OxfordAQA IGCSE Biology: Organisation — Revision Notes

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

Cell structure (3.1.1)

Most animal cells (eukaryotic) have:

• Nucleus — controls the activities of the cell.
• Cytoplasm — where most of the chemical reactions take place.
• Cell membrane — controls the passage of substances into and out of the cell.
• Mitochondria — where most energy is released in respiration.
• Ribosomes — where protein synthesis occurs.

Plant cells (eukaryotic) often also have chloroplasts (absorb light energy to make food) and a permanent vacuole filled with cell sap. Plant and algal cells have a cell wall made of cellulose, which strengthens the cell.

Bacterial cells (prokaryotic) have cytoplasm and a membrane surrounded by a cell wall; the genes are not in a distinct nucleus, and some genes are in small circular structures called plasmids.

Cells may be specialised for a particular function: be ready to link a named cell’s structure to its job in a tissue, organ or organism.

Levels of organisation (3.1.2)

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

During the development of a multicellular organism, cells differentiate so that they can perform different functions.

• Tissue — a group of cells with similar structure and function.
• Organ — made of tissues; one organ may contain several different tissues.
• Organ system — a group of organs that perform a particular function.

Animal tissues, organs and systems (3.1.3)

Tissue	What it does
Muscular	Contracts to bring about movement
Glandular	Produces substances such as enzymes and hormones
Epithelial	Covers some parts of the body

The stomach is the standard example of an organ: muscular tissue moves the contents through the digestive system, glandular tissue produces digestive juices, and epithelial tissue covers the outside and the inside.

The digestive system is the standard example of an organ system:

• Glands (pancreas, salivary glands) produce digestive juices.
• Stomach and small intestine — where digestion occurs.
• Liver — produces bile.
• Small intestine — where soluble food is absorbed.
• Large intestine — where water is absorbed from the undigested food, producing faeces.

Plant tissues and organs (3.1.4)

Tissue	What it does
Epidermal	Covers the plant
Palisade mesophyll	Carries out photosynthesis
Spongy mesophyll	Has air spaces that help gases diffuse
Xylem and phloem	Transport substances around the plant

Plant organs include stems, roots and leaves. Internal structure is limited to the leaf and the position of the xylem and phloem in a dicotyledonous primary root and primary stem.

Transport in cells (3.1.5)

Process	What moves	Direction	Energy from respiration?
Diffusion	Particles in solution or of a gas	Higher → lower concentration (net movement)	No
Osmosis	Water, through a partially permeable membrane	Dilute → more concentrated solution	No
Active transport	Substances such as mineral ions or sugar	Against a concentration gradient	Yes

• The greater the concentration difference, the faster the rate of diffusion. Oxygen for respiration enters cells by diffusion.
• Osmosis terms: a solution that is isotonic with the cell contents causes no net movement of water; in a hypotonic (more dilute) solution water enters the cell; in a hypertonic (more concentrated) solution water leaves. A plant cell that has taken in water presses its contents against the cell wall (turgor); one that loses a lot of water shrinks away from its cell wall (plasmolysis).
• Active transport lets root hair cells absorb ions from very dilute soil solutions, and lets sugar be absorbed from low concentrations in the intestine and the kidney tubules.
• Required practical (3.1.5): investigate the effect of different concentrations of solutions separated by a partially permeable membrane (for example, potato pieces in sugar solutions; the concentration giving no change in mass is isotonic with the cells).

Surface area to volume ratio. A single-celled organism has a relatively large surface area to volume ratio, so all its exchanges happen across its surface membrane. Larger, more complex organisms need specialised exchange surfaces, which work better when they have:

1. a large surface area that is thin, giving a short diffusion path;
2. (in animals) an efficient blood supply;
3. (in animals, for gas exchange) ventilation.

Exchange surface	Adaptations
Small intestine	Long and folded, covered in villi and microvilli (large surface area); thin wall one cell thick (short diffusion path); rich blood supply (keeps the concentration gradient steep)
Lungs	Millions of alveoli (very large surface area); walls one cell thick (short diffusion path); dense capillary network; ventilated, so fresh air keeps the gradient steep
Roots	Root hair cells greatly increase the surface area for absorbing water and mineral ions
Leaves	Flat, thin shape and air spaces inside (large surface area, short diffusion path for gases)

Worked example. Explain why the small intestine is not ventilated but the lungs are. Both are exchange surfaces with a large surface area, a short diffusion path and a rich blood supply. The lungs exchange gases with the air, so fresh air must keep arriving or the air in the alveoli would reach the same concentrations as the blood and diffusion would stop; the small intestine exchanges dissolved substances with the blood, so blood flow alone keeps its gradient.

Linked Topic 2 notes: enzymes, digestion, the heart and the lungs

The 9201 specification places the next three sections in Topic 2 Bioenergetics (3.2.4 Digestion, 3.2.3 Circulation in humans, 3.2.5 Breathing), not in Topic 1. They are kept here as linked recall because Topic 1 names the digestive system and uses the small intestine and lungs as exchange surfaces. For the full Topic 2 coverage, use the Bioenergetics study guide.

Enzymes and digestion (Topic 2, 3.2.4)

Enzymes are large proteins that act as biological catalysts, speeding up reactions without being used up.

The active site has a shape complementary to the substrate, so each enzyme is specific to its 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	Sugars (maltose)	Salivary glands, pancreas, small intestine
Protease	Protein	Amino acids	Stomach, pancreas, small intestine
Lipase	Lipids	Fatty acids + glycerol	Pancreas, small intestine

The stomach produces hydrochloric acid; the enzymes in the stomach work best in acid conditions.

Bile is made in the liver, stored in the gall bladder, released into the small intestine, and does two things:

1. Neutralises stomach acid, giving the alkaline conditions in which the enzymes in the small intestine work best.
2. Emulsifies fats — breaks large drops into smaller droplets, 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 sugar is produced. The high temperature changes the shape of the enzyme’s active site permanently; starch can no longer fit, so it is not broken down; the enzyme is denatured. Describing the enzyme as “killed” rather than denatured scores nothing on this kind of question.

The heart and blood vessels (Topic 2, 3.2.3)

The heart has four chambers (right and left atria, right and left ventricles). Blood enters the atria; the atria contract and force blood into the ventricles; the ventricles contract and force blood out of the heart. Valves make sure blood flows in the correct direction.

Double circulation: one circuit to the lungs and one to all the other organs, so blood passes through the heart twice on each complete journey round the body. The left ventricle has a thicker muscular wall because it pumps blood around the whole body, while the right ventricle pumps only to the nearby lungs.

Vessel	Wall	Lumen	Valves
Artery	Thick, with muscle and elastic fibres	Narrow	No
Vein	Thinner	Wide	Often
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 (Topic 2, 3.2.5)

Trachea → bronchi → bronchioles → alveoli.

• Inhaling: intercostal muscles contract and pull the ribcage up; the diaphragm contracts and flattens; the volume of the thorax increases, the pressure falls below atmospheric pressure, and air enters the lungs.
• Exhaling: intercostal muscles and diaphragm relax; the volume of the thorax decreases, the pressure rises, and air leaves the lungs.

The alveoli provide a very large surface area, richly supplied with blood capillaries, so gases readily diffuse into and out of the blood.

Exam traps

Topic 1

• Confusing tissue with organ, or giving a definition without a named example.
• Saying all cells have a cell wall (animal cells do not), or that a bacterial cell has a nucleus.
• Describing osmosis as “diffusion of water” without the partially permeable membrane or the direction (dilute to more concentrated).
• Saying diffusion or osmosis needs energy; only active transport uses energy from respiration.
• Naming an exchange-surface adaptation without saying what it increases or shortens.

Linked Topic 2

• 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.

Self-test

1. Define tissue and organ, with an example of each.
2. Name two structures found in a plant cell but not in an animal cell, and state where the genes are in a bacterial cell.
3. State one difference between osmosis and active transport.
4. A potato piece gains mass in a sugar solution. Is the solution hypotonic or hypertonic to the potato cells, and why does the mass increase?
5. Give three ways the small intestine is adapted for absorbing soluble food.
6. (Linked Topic 2) What two functions does bile perform, and why is it not an enzyme?
7. (Linked Topic 2) Why is the left ventricle wall thicker than the right?
8. (Linked Topic 2) Name the products of lipid digestion and the enzyme responsible.

Answers: 1. A tissue is a group of cells with similar structure and function, e.g. muscular tissue; an organ is made of several different tissues, e.g. the stomach. 2. Any two of chloroplasts, a permanent vacuole, a cellulose cell wall; bacterial genes are not in a distinct nucleus, and some are in plasmids. 3. Any one: osmosis is the movement of water only, through a partially permeable membrane, while active transport moves substances such as ions or sugar; osmosis does not need energy from respiration, active transport does; osmosis moves water from a dilute to a more concentrated solution, active transport moves substances against their concentration gradient. 4. Hypotonic (more dilute than the cell contents); water enters the cells by osmosis, so the mass increases. 5. Any three: it is long and folded, with villi and microvilli, giving a large surface area; its wall is thin (one cell thick), giving a short diffusion path; it has a rich blood supply, keeping the concentration gradient steep. 6. It neutralises stomach acid to give the alkaline conditions the enzymes in the small intestine need, and emulsifies fats to increase the surface area for lipase; it is not an enzyme because it does not catalyse a reaction. 7. It must pump blood around the entire body, whereas the right ventricle pumps only to the nearby lungs. 8. Fatty acids and glycerol, produced by lipase.