OxfordAQA IGCSE Biology: Organisation — Revision Notes

Condensed for the final weeks. For the full explanation, use the Organisation study guide.

Levels of organisation

cell  ->  tissue  ->  organ  ->  organ system  ->  organism

• Tissue — a group of similar cells working together for a shared function.
• Organ — several different tissues working together.
• Organ system — several organs working together.

The stomach is the standard example: muscular tissue to churn, glandular tissue to secrete enzymes and acid, epithelial tissue to line and cover.

Enzymes

Biological catalysts — proteins that speed up reactions without being used up, by lowering the activation energy.

The active site has a shape complementary to the substrate, so each enzyme is specific to one substrate.

Factor	Effect
Temperature	Rate rises to an optimum (~37 °C in humans), then falls sharply as the enzyme denatures
pH	Optimum varies by enzyme; away from it the active site changes shape

Denaturation is the change in shape of the active site so the substrate no longer fits. Enzymes are not alive and are not “killed”.

Enzyme	Substrate	Products	Made in
Amylase	Starch	Maltose	Salivary glands, pancreas, small intestine
Protease	Protein	Amino acids	Stomach, pancreas, small intestine
Lipase	Lipids	Fatty acids + glycerol	Pancreas, small intestine

Bile is made in the liver, stored in the gall bladder, and does two things:

1. Neutralises stomach acid, providing the alkaline pH that pancreatic enzymes need.
2. Emulsifies fats — breaks large droplets into small ones, increasing the surface area for lipase.

Bile is not an enzyme. Saying it digests fat loses the mark; it increases surface area so lipase can work faster.

Worked example. Explain why boiling amylase before adding starch means no maltose is produced. The high temperature breaks the bonds holding the enzyme’s tertiary structure, so the active site changes shape permanently; starch can no longer bind, so no enzyme-substrate complexes form; the enzyme is denatured, so no maltose is produced. Describing the enzyme as “killed” rather than denatured scores nothing on this kind of question.

The digestive system

Mouth → oesophagus → stomach → small intestine → large intestine → rectum.

• Stomach — protease (pepsin) and hydrochloric acid; the acid gives the optimum pH for pepsin and kills bacteria.
• Small intestine — where absorption occurs; it is long and folded, and covered in villi and microvilli, all increasing surface area; it has a thin (one-cell) wall for a short diffusion path, and a rich blood supply to maintain the concentration gradient.

Those adaptations — large surface area, thin wall, good blood supply — recur for every exchange surface in biology.

The heart

Double circulation: blood passes through the heart twice per circuit — once to the lungs, once to the body. This keeps oxygenated and deoxygenated blood separate and allows the body circuit to be at a higher pressure, so blood travels faster and delivers oxygen more efficiently.

Right side → lungs (deoxygenated). Left side → body (oxygenated). The left ventricle has a thicker muscular wall because it must generate enough pressure to pump blood around the whole body, while the right ventricle only pumps to the nearby lungs.

Valves prevent backflow, ensuring one-way flow.

Vessel	Wall	Lumen	Valves
Artery	Thick, muscular, elastic	Narrow	No
Vein	Thin	Wide	Yes
Capillary	One cell thick	Very narrow	No

Arteries carry blood away from the heart, veins towards it — the pulmonary artery carries deoxygenated blood, so “arteries carry oxygenated blood” is wrong.

The lungs

Trachea → bronchi → bronchioles → alveoli.

Alveoli are adapted for gas exchange by being: numerous, giving a very large surface area; one cell thick, for a short diffusion distance; moist, so gases dissolve; and surrounded by a dense capillary network, maintaining a steep concentration gradient.

Ventilation maintains the gradient by continually replacing the air.

Exam traps

• Calling bile an enzyme.
• Saying enzymes are killed rather than denatured.
• Giving “glycerol and fatty acid” as one product rather than two.
• Saying all arteries carry oxygenated blood.
• Giving only one alveolar adaptation when three are needed.
• Confusing tissue with organ.

Self-test

1. Define tissue and organ, with an example of each.
2. What two functions does bile perform, and why is it not an enzyme?
3. Why is the left ventricle wall thicker than the right?
4. Give three adaptations of an alveolus and what each achieves.
5. Name the products of lipid digestion and the enzyme responsible.

Answers: 1. A tissue is a group of similar cells with a shared function, e.g. muscular tissue; an organ is several different tissues working together, e.g. the stomach. 2. It neutralises stomach acid to give the alkaline pH pancreatic enzymes need, and emulsifies fats to increase the surface area for lipase; it is not an enzyme because it does not catalyse a reaction. 3. It must pump blood at high pressure around the entire body, whereas the right ventricle pumps only to the nearby lungs. 4. Very large total surface area from the huge number of alveoli; walls one cell thick, giving a short diffusion path; moist lining so gases dissolve; dense capillary network maintaining a steep concentration gradient — any three. 5. Fatty acids and glycerol, produced by lipase.