues with blood, they branch from the aorta. CHD develops when cholesterol layers build on the walls of the coronary arteries, partially blocking the path of blood, thus this tissue of the heart is not supplied with oxygen nor nutrients, so it stops working properly. If it is not treated at this age, a blood clot may form near the partially blocked area, completely blocking the artery, when this happens, the blood cannot function anymore, a heart attack occurs, which is extremely fatal. The causes of CHD are mostly in the diet. A diet with lots of fats, increases the chance of cholesterol building up on the walls of the artery, causing CHD, Same thing with salts. Smoking also increases the rate of fat deposition. It was also said that Causes Of CHD are:

• Diet full of fats increases the fats level in blood
• Diet full of salts, salts can be deposited in the artery leading to CHD, same as fats or cholesterol
• Smoking, carbon monoxides increases fat deposition
• Stress was also said to contribute to CHD by raising blood pressure
• Lack of exercise, regular workouts improve the blood flow wearing layers of fats or salts deposited on the walls of arteries away.

So to protect yourself from CHD you need to avoid diets full of fats and salts, avoid smoking, try to be less stressed out, and exercise regularly.

Tissue Fluid And Lymph:

Tissue fluid is a fluid surrounding the cells of a tissue. It supplies them with all their needs of oxygen and nutrients, and takes away all their waste products including carbon dioxide. Tissue fluid plays a very big role in substance exchange between blood and cells.

Plasma from the blood capillaries move to the tissue through gaps in the walls. They become tissue fluid. They exchange their content of oxygen and nutrients with the cells and take carbon dioxide and waste products. At the end of the capillary bed, the tissue fluid leaks back into the blood, and becomes plasma again, but not all of it. A little of it is absorbed by the lymphatic vessel and becomes lymph. The lymphatic vessel takes the lymph to the blood stream by secreting them in a vein near the heart, called subclavian vein. The lymph in the lymphatic vessels are moved along by the squeeze of muscles against the vessel, just like some veins.

The lymphatic system plays a big role in the protection against disease. It produces the white blood cells lymphocytes. Which kill any cell with a different antigens than the ones in your body cells. So if bacteria get into your body, your lymphocytes quickly recognise them as foreigners and will divide and kill them.

Lymphocytes are considered a problem when it comes to organs transplant. For example if someone (recipient) with renal failure receives a kidney from another person (the donor), the cells of the kidney will have different antigens than the other cells in the patient’s body. The lymphocytes will consider the cells of the kidney an enemy and start attacking it, this is called tissue rejection. Organ transplant is perfect in one case, this is when the donor and the recipient are identical twins, because the antigens of their cells perfectly match. In other cases the recipient is given immunosuppressant drugs to actually weaken their immune system to prevent tissue rejection.

Brief Summary Of Functions Of The Lymphatic System:

• Production of white blood cells lymphocytes
• Transport of digested fats from villi to blood stream
• Transport of lymph from the tissue fluids to the blood stream at the subclavian vein.

Plant Nutrition

Plants are living organisms, they need food in order to keep living. The way they obtain their nutrients however, is completely different than that of ours. Plant make most of their nutrients by them selves, they just need 2 raw materials, these water and carbon dioxide.

The leaf of a plant is considered the kitchen of it. It is where food is made, later on you will see how the leaf is adapted to making food.

Upper Epidermis: it is a layer of cells that cover the leaf and protect it, it is covered by a layer of wax calledcuticle.

Mesophyll Layer:

• Palisade Mesophyll: a layer of palisade cells which carry out most of photosynthesis
• Spongy Mesophyll: a layer of spongy cells beneath the palisade layer, they carry out photosynthesis and store nutrients.

Vascular Bundle: it is a group of phloem and xylem vessels that transport water and minerals to and from the leaves. Lower Epidermis: similar to the upper epidermis, only that it contains a special type of cells called guard cells. Guard cells are a specialised type of cells that control the passage of carbon dioxide into the cell and the passage of oxygen out of the cell by opening and closing the stomata (a hole in the leaf through which gases pass) so guard cells are responsible for the gas exchange.

Photosynthesis: Photosynthesis means “making with light”. It is the process by which plants make useful glucose out of the raw materials water and carbon dioxide, using light energy from the sun. Water is essential for photosynthesis, it is sucked up from the soil by the roots and transported up the stem to leaves where it is put into use.  Carbon dioxide, just like water is essential for photosynthesis. It moves into the leaf from the air by diffusion, through the stomata (tiny wholes in the leaf). Once carbon dioxide and water are present in the leaf, one condition for photosynthesis is needed, that is light. The two cells in the diagrams are called palisade cells (the rectangular one) and spongy mesophyl cell (the circular one), these are the cells where photosynthesis takeplace. They a structure called chloroplasts, these structures contain a green pigment named chlorophyll, this is to trap sunlight to be used in energy, a large number of chloroplasts is required for photosynthesis.

How photosynthesis happen:

• Carbon dioxide and water enter the cell
• The cell traps light energy using chloroplasts
• The energy is used to split water (H2O) into hydrogen and oxygen
• The oxygen is excreted outside the leaf to the atmosphere as a waste product
• The hydrogen reacts with carbon dioxide forming glucose.

Overall equation for the Photosynthesis

Carbon Dioxide Supply:

The carbon dioxide moves to the leaf from the atmosphere by diffusion through tiny holes in the leaf called stomata. Carbon dioxide is not present in a high concentration in air, but compared to its concentration inside the leaf, it is more in the air. This is because the cells inside the leaf are always doing photosynthesis (at daytime), converting the carbon dioxide into the glucose quickly, thus the concentration of it inside the leaf decreases, making a concentration gradient for diffusion from the atmosphere to the leaf.

Water Supply:

The water is absorbed by the roots of the plants, then they are transported upwards through a hollow tube called the xylem vessel, till it reaches the leaf where photosynthesis takes place, it enters the leaf through holes in the xylem. Excess water leaves the cell through the stomata, this is called “transpiration”

Sunlight Supply:

The leaves are always exposed to sunlight at daytime. The sun penetrates the transparent layers on the leaf till it reaches the mesophyll layer, where photosynthesis take place. Palisade cells are nearer to the surface of the leaf than the spongy cells, so they receive more of the light and make more photosynthesis.

Factors Needed For Photosynthesis:

• Water
• Carbon Dioxide
• Light

Factors Affecting The Rate Of Photosynthesis:

• Amount of water: the rate increases as it increases
• Concentration of carbon dioxide: the rate increases as it increases
• Light intensity: the rate increases as it increases

Plants at night:

At night, the plant performs several process to convert the stored starch into many useful nutrients like:

• Sugars for respiration
• Cellulose and proteins for making cells
• Vitamins to help in energy action
• Fats as a long term storage material
• Remaining starch is temporarily stored.

Mechanism of Guard Cells:

At daytime, the guard cells open the stomata to allow gaseous exchange, this occurs according to the following steps:

• Sunlight increases the potassium concentration in the vacuoles of the guard cells, the water potential decreases making a gradient between the guard cells and the surround epidermal cells,
• Water moves by osmosis into the guard cells from the epidermal cells,
• The water raises the pressure inside the guard cells,
• The cell wall adjacent to the stomata is thicker and less stretchable then the cell wall on the other side,
• The pressure expand the whole cell except for the inner cell wall (adjacent to the stomata) creating a curve and a pore between the two guard cells,
• The stoma opens.

At night however, the mechanism is opposite:

• Potassium level decreases in the vacuole of the guard cells,
• Water potential increases in the cell and water diffuses out of it,
• The guard cells straighten up because of low pressure closing the stoma.

Mineral Requirements:

The plant is also in need for mineral ions to control chemical activities, grow, and produce materials. The most important minerals are:

• Mg+2  (Magnesium ions): they are important for the production of the green pigment chlorophyll. Lack of it results in lack of photosynthesis and wilting of the leaves,
• Nitrates: these are the sources of nitrogen, they are required to make amino acids and proteins by combining with glucose. Lack of it results in deformation of the plant structure making it small and weak.

Both mineral ions are absorbed from the soil.

Fertilisers:

Sometimes the soil is lacking of the mineral ions needed, this problem can be solved by adding fertilisers to the soil. Fertilisers are chemical compounds rich in the mineral ions needed by the plants. They help the plants grow faster, increase in size and become greener, they simply make them healthier and increase the crop yield. But there are disadvantages of fertilisers, such as: Excess minerals and chemical can enter a nearby river polluting it and creating a layer of green algae on the surface of it, causing lack of light in the river, thus preventing the aqua plants photosynthesizing. When living organisms in the river or lake die, decomposers such as bacteria multiply and decay, respire using oxygen. Eutrophication takes place eventually.

Green House:

A green house is a placed covered by transparent polythene. In green houses, the limiting factors of photosynthesis are eliminated, and the plants are provided the most suitable conditions for a healthy, rapid growth. The soil in green houses is fertilised and very rich in mineral ions, assuring healthy, large yields. More carbon dioxide is supplied to the crops for faster photosynthesis. The polythene walls and ceiling allow heat waves and light rays only to enter and prevent harmful waves, thus providing a high light intensity and optimum temperature, sometimes a heating system is used too. A watering system is also present. The disadvantages of green houses are that it is too small to give a large yield and that it is expensive.

The Respiratory System

We rarely think about breathing unless we’re out of breath. The act of breathing is part of the respiratory system, a complex process where air travels into and out of the lungs.

Respiration is slightly different, where exercise causes muscles to release energy in the form of glucose. Both systems are co-dependent, especially when you’re playing sport and inhaling greater quantities of oxygen.

Human Respiratory System:

The human respiratory system is made up of air passages, lungs and the respiratory muscles. Nose: most breathing and gas exchange occur through the nose. It is lined by a layer of mucus and hair to trap the dust and germs in the air. It is also supplied with a dense network of blood capillaries to warm the air entering the body. Pharynx: Works together with the epiglottis to block the nasal cavity and the trachea during swallowing food, to prevent it from entering the respiratory system. Trachea (windpipe): this is a tube that connects the nasal cavity and larynx to the lungs. It is lined with a layer of ciliated epithelium cells and goblet cells which secrete mucus that traps bacteria and dust from inhaled air and gets moved upwards to the larynx by the cilia.

It is then either spit out or swallowed to the stomach where it is eliminated by acid.

Bronchi: when the trachea reaches the lungs, it is divided into two tubes, one goes to the right lung and one goes to the left lung. These are called the bronchi. The bronchi are then divided bronchioles that extended deeper into the lungs.

Alveoli (air sacs): these are tiny bags full of gas; they are present in the lungs in large amounts (several million alveolus in each lung). They give the lungs a much larger surface area (about 70 m2) for faster diffusion of gases between them and the blood. Rib Cage: the lungs are protected by this cage of bones. It surrounds all the thoracic cavity. They are 12 pairs of ribs; one pair extends from one of the first 12 vertebrae of the vertebral column. All of the ribs except for the last two pairs are connected to the sternum, the chest bone. Each pair of ribs is connected to the pairs above it and below it by muscle fibres called inter costal muscles. The rib cage and the lungs are separated by an elastic layer called pleural membrane, or pleura for short. It protects the lungs from damage caused by friction with the rib cage during breathing. Diaphragm: this is a sheath of muscles that separates the thoracic cavity from the abdominal cavity. Together with the ribs and the inter costal muscles, it plays a big role in breathing and gas exchange.

 

Gas Exchange (Breathing):

Breathing is different from respiration. Breathing is just the exchange of waste gases from the body with fresh air from the atmosphere. The action of breathing fresh air in is called inhaling; the action of breathing waste gases out is called exhaling.

During Inhaling, the brain sends electric impulses by nerves to the diaphragm and the inter costal muscles. The diaphragm contracts becoming flatter. The inter costal muscles also contract and move the ribs in an outer upwards directions. These actions expand the thoracic cavity making the lungs expand, thus increasing the increasing the volume, with the volume increasing the internal pressure decreases which makes air enter the lungs through the mouth, nose and trachea.

During Exhaling, the diaphragm and the inter costal muscles relax again, contracting the thoracic cavity thus squeezing the air out of the lungs to the trachea and mouth and nose to the atmosphere.

Respiratory System in Action:

Inhaling occurs, air is absorbed by lungs, it enters the nose where bacteria and dust in it are trapped by mucus and warmed by blood capillaries. The air enters the trachea where it is cleaned again by cilia. The bronchi take the air from the trachea to each lung. Bronchi divide into several bronchioles; each one has a group of alveoli at the end of it. In the alveoli gas exchange takes place where the oxygen rich air diffuses into the blood capillaries of the pulmonary arteries and the carbon dioxide rich gas diffuses into the alveoli to be exhaled.

The pulmonary vein carries the oxygenated fresh air to the heart where it is pumped to all the body cells. The inter costal muscles and diaphragm relax squeezing the waste gases out of the lungs, this is exhalation.

 

Gas Exchange in Alveoli:

Each alveolus is supplied with blood capillaries. These come from the pulmonary artery and they contain deoxygenated blood rich in carbon dioxide. The concentration of oxygen is very high inside the alveolus and very low in the blood, so oxygen molecules diffuse from the alveolus to the red blood cells and combine with haemoglobin. At the very same time this occurs, carbon dioxide diffuses from the blood to the alveolus because the concentration of it is very high in the blood and low in the alveolus.

 

Adaptations of Alveoli:

Gas exchange happens because of several factors in the alveolus and the blood capillaries that control the rate of gas exchange:

• Very thin wall of both the alveolus and the capillary, they are one cell thick which makes the diffusion distance shorter, increasing the rate.
• The difference in concentration of gases between the alveolus and the capillary is very large, increasing the diffusion rate of gases.
• The alveolus are balloon shaped which gives it a very large surface area for faster diffusion.
• The walls of the alveolus are lined by a thin film of water in which gases dissolve in during diffusion, this makes it faster.

 

Composition of Inspired and Expired Air:

Gas	Inspired Air	Expired Air
Oxygen	21%	16%
Carbon Dioxide	0.04%	4%
Nitrogen	79%	79%
Water Vapour	Variable	High

 

Lung Capacity:

When lungs of an adult are fully inflated they have a volume of about 5 litres.

Tidal Volume: This is the volume of air breathed in and out at rest, this is 0.5 litres.

Vital Volume: The maximum volume of air that can be breathed in and out, at exercise for example is 3 litres.

Residual Volume: The lungs have to have a certain volume of air inside them all the time to keep shape. This is the residual volume and it is 1.5 litres. This air is renewed through breathing.

Aerobic Respiration:

A chemical, metabolic reaction that burns down glucose with oxygen producing carbon dioxide, water vapour and lots of energy

Aerobic Respiration: the release of relatively large amounts of energy in cells by the breakdown of food substances in the presence of oxygen.

Anaerobic Respiration:

Some organisms are able to respire and release energy when oxygen is lacking. This is anaerobic respiration. These are like yeast, bacteria and other organisms. Humans can also respire anaerobically for a short period of time. The amount of energy produce is much smaller than that produced during aerobic respiration though.

Anaerobic respiration: the release of relatively small amount of energy by the breakdown of food substances in the absence of oxygen.

C₆H₁₂O₆ (aq) + 6O₂ (g) → 6 O₂ (g) + 6H₂O (l)

Anaerobic Respiration I Yeast: Yeast is able to respire anaerobically by breaking down glucose molecules into ethanol and carbon dioxide.

C₆H₁₂O₆ → 2C₂H₅OH + 6CO₂

Ethanol is produced here, so it is a fermentation reaction. Do remember that glucose is the only reactant.

Anaerobic Respiration in Humans:

When the amount of oxygen received by the muscle cells of the body is not enough to carry out all respiration aerobically, the cells respire anaerobically. But they cannot go like that for a long time. The anaerobic respiration in humans is different than of yeast. Lactic acid is produced instead of ethanol, and no carbon dioxide is produced.

C₆H₁₂O₆ → 2C₃H₆O₃

The lactic acid produced is very toxic and harmful to the body. That is why it has to be broken down with oxygen as soon as possible. This is called oxygen debt. Breaking down lactic acid releases energy too, if you add up the amount of energy produced during breaking down lactic acid and anaerobic respiration, you will find that it is the same as the amount produced during aerobic respiration.

 

Effects of Smoking:

Short Term Effects:

• Cilia can’t vibrate anymore, the air inhaled isn’t clean. Goblet cells release more mucus which makes the trachea narrower.
• Nicotine increases heart beat rate and blood pressure.
• Carbon monoxide combines with haemoglobin instead of oxygen combining with it.
• Carboxyhaemoglobin is formed which is stable.
• Less oxygen transported to cells.

Diseases Caused By Tar:

Chronic Bronchitis:

• Tar makes goblet cells in trachea produce excess mucus
• Mucus falls into lungs
• Bacteria in mucus breed causing infections like bronchitis
• The layer of excess mucus lining the walls of the alveoli increase the diffusion distance of gases making gas exchange slower

Emphysema:

• The excess mucus lining the alveoli irritates it, causing strong coughs which damage the alveoli.
• The alveoli lose its shape and surface area making gas exchange much slower.
• This cause short breathes and sounds while breathing.

Lung Cancer:

• When tar reaches the lungs, it is absorbed by cells of the bronchi, bronchioles and the lungs.
• The tar causes excessive division and reproduction of these cells which develops into cancer
• The cancer can be spread to other organs too.

Diseases Caused By Nicotine:

Coronary Heart Disease:

• Nicotine helps cholesterol deposition on walls of coronary arteries. This causes atheroma.
• Carbon monoxide also increases risk of blood clots forming which might results in blocking the artery.
• Less oxygen is delivered to heart cells, a heart attack or failure can take place leading to death.

Excretion in Humans

Excretion is the removal of toxic materials, the waste products of metabolism and substance in excess of requirements from organisms. Metabolism is chemical reactions taking place inside cells, including respiration. The body excretes three main waste materials. These are Carbon Dioxide, Urea and Water. Excretion is a very important feature to us because without it toxic substances will build up in our bodies and kill us. It also helps in maintaining the composition of body fluids. The Excretory System of humans is made up of 4 structures: Two kidneys, two ureters, a bladder, and the urethra. The kidneys act as a filter to filter the waste products from the blood, the ureters are tubes that transport the main waste products (urine) from the kidneys to the bladder, where it is stored until it is excreted out of the body through the urethra.

 

Formation of Urea:

• When you eat a food high in protein, it is digested in the small intestine into amino acids.
• The villi on the walls of the small intestine absorb the amino acids into the hepatic portal vein.
• Hepatic portal vein is a special vein that transports digested material from the small intestine to the liver.
• The liver plays a big role in maintaining the level of protein in our body. It absorbs all amino acids from the hepatic portal vein. If the body needs proteins, they will pass through the liver into the blood stream to be used by the body cells to make protein.
• If the body does not need proteins. The liver will absorb excess amino acids and break them down into carbohydrates and nitrogen. The formula of amino acids is CHON; here we remove the nitrogen from the molecule, to get a carbohydrate. This is called deamination. Nitrogen is made into urea which is a nitrogenous waste product.
• The products are then released to the blood stream.

Kidneys Structure: A kidney consists of two main structures: Cortex (outer layer) Medulla Between the cortex and the Medulla, there is a structure called thenephrone.  The nephrone is the where filtration of toxic materials from the blood takes place. We have many of them in each kidney. In the centre of the kidney there is a cavity called the pelvis which leads to the ureter.

Structure of Nephrone:

The nephrone starts with a cup shaped structure called Bowman’s capsule. Inside the Bowman’s capsule there is a very dense network of blood capillaries entering as capillaries from the renal artery and exiting as capillaries from the renal vein. This dense network of capillaries is called Glomerulus. The rest of the nephrone is a long coiled tube where materials filtered from the blood flow in. At some point the coiled tube becomes straight and is bent in a U shape tube, this part is called loop of Henle and it is surrounded by a network of capillaries from the renal vein, it is where reabsorption takes place. All nephrones end at a large tube called the Collecting duct where content of the nephrones are transported to the pelvis, to be secreted in the ureter.

Mechanism of the Kidneys:

Ultrafiltration:

The blood in the renal artery contains large amounts of urea, glucose, water, mineral ions and some amino acids. When it reaches the glomerulus, the high pressure of the blood and the concentration gradient of these materials between the blood and the nephrone cause most of these substances to diffuse from the blood to the bowman’s capsule and become content of the nephrone, which is called glomerular filtrate (glomerular filtrate is a mixture of urea, water, glucose and mineral ions that diffused from the blood to the nephrone).

Reabsorption:

The glomerular filtrate moves in the nephrone till it reaches the loop of henle, which is surrounded by a dense network of blood capillaries of the renal vein. Here there is a concentration gradient of the content of the content of nephrone between the nephrone and blood. Both diffusion and active transport occur to ensure the complete reabsorption of valuable substances from the glomerular filtrate back to the blood; these substances are glucose and amino acids. Some water also moves by osmosis to the blood, as well as minerals.

That leaves urea, excess water and minerals to continue in the nephrone till it reaches the collecting duct and the pelvis. This mixture is called urine. Urine is transported from the pelvis to the urinary bladder by the ureters. It is them secreted out of the body through the urethra.

Dialysis: If a person gets a kidney failure, which means his kidneys cannot function anymore, they have to wash their blood on regular basis with a machine that is an alternative to the damaged kidneys. This process is called dialysis. During this process, a tube is attached to the patient’s vein; the tube is attached to the dialysis machine on the other end. There is another tube coming out of the machine to the patient’s vein. The blood is sucked from the patient’s vein, it goes through the machine, and out from the other side back to the patient’s vein. When the blood enters the dialysis machine, it is very rich in waste materials (urea, excess water and minerals).  The tubes inside the dialysis machine are made of a partially permeable membrane to allow diffusion. The tubes are also surrounded with dialysis fluid

which is the same as blood plasma. The concentration of waste products in the blood is much higher in the blood than in the dialysis fluid. This creates a concentration gradient, diffusion occurs and waste products leave the blood to the dialysis fluid, which then exists the machine and gets disposed. The dialysis fluid has to be renewed continuously to keep the concentration gradient of waste products higher in the blood, thus ensuring that all waste products leave the blood. The clean blood is then returned to the patient’s vein.

Homeostasis

The human body has the ability to maintain a constant internal environment so that every organ and cell is provided the perfect conditions to perform its functions. This is called homeostasis. There is no organ system for this function. However, every organ plays a role in maintaining a constant internal environment. For example the lungs are responsible for the supply of oxygen to cells. The liver is to maintain a constant level of glucose and amino acids, and so on..

Temperature Regulation:

A healthy human should have a body temperature of 37°C.  If the body temperature drops below 37°C, metabolic reactions become slower because molecules move slower and have less kinetic energy. If the temperature rises above 37°C, the enzymes of the body begin to get denatured and metabolic reactions will be much slower.

Sometimes, the temperature of the area you are at is low enough to decrease your body temperature. Sometimes it is high enough to raise your body temperature. This is why the body has the ability to control its body temperature. Our skin is responsible for this process.

The Human Skin: The skin is an organ that coats your entire body. The skin is made up of two layers, the Epidermis and the dermis. The epidermis’s main function is to protect the dermis which contains most of the structures, and protect the body from ultra-violet rays. The surface of the epidermis is made of tough, dead cells. The dermis contains many useful structures. Hairs, sweat and sebaceous glands, sense receptors and erector muscles are responsible for controlling the body temperature. Blood vessels transport oxygen and nutrients to the cells of the skin.

A healthy body is continuously gaining and losing heat. Metabolic reactions like respiration release a lot of heat energy, muscular activity increase the metabolic rate and release more heat energy. The body can also gain temperature from the surroundings like the sun or by eating hot food. Heat is lost by the body through exposed skin by conduction. If there is sweat or water on the skin, it will absorb body heat to evaporate which drops the temperature. All these factors are normal however, but it is considered dangerous when the body temperature keeps on dropping or rising severely.

Cooling Down the Body:

When the body is overheated, the body takes several actions to drop it by trying to lose heat in several ways: Vasodilation: this action causes the body to lose heat quickly. It involves widening the lumen of blood vessels of the skin, this increases blood flow and rate of heat loss. The vessels are also brought near the surface of the skin to reduce the distance heat has to travel to escape. Sweating: Sweat glands near the skin begin to secret sweat on the surface of the skin through the pores. This sweat acts as a heat consumer to absorb the body heat and use it in evaporation. The activity of sweat glands is increased when the temperature of the body rises.

Hairs lie flat: The muscle erectors of the hairs relax making the hairs lay flat of the skin. When the hairs are erect, they trap air in the gaps between them, this acts as an insulation and prevents heat loss. But when the hairs are flat, less air is trapped between them so there is no insulation and more heat can be lost.

Heating Up the Body:

When the body temperatures drop, the body takes several actions to regulate its temperature by insulation to prevent heat loss and producing heat energy: Vasoconstriction: this causes the blood vessels to become narrower to reduce heat loss. They also sink deep into the skin to increase the distance heat has to travel to escape thus reducing heat loss. Shivering: the muscles in the limbs start to contract and relax rapidly, thus increasing the rate of respiration and amount of heat energy released by it. Hairs become erect: muscle erectors contract and make the hairs erect and stand up vertically trapping air in the gaps between them. This acts as insulation to reduce heat loss.

How the Body Senses Change in internal environment:

When the body’s internal temperature changes the temperature of the blood changes with it. When the blood flows through the brain, a part of it called the hypothalamus detects the drop or rise in temperature. The brain then starts sending electrical impulses to the rest of the body so that it works on heating or cooling its self.

This process is called Negative Feedback. Negative feedback is not for change in temperature only though, it is for any change in the internal temperature including the blood glucose level.

Regulating Blood Glucose Level:

For blood glucose level however, the pancreas is the organ which monitors its level not the hypothalamus. When the blood flows through the pancreas, the pancreas detects the level of glucose in it. If it is higher than normal, the pancreas secretes a hormone called insulin. Insulin flows in the blood till it reaches the liver. When it reaches the liver, insulin hormone will make it convert excess glucose in the blood into glycogen and store it in the liver cells. When the blood glucose level becomes normal, the pancreas will stop secreting insulin so that the liver stops converting glucose. If the blood glucose level decreases below normal, the pancreas secretes another hormone called glucagon. When glucagon reaches the liver, it makes the liver convert the glycogen it made from excess glucose back into glucose and secrete it into the blood stream so that the blood glucose level goes back to normal. When this happens the pancreas stops secreting glucagon.

Normal Blood Glucose Level: 80-100 mg per 100cm3.

Co-ordination and Response

You have previously learned that one of the 7 characteristics of living organism is irritability or sensitivity. And this is the ability to detect a change in the outer environment and respond to it. A change in the environment is also called a stimulus (plural stimuli). Actions taken by the body in order to co-operate with a stimuli are called responses. The body detects a stimulus by parts in the body called receptors and is able to respond to it through other parts called effectors. Two organ systems are continuously working to detect and respond to stimuli, these organ system are called the nervous system and the endocrine system.

The Nervous System: The nervous system is a system of organs working together to detect and respond to stimuli. The nervous system is made up of two systems, the Central Nervous System (C.N.S) and the Peripheral Nervous System (P.N.S) the peripheral nervous system connects the central nervous system to the other parts of the body. Central Nervous System (CNS): The central nervous system is made up of the brain and the spinal cord. The spinal cord is basically a big bundle of nerve cells running through a tunnel inside the backbone which protects it while the brain is protected by the skull. The central nervous system is what gives out orders to other parts of the body to perform certain jobs. The Peripheral Nervous System PNS: The peripheral nervous system is the other part of the nervous system. The main job of the PNS is to detect stimuli and send impulses to the CNS according to the stimuli. The PNS is made of receptors and nerves that carry the impulses.

Receptor cells are ones whose function is to detect something about its environment. There are many receptors in the body that are able to detect many changes like temperature, touch, light, sound and chemicals. There are some organs in the body that are there to detect just one stimulus, like the eye for example. These are called sensory organs and they can be defined as a group of receptor cells responding to specific stimuli.

Effectors are the opposite of receptors. Receptors are two detect the stimuli while effectors are two respond to it. Effectors are usually muscles and glands.

Neurons (Nerve Cells):

Neurones are one of the most important structures of the nervous systems. Neurones act as a wire that transmits electrical impulses all over the body. Like a cable that consists of many wires, a bundle of neurones is called a nerve. There are 3 types of neurones, each type is to transmit electrical impulses from a specific place to another.

Motor Neurone: This is a neurone that transmits electrical impulses from the Central nervous system to the effectors.

This neurone is made up of three segments; the cell body which is the start of the motor neurone and is in the CNS, axon which stretches out from the cell body all the way to end of the neuron, and the motor plate which is the end of the neurone and is in the effector muscle. 
Neurones have features that are common between most animal cells like a nucleus, cymiddlelasm and cell surface membrane, but they also have some exclusive features like the axon. The axon is an extended cymiddlelasm thread along which electrical impulses travel. Some motor neurones have axons of length 1 metre. Axons are coated by a layer of myelin called myelin sheath, this is an electrically insulating layer which is essential for the proper functioning of the nervous system.

Another exclusive feature of neurones is dendrites, these are several short threads of cymiddlelasm coming out of the cell body. Their function is to pick up electrical impulses from other cells.

The last exclusive feature of motor neurones only is motor end plate. This is just the end of the axon which is in the muscle. It passes the electrical impulses from the neurone to the muscle fibres.

Sensory Neurones: like other neurones, sensory neurones carry electrical impulses from one place to another. But sensory neurones carry electrical impulses in the direction different to that of motor neurones, from the receptors to the CNS.

The sensory neurone’s shape is unique. This is because it is made of a cell body, with two arms extending out of it. The first arm is the axon which’s other end is in the CNS. The second arm is dendrite which’s other end is in the receptor. The dendrite is similar in structure to the axon except that it joins the receptor with the cell body. The electrical impulses of the sensory neurone flow from the receptor, through the dendrite to the cell body, then from the cell body to the CNS through the axon.

Relay Neurone: Relay neurones are located in the CNS. Their job is to pass electrical impulses from the sensory neurone onto the motor neurone, so it acts like a diversion.

Where neurones meet, they are not actually touching each other. Instead there is a gap called synapse or junction box. When the electrical impulses reach the end of a neurone, the neurone secretes a chemical transmitter which passes by diffusion to the other neurone causing the impulses to be carried from the first neurone to the second.

Reflex Arc (Nervous System in Action):

If your finger touches a hot surface, receptor cells in the skin of your finger detect a stimulus, which is a sudden rise in the temperature. The receptor uses the energy of the stimulus to generate electrical impulses. These impulses are then carried by the axons of the dendrites of the sensory neurone through cell body to axon and from the axon to the CNS. At the CNS the electrical impulses travel through the synapse to the relay neurone, which passes it onto the motor neurone. The nerve impulses are transmitted through the axon of the motor neurone to the targeted muscle which contracts when electrical impulses reach it, resulting in your finger being pulled away from the hot surface. This pathway is called the reflex arc and happens in about a fraction of a second.

Reflex Arc: RECEPTOR → Sensory Neurone → CNS → Motor Neurone → EFFECTOR

 

Voluntary and Involuntary Actions:

The reflex arc is a reflex action. Reflex means it is automatically done without your choice. This is because when the electrical impulses reach the relay neurone in the CNS from the receptors, some impulses are carried by other neurones to the brain, and some impulses are passes onto the motor neurone to the effector muscle and the response takes place. The electrical impulses going to your brain are much slower that the ones going to the effector muscle directly. This is why the reflex action takes place before you realise it, it is uncontrollable. Reflex actions are said to be involuntary actions. Involuntary actions start at the sense organ heading to the effector. They are extremely quick. Voluntary actions are the ones that you make the choice to do. Like picking up a bag from the floor for example. Your brain sends electrical impulses to the effector muscles ordering them to contract so you could pick the bag up. Voluntary actions are slower than involuntary actions and they start at the brain.

The Human Eye:

The human eye is a sensory organ. This means it is an organ of tissues working together to detect and respond to a specific stimulus, which is light.

Features of the Human Eye:

• Lens: changes shape to focus light on retina
• Ciliary muscles: contracts and relaxes to adjust thickness of the lens
• Suspensory ligaments: loosens and tightens to adjust thickness of lens
• Iris: widens and narrows to control amount of light entering the eye depending on light intensity
• Choroid: middle layer surrounding the eye. It contains many blood vessels
• Sclera: outer most tough, protective layer of the eye.
• Retina: inner most layer. It is sensitive to light and it is where the fovea is and it has rods and cons
• Fovea: very light sensitive spot
• Blind spot: Where the optic nerve touches the eye. No light sensitive cells in this area.

How We See:

When the light hits an object, it is reflected in all directions. When a light ray reflected from the object hits your eye you see that object. At the back of your eye, there is a spot on the retina called the fovea (blind spot). This spot is full of light sensitive cells. When the light ray falls on the fovea, the light sensitive cells generate electrical impulses that travel through the optic nerve to brain. When the electrical impulses reach the brain, the brain generates the image you see. This all happens in less than a fraction of a second.

But this is the general idea only. Light rays enter the eye from every direction. If they are not focused on the fovea, they will most probably not hit it and we won’t see. Here comes the role of the front part of the eye. When the light ray hits the eye at an angle, it first has to penetrate the cornea which refracts (bends) the light ray inwards. The cornea acts as a converging (convex) lens. Then the light penetrates the lens which refracts the ray a little more inwards focusing the light ray on the fovea. And thus the light ray is focused on the retina. When the ray hits the retina, the closer to the fovea the sharper the image is.

Accommodation:

The angle at which the light ray hits the hits the eye depends on the distance of the object. Every light ray that hits the eye needs a certain amount of refraction in order to be directed to the fovea. This is why the lens has the ability to widen and narrow according to the distant of the object you’re looking at in order to make the light ray hit the retina at the right spot. This is called accommodation. Light rays refracted from close objects are diverging (spreading out), they need to be refracted inwards to be focused on the fovea. When you look at a close object, it takes some time till the vision becomes clear. This is because at first, the light ray is not correctly refracted, so it hits the retina away from the fovea. The electrical impulses are generated and sent to the brain which realises that the image is not clear. The brain then sends electrical impulses to the ciliary muscles making them contract. When the ciliary muscles contract the suspensory ligaments become loose, this makes the lens become thicker and rounder for more refraction of the light rays. Now the light rays are correctly refracted and hit the retina at the fovea, the image becomes clear.

For far visions it is the exact opposite. The rays reflected from far objects are almost parallel. Very little refraction should be done. The brain sends electrical impulses to ciliary muscles making them relax, the suspensory ligaments now tighten up and pull the lens which become narrow.

Distance	Ciliary muscles	Suspensory ligaments	Lens
Near	Contract	Loosen	Widens
Far	Relaxes	tighten	narrows

Rods and Cones:

The retina is full of light sensitive cells called photoreceptors. There are two types photoreceptors, they are rods and cones. Rods and cones are specialised types of neurons. They look alike but they are a little different in function.

Rods are sensitive to dim light. At night or in dark places, most light detection electrical impulses transmission is done by rods. Vitamin A is essential for proper functioning of rods, if Vitamin A lacks it can lead to night blindness. Rods are spread all over the retina.

Cones are sensitive to bright and coloured light. All cones are packed in one area, the fovea.

The Pupil:

The pupil of the eye is the dark round area in the centre of it. It is surrounded by a coloured ring structure called the iris. The pupil and the together play a big role in protecting the eye from damage by limiting the amount of light entering the eye. If too much light fall on the retina, the rods and cones get damaged. The iris and pupil change their size to smiddle that happening. The iris contains two sets of muscles; Circular and Radial muscles. Circular muscles run around the iris and radial muscles run from the centre to the outside. When circular muscles contract they make the pupil smaller. When the radial muscles contract the stretch the pupil outwards making it wider.

In bright light, too much light starts entering the eye, which is dangerous for the rods and cones, which detect the high light intensity. The rods and cones start a reflex arc by sending electrical impulses to the brain via sensory neurone. The brain responds by sending electrical impulses to the muscles of the iris via motor neurone. These impulses make the circular muscles contract and the radial muscles relax limiting the amount of light entering the eye, thus protecting the rods and cones from damage.

If you walk into a dark room, the rods and cones sense the little amount of light. They start another reflex arc and send electrical impulses to the brain which responds by sending electrical impulses the muscles of the iris. The radial muscles contract and the circular muscles relax widening the pupil to let more light in.

Antagonistic Muscles:

You have just learned that in order for the pupil to get narrower or wider, two muscles work simultaneously, when one contracts the other relaxes. Pairs of muscles like that are called antagonistic muscles.

The most known antagonistic muscle pair is the biceps and triceps of the arm. The bi and the tri for short, they are what causes the movement of the arm. They work simultaneously to bend or straighten the arm. The biceps is located in front of the humerus bone of the upper arm. The biceps is joined to the radius bone of the lower arm and the triceps is joined to the ulna bone of the lower arm. Muscles are attached to bones by strong fibres called tendons.

When you want to bend your arm the brain send two electrical impulses, one to the bi making it contract and one to the tri telling it to relax. When the bi contracts, it becomes shorter pulling the bones to which it is attached close and bending the arm. This causes the fibres of the tri to stretch while they are relaxed. To straighten your arm, the brain send electrical impulses to both muscles making the bi relax in order to leave the muscle it is attached to free. The tri contracts and becomes shorter pulling the muscle it is attached to into place and straightening the arm.

The biceps can be called a flexor because it flexes (bends) the arm. The triceps can be called an extensor because it extends (straightens) the arm.

Drugs:

A drug is a chemical substance that modifies and affects chemical reactions of the body when taken in. Many drugs are useful to us like antibiotics, painkillers and caffeine.

Some drugs however are abused by users to feel relaxed, or reach euphoria. Euphoria is a state of mind at which the abuser feels extremely happy and relaxed. These drugs include alcohol and heroin.

Alcohol:

Alcohol is a depressant drug. This means that it reduces the activity of the brain and slows down the nervous system and reflex actions. Alcohol can be extremely dangerous when the user is in a situation in which they need fast reflex actions.  Alcohol is addictive. The more you drink it the more you need it. The user may reach a point where they cannot do without alcohol, this is when they become alcoholics. Alcohol is broken down into fats by the liver. If the abuser drinks too much alcohol, the cells of the kidney may die shortening their life.

Heroine:

Heroine is a narcotic drug. This means that it relieves pain and induces sleep. Heroine is extracted from a plant called opium poppy. Most heroine abusers become addicts. For the addicts heroine become the number one priority in their lives. They would do anything to get the drug even become criminals and possess a threat to their society. If not rehabilitated, a heroine abuser will end up homeless or dead. Some heroine users inject the drug in their veins by an unsterilized, shared needle, this increases the risk of getting AID/HIV.

The Endocrine System:

You have previously learned that messages are delivered around body as electrical impulses by the nervous system. Another way messages are transported around the body is by chemicals calledhormones secreted by the endocrine system. Hormones are chemical substances produced by a gland, carried by the blood, which alters the activity of one or more specific target organs and is then destroyed by the liver. Hormones are produced in organs called endocrine glands which make up the endocrine system. The following diagram shows the glands of a human body. Glands are organs made of secretory cells which’s function is to produce hormones and secret them into the bloodstream. Glands have a dense network of blood capillaries in them to secret the hormones in. hormones are carried around the plasma like all other content of the blood but certain organs are able to use them, these are target organs.

Gland	Hormone produced	Function of hormone
Adrenal gland	Adrenaline	Prepares the body for activities that need energy and quick reflex actions
Pancreas	Insulin	Makes liver reduce blood glucose level
	Glucagon	Makes liver increase blood glucose level
Testis	Testosterone	Produces male secondary sexual characteristics
Ovary	Oestrogen	Produces female secondary sexual characteristics
	Progesterone	Helps control menstrual cycle and maintain pregnancy

 

Adrenaline:

When you get a fright you feel some changes in your body like a sudden increase in heart beat rate, blood flowing quickly in veins and your breath becomes deeper and faster. This is because the fright you got caused the brain to send electrical impulses to the adrenal glands making them secrete adrenaline hormone in your bloodstream. Adrenaline is a hormone that is secreted from the adrenal glands to prepare the body for situations that need lots of energy and fast reflex action, like fights or running away for example. Adrenaline’s main objective is to increase your metabolic rate so that you have enough energy for fighting or running away etc. This is why adrenaline is called the three Fs hormone (Fight, fright, flight). One of adrenaline’s target organs is the heart. When adrenaline reaches the heart it causes the cardiac muscle to contract and relax much rapider so that oxygen and glucose reach the muscles of the body faster. Adrenaline also makes the liver convert glycogen into glucose and secret it in the blood to be used in respiration. When adrenaline reaches the diaphragm and the intercostals muscles of the ribs, they make it contract and relax faster too to increase rate of breathing. These changes cause an increase in the respiration rate so that lots of energy is being released. Generally, adrenaline is secreted when you are nervous or anxious.

Use of Hormones in Food Industries:

Technologies and science have advanced enough that we can now gut much more money out of farming and animal keeping. Hormones are now being used in farms to increase milk yields in cows and growth rate in cattle and fish.

In farms, the cows are being injected with a hormone called Bovine Somatotropin or BST. BST is a hormone that is naturally produced in cows. The function of BST is to produce milk. Injecting cows with extra BST will boost milk production and bring in more money for the farmers. Some people however are against the use of BST and claim it is safer for both the cows and the consumer to keep it natural and keep more cows if we want an increased milk yield.

Growth hormones are also being mixed with the food fed to cattle to increase their growth rate and make them grow larger. But again many people are against this and prefer buying meat and fish that were naturally grown.

Comparing Nervous and Endocrine Systems:

Nervous System	Endocrine System
Information sent in form of electrical impulses	Information sent in form of chemical hormones
Information travel neurones	Information travel in bloodstream
Information travels extremely rapidly	Information travels relatively slow
Information is headed to one target (effector)	Information may be used by several targeted organs
Electrical signals have an effect that ends quickly	Hormones have a longer lasting effect

Coordinates and Responses in Plants:

Plants cannot move themselves to areas of preferable conditions. This is why plants have the ability to detect a stimulus and respond to it by growing or bending in its direction or away from it. These responses are called tropisms. For example a plant tends to grow its stem in the direction of sunlight for more photosynthesis, this is atropism. There are two types of tropism, these are phototropism and geotropism.

• Phototropism: the response in which a plant grows towards or away from the direction from which light is coming.
• Geotropism: the response in which a plant grows towards or away from gravity.

A tropism can be either positive or negative. If a tropism is in the direction of the stimulus, it is positive. If the tropism is away from the stimulus it is negative.

For example, a plant’s shoot tends to grow in the direction of sunlight, this is positive phototropism. But the plant’s root grows in the opposite direction deeply into the soil, this is negative phototropism. However, positive phototropism can also be described as negative geotropism because it involves the plant growing in the direction opposite to gravity. And negative photo tropism can be described as positive geotropism because it involves the plant growing towards gravity.

Auxins:

Tropisms are controlled by a chemical called Auxin. Auxin is a plant hormone. It is produced by cells at the tip of roots and shoots of plants. At the tip of a shoot, there is an area in which cells are being produced by dividing so that the shoot grows. Old cells do not divide, but they grow longer instead. The growth of these cells longer is controlled by auxins. Auxins is what makes the plant grows this is why a plant doesn’t grow if you cut it’s tip off.

Auxins’ Role in Phototropism:

If the sun shines on the right side of a plant’s shoot, auxins will accumulate on the dark opposite left side. Auxins accumulating there makes the cells on the left side grow much faster than the cells on the right side. When the left side of the shoot starts growing faster than the right side, the shoot will start to bend to the right side towards sunlight. This is phototropism.

Auxins’ Role in Geotropism:

Auxins tend to settle at the bottom end of the root. However, this does not make the sells of the tip of the root grow longer. Instead, auxins prevent the cells at the bottom tip of the root from growing, making the cells at the middle of the root grow faster. When the cells of the middle of the root grow faster, they push the root deeper into the soil and the root gets longer. The root grows in the direction of the gravitational pull. This is geotropism.

Roots show positive geotropism and negative phototropism because they grow towards gravity and away from sunlight at the same time. Shoots show positive phototropism and negative geotropism because they grow towards the sunlight and away from gravity at the same time.

Advantages of Positive Phototropism:

• Leaves exposed to more sunlight and are able to do more photosynthesis,
• Flowers can be seen by insects for pollination,
• The plant gets higher for better seed dispersal.

Advantages of Positive Geotropism:

• By growing deeply into the soil, the root fixes the plant into the ground firmly,
• Roots are able to reach more water,
• Roots have a larger surface area for more diffusion and osmosis.

Selective Weed Killers:

Auxins can be used to kill weeds that grow over grass or cereal crops. If weed grows on crops, auxins are sprayed everywhere. Weeds absorb auxins faster than crops or grass. Auxins accumulate in the weeds making them grow very rapidly. Fast growth of weed kills it leaving the crops or grass alive. Auxins are used ass selective weed killers.