O Level Biology 5090 — all 19 topics, free.

A complete study guide for Cambridge O Level Biology 5090, mapped to all 52 sub-topics of the official syllabus for exams in 2026–2028 (version 4), plus the practical skills section worth 20% of your grade.

How to use it: biology is marked on precision. Most lost marks are vague answers where an exact term was needed. Each unit gives you the definitions in examiner wording, the process step by step, and a skill check — attempt it before opening the answer.

📄 19 plain-English chapter handouts →✎ Practice & self-test →

CAIE 5090 · exams 2026–202852 syllabus units19 topicsFood tests & geneticsPractical/ATP included

Start here

The three papers

Paper	Format	Time / marks	Weight
Paper 1 — Multiple Choice	40 four-option multiple-choice questions	1 hour · 40 marks	30%
Paper 2 — Theory	Short-answer and structured questions	1 h 45 min · 80 marks	50%
Paper 3 — Practical Test or	Experiments carried out in a laboratory	1 h 30 min · 40 marks	20%
Paper 4 — Alternative to Practical	Written questions on experiments; no lab work	1 hour · 40 marks	20%

Assessment objectives: AO1 Knowledge with understanding, AO2 Handling information and problem-solving, AO3 Experimental skills — with AO3 carrying 20% through Paper 3 or 4 alone.

Biology has no formula sheet and no data booklet — but it also has very few equations. What it does have is a large vocabulary, and marks hinge on using the exact term: "active transport" not "absorption", "villi" not "folds", "denatured" not "killed". Build a glossary as you revise; the definitions bank at the end of this guide is a starting point.

Learn these cold

Food tests and the few equations you need

Food tests — required for both theory and practical papers

Test for	Reagent and method	Positive result
Starch	Add iodine solution	Orange-brown → blue-black
Reducing sugars (glucose, maltose)	Add Benedict's solution and heat in a water bath	Blue → green → yellow → orange → brick red (the more red, the more sugar)
Protein	Biuret test — add sodium hydroxide, then copper(II) sulfate solution	Blue → purple/violet
Lipids (fats and oils)	Ethanol emulsion test — shake with ethanol, then pour into water	Clear → cloudy white emulsion

Forgetting that Benedict's test needs heating in a water bath. An unheated Benedict's test stays blue whatever sugar is present, and the heating step is nearly always worth a mark of its own.

The equations and formulae to know

Photosynthesis: carbon dioxide + water → (light energy, chlorophyll) → glucose + oxygen
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Aerobic respiration: glucose + oxygen → carbon dioxide + water (+ energy)
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Anaerobic respiration in muscle: glucose → lactic acid (+ a little energy)

Anaerobic respiration in yeast: glucose → alcohol (ethanol) + carbon dioxide (+ a little energy)

magnification = image size ÷ actual size

Worked example — magnification

A drawing of a cell measures 60 mm across. The actual cell is 0.05 mm across. Find the magnification.

Magnification = image size ÷ actual size = 60 ÷ 0.05
= ×1200 (magnification has no units — it is a ratio)

Rearranged: actual size = image size ÷ magnification. Always convert both measurements to the same unit first (1 mm = 1000 µm).

Start here

Study planner & progress

All 52 syllabus units plus the practical skills section. Tick one when you can answer a past-paper question on it unaided. Your ticks are saved on this device only — nothing is sent anywhere, and there is no account to create.

Loading…

Topic 1 · 2 units

Cells

1.1Cell structure and function

Structure	Function	Animal	Plant	Bacterial
Cell membrane	Controls what enters and leaves the cell (partially permeable)	✓	✓	✓
Cytoplasm	Site of most chemical reactions	✓	✓	✓
Nucleus	Contains DNA; controls the cell's activities	✓	✓	✗ (circular DNA instead)
Ribosomes	Site of protein synthesis	✓	✓	✓
Mitochondria	Site of aerobic respiration, releasing energy	✓	✓	✗
Cellulose cell wall	Supports the cell and stops it bursting	✗	✓	✓ (not cellulose)
Chloroplasts	Contain chlorophyll; site of photosynthesis	✗	✓	✗
Sap vacuole (large, permanent)	Stores cell sap; keeps the cell turgid for support	✗	✓	✗
Plasmids	Small rings of extra DNA	✗	✗	✓

Bacterial cells have ribosomes, circular DNA, plasmids, cytoplasm, a cell membrane and a cell wall — but no nucleus, no mitochondria and no chloroplasts.

Preparing a slide: place the specimen on a slide, add a drop of stain (methylene blue for animal cells, iodine solution for plant cells), then lower the coverslip slowly at an angle using a mounted needle — this avoids trapping air bubbles, which look like dark-edged circles and can be mistaken for cells.

Rules for biological drawings — worth several marks

Sharp pencil, clear continuous lines — no sketchy or shaded lines.
No shading or colouring.
Drawing should be large — at least half the space provided.
Label lines drawn with a ruler, not crossing each other, ending exactly on the structure.
Labels written horizontally, outside the drawing.
Include a title and the magnification if asked.

Skill check: Give three structures found in a plant cell but not an animal cell, and state one function of each.

Solution: Cellulose cell wall — supports the cell and prevents it bursting; chloroplasts — contain chlorophyll and are the site of photosynthesis; large permanent sap vacuole — stores cell sap and keeps the cell turgid for support.

1.2Specialised cells, tissues and organs

Levels of organisationCell → tissue (a group of similar cells working together) → organ (several tissues working together) → organ system (several organs) → organism.

Specialised cell	Adaptation	Function
Red blood cell	Biconcave disc (large surface area), no nucleus (more room), contains haemoglobin	Transports oxygen
Root hair cell	Long extension giving a large surface area; many mitochondria	Absorbs water and mineral ions
Xylem vessel	Hollow dead tubes, no end walls, lignified walls	Transports water and ions; supports the plant
Palisade mesophyll cell	Many chloroplasts, column-shaped, near the leaf surface	Photosynthesis
Nerve cell (neurone)	Very long axon; branched endings	Transmits electrical impulses
Sperm cell	Tail (flagellum) for swimming; many mitochondria for energy; enzymes in the acrosome	Fertilises the egg
Egg cell	Large food store in cytoplasm; jelly coat that changes after fertilisation	Provides nutrients for the embryo

When asked "explain how this cell is adapted", always link structure to function explicitly: "the root hair cell has a long, thin extension which gives a large surface area, so water can be absorbed by osmosis faster." The structure alone earns nothing.

Topic 2 · 2 units

Classification

2.1Classification systems 2.2Features of organisms

DefinitionsA species is a group of organisms with similar features that can breed together to produce fertile offspring.
The binomial system gives every organism a two-part Latin name: genus (capital letter) then species (lower case), e.g. Homo sapiens.
Organisms are classified by shared features; the more features shared — and the more similar their DNA base sequences — the more closely related they are.

Vertebrate group	Key features
Fish	Wet scales, gills, fins, external fertilisation, cold-blooded
Amphibians	Moist permeable skin, larvae with gills, external fertilisation, cold-blooded
Reptiles	Dry scaly skin, soft-shelled eggs on land, internal fertilisation, cold-blooded
Birds	Feathers, beak, hard-shelled eggs, warm-blooded
Mammals	Fur/hair, mammary glands, external ears, give birth to live young, warm-blooded

Arthropods (jointed legs, exoskeleton) divide into: insects (3 body parts, 6 legs, usually wings, one pair of antennae), arachnids (2 body parts, 8 legs, no antennae), crustaceans (chalky exoskeleton, more than 4 pairs of legs, two pairs of antennae) and myriapods (many segments, one or two pairs of legs per segment).

Plant groups: ferns (leaves called fronds, reproduce by spores, no flowers) and flowering plants (reproduce by seeds in flowers) — divided into monocotyledons (one cotyledon, narrow leaves with parallel veins) and dicotyledons (two cotyledons, broad leaves with branching veins).

Using a dichotomous keyStart at question 1. Each step gives two alternatives; choose the one matching your organism and follow the instruction to the next numbered question, until you reach a name. Read both options before choosing — the distinguishing feature is often the second half of the statement.

Skill check: Two organisms look almost identical but their offspring are always sterile. Are they the same species?

Solution: No. A species is defined by the ability to breed together to produce fertile offspring. Sterile offspring (like a mule from a horse and a donkey) show the parents are different species.

Topic 3 · 2 units

Movement into and out of cells

3.1Diffusion and osmosis 3.2Active transport

	Diffusion	Osmosis	Active transport
What moves	Any particles (gases, dissolved solutes)	Water only	Dissolved ions/molecules
Direction	High → low concentration (down the gradient)	High → low water potential, through a partially permeable membrane	Low → high concentration (against the gradient)
Energy needed?	No — passive	No — passive	Yes — energy from respiration, via protein carriers
Example	Oxygen into blood in the alveoli	Water into root hair cells	Mineral ions into root hairs; glucose from the gut when concentration is low

Definitions — use this exact wordingDiffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random movement.
Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, using energy from respiration.

Factors increasing the rate of diffusion: a steeper concentration gradient, higher temperature, larger surface area, shorter distance (thinner membrane).

Cell in…	Plant cell	Animal cell
Dilute solution (water enters)	Becomes turgid — the cell wall prevents bursting and provides support	Bursts (lyses) — no cell wall to resist
Concentrated solution (water leaves)	Becomes flaccid, then plasmolysed (membrane pulls away from the wall); the plant wilts	Crenated — shrinks and shrivels

The classic osmosis experiment: cut potato cylinders of equal length, blot dry, measure mass, place in sucrose solutions of different concentrations for a fixed time, blot and re-weigh. Calculate percentage change in mass so different starting masses can be compared fairly. Where the line crosses zero change, the sucrose concentration equals the cell contents' concentration.

Saying "water moves from high to low concentration" in osmosis without saying water concentration or water potential — a solution that is concentrated in sugar is dilute in water. Examiners look for the direction to be stated unambiguously.

Skill check: Potato cylinders lose mass in a strong sucrose solution. Explain why, and predict what happens to the cells.

Solution: The sucrose solution has a lower water potential than the potato cell contents, so water moves out of the cells by osmosis through the partially permeable membranes — the cylinders lose mass. The cells become flaccid and, if enough water leaves, plasmolysed, with the cell membrane pulling away from the cell wall.

Topic 4 · 1 unit

Biological molecules

4.1Biological molecules

Molecule	Elements	Built from	Examples
Carbohydrates	Carbon, hydrogen, oxygen	Glucose (simple sugars)	Starch, glycogen, cellulose
Lipids (fats and oils)	Carbon, hydrogen, oxygen	Fatty acids and glycerol	Fats, oils
Proteins	Carbon, hydrogen, oxygen, nitrogen (and often sulfur)	Amino acids	Enzymes, haemoglobin, antibodies
DNA	Carbon, hydrogen, oxygen, nitrogen, phosphorus	Nucleotides	Chromosomes

Storage carbohydrates: starch in plants, glycogen in animals. Cellulose is the structural carbohydrate of plant cell walls. All three are made from glucose but joined differently.

Food tests for these molecules are in the food tests table near the top of this guide.

Skill check: Which element distinguishes proteins from carbohydrates and lipids, and why does that matter to plants?

Solution: Nitrogen. Carbohydrates and lipids contain only carbon, hydrogen and oxygen. This is why plants need nitrate ions from the soil — to make amino acids and hence proteins, which they cannot build from photosynthesis products alone.

Topic 5 · 2 units

Enzymes

5.1Enzyme action 5.2Effects of temperature and pH

DefinitionEnzymes are proteins that function as biological catalysts — they speed up chemical reactions in living organisms and are not used up in the reaction.

The lock-and-key explanationEach enzyme has an active site with a specific shape. Only a substrate with a complementary shape fits, forming an enzyme–substrate complex — this is why enzymes are specific. The reaction occurs, products are released, and the enzyme is free to be used again.

Factor	Effect	Explanation
Temperature (below optimum)	Rate increases	Molecules have more kinetic energy → more frequent successful collisions between enzyme and substrate
Temperature (above optimum)	Rate falls sharply to zero	The enzyme is denatured: the active site changes shape, so the substrate no longer fits
pH (away from optimum)	Rate falls	Extremes of pH also denature the enzyme, changing the active site shape
Substrate concentration	Rate increases, then levels off	Eventually all active sites are occupied — the enzyme concentration becomes limiting

Writing that a high temperature "kills" the enzyme. Enzymes are molecules, not living things — the correct word is denatured, and you must add that the active site changes shape so the substrate no longer fits. "Killed" scores zero.

Optimum pH examples: pepsin in the stomach works best at about pH 2 (acidic); amylase in the mouth and small intestine works best around pH 7; enzymes in the small intestine work in the alkaline conditions produced by bile.

A standard enzyme practical: amylase digesting starch. Test samples at intervals with iodine solution on a spotting tile — the blue-black colour disappears when the starch has been broken down, and the time taken measures the rate. Control the pH with a buffer, use a water bath for temperature, and keep enzyme and substrate volumes constant.

Skill check: An enzyme's optimum temperature is 40 °C. Explain the shape of a graph of rate against temperature from 0 °C to 70 °C.

Solution: From 0 to 40 °C the rate rises: molecules gain kinetic energy, so enzyme and substrate collide more frequently and more successfully. At 40 °C the rate is at a maximum (optimum). Above 40 °C the rate falls steeply to zero, because the enzyme is denatured — its active site changes shape so the substrate can no longer bind.

Topic 6 · 3 units

Plant nutrition

6.1Photosynthesis 6.2Leaf structure 6.3Mineral nutrition

DefinitionPhotosynthesis is the process by which plants make carbohydrates from raw materials, using energy from light absorbed by chlorophyll.

carbon dioxide + water → glucose + oxygen (light energy, chlorophyll)
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Limiting factorsA limiting factor is the factor present in the shortest supply, which limits the rate of the process. For photosynthesis these are light intensity, carbon dioxide concentration and temperature. On a rate graph, the curve rises then plateaus when some other factor becomes limiting.

Leaf adaptations: broad and thin (large surface area, short diffusion distance), palisade cells packed with chloroplasts near the top, air spaces in the spongy layer, and stomata for gas exchange.

Leaf feature	Adaptation for photosynthesis
Broad and flat	Large surface area to absorb light and carbon dioxide
Thin	Short diffusion distance for gases
Palisade cells at the top, packed with chloroplasts	Maximum light absorption
Air spaces in the spongy mesophyll	Allow gases to diffuse to and from cells
Stomata (opened by guard cells)	Allow CO₂ in and O₂ out
Network of veins (xylem and phloem)	Supply water and remove the products of photosynthesis

Mineral nutrition: plants need nitrate ions to make amino acids and proteins (deficiency → stunted growth and yellow older leaves) and magnesium ions to make chlorophyll (deficiency → yellowing between the veins, called chlorosis).

Classic photosynthesis practicals: destarch the plant first by keeping it in the dark for 24–48 hours, so any starch found afterwards must be newly made. To test a leaf for starch — boil in water (kills tissue, breaks down membranes), boil in ethanol in a water bath to remove chlorophyll (never heat ethanol directly, it is flammable), rinse in water to soften, then add iodine solution. Blue-black means starch, so photosynthesis occurred. Variegated leaves test the need for chlorophyll; a leaf half-covered in foil tests the need for light.

Skill check: On a graph of photosynthesis rate against light intensity, the curve rises then flattens. Explain both parts.

Solution: At low light intensity, light is the limiting factor — increasing it increases the rate. When the curve plateaus, light is no longer limiting; some other factor such as carbon dioxide concentration or temperature has become the limiting factor, so extra light makes no difference.

Topic 7 · 2 units

Transport in flowering plants

7.1Uptake and transport of water and ions 7.2Transpiration and translocation

	Xylem	Phloem
Transports	Water and mineral ions	Sucrose and amino acids (assimilates)
Direction	Roots → leaves only (upwards)	Both directions, source → sink
Cells	Dead, hollow, lignified, no end walls	Living sieve tubes with companion cells
Process name	Transpiration stream	Translocation

Water uptake: water enters root hair cells by osmosis (they have a large surface area); mineral ions enter by active transport (against the concentration gradient, using energy from respiration).

TranspirationTranspiration is the loss of water vapour from the leaves of a plant by evaporation and diffusion through the stomata. It creates a transpiration pull that draws water up the xylem in a continuous column.

Factor increased	Effect on transpiration rate	Why
Temperature	Increases	Faster evaporation; water molecules have more energy
Wind speed	Increases	Removes water vapour, maintaining a steep diffusion gradient
Light intensity	Increases	Stomata open for photosynthesis, so more water escapes
Humidity	Decreases	Reduces the water potential gradient between leaf and air

A potometer measures the rate of water uptake (used as a measure of transpiration). Set it up under water and cut the shoot stem at an angle under water, to prevent air bubbles blocking the xylem. Measure how far the air bubble moves in a set time. Note it measures uptake, which is slightly more than transpiration because some water is used in photosynthesis.

Skill check: A plant wilts on a hot, windy day even though the soil is moist. Explain why.

Solution: High temperature and wind both increase the rate of transpiration, so water is lost from the leaves faster than the roots can absorb and transport it. The cells lose water, become flaccid and lose their turgor pressure, so they no longer support the leaves and stem — the plant wilts.

Topic 8 · 3 units

Human nutrition

8.1Diet

Nutrient	Why needed	Deficiency causes
Carbohydrates	Main source of energy	Lack of energy
Proteins	Growth and repair of tissues	Kwashiorkor; poor growth
Lipids	Energy store; insulation; cell membranes	Lack of energy store
Vitamin C	Healthy skin and gums; forms connective tissue	Scurvy
Vitamin D	Helps absorb calcium for bones and teeth	Rickets
Calcium	Strong bones and teeth; blood clotting	Weak bones, rickets
Iron	Making haemoglobin	Anaemia
Fibre (roughage)	Provides bulk so muscles can push food along by peristalsis	Constipation
Water	Solvent for reactions and transport	Dehydration

A balanced diet contains all nutrients in the correct proportions. Requirements vary with age (children need more protein and calcium for growth), activity (more energy needed), sex, and pregnancy/breastfeeding (more protein, calcium and iron).

8.2Human digestive system 8.3Absorption and assimilation

The five stagesIngestion (taking food in) → digestion (breaking large insoluble molecules into small soluble ones) → absorption (soluble molecules pass into the blood) → assimilation (absorbed molecules are used by cells) → egestion (undigested waste removed).
Mechanical digestion breaks food into smaller pieces (teeth, stomach churning), increasing surface area for enzymes. Chemical digestion breaks the molecules with enzymes.

Enzyme	Made in	Acts on	Produces
Amylase	Salivary glands, pancreas	Starch	Maltose
Maltase	Small intestine	Maltose	Glucose
Protease (e.g. pepsin)	Stomach, pancreas	Proteins	Amino acids
Lipase	Pancreas, small intestine	Lipids	Fatty acids and glycerol

Hydrochloric acid in the stomach has two jobs: it kills bacteria in food, and it provides the acidic pH (about 2) that is the optimum for pepsin.

BileBile is made in the liver and stored in the gall bladder. It is not an enzyme. It (1) emulsifies fats — breaking large fat droplets into smaller ones, increasing the surface area for lipase to act on, and (2) neutralises the acid from the stomach, providing the alkaline pH needed by intestinal enzymes.

Absorption happens mainly in the small intestine, which is adapted with: villi and microvilli (very large surface area), a thin wall (one cell thick) for a short diffusion distance, a rich blood supply maintaining a steep concentration gradient, and lacteals to absorb fatty acids and glycerol. Water is absorbed mainly in the colon.

Assimilation: glucose is used in respiration or stored as glycogen in the liver and muscles; amino acids build new proteins, with excess deaminated in the liver.

Calling bile an enzyme, or saying it "digests fats". Bile emulsifies fats — a physical change increasing surface area. The chemical digestion is done by lipase.

Skill check: Explain two ways a villus is adapted for absorption.

Solution: Any two: it is one of millions of finger-like projections, giving a very large surface area (increased further by microvilli) so more nutrients are absorbed; its wall is only one cell thick, giving a short diffusion distance; it has a dense capillary network that carries absorbed molecules away, maintaining a steep concentration gradient; and it contains a lacteal for absorbing the products of fat digestion.

Topic 9 · 1 unit

Human gas exchange

9.1Human gas exchange

Pathway of air: nose/mouth → trachea → bronchi → bronchioles → alveoli.

Alveolus adaptation	Effect
Millions of tiny air sacs	Very large surface area for gas exchange
Wall one cell thick, and capillary wall one cell thick	Short diffusion distance
Dense network of capillaries	Maintains a steep concentration gradient
Moist lining	Gases dissolve before diffusing

	Inspiration (breathing in)	Expiration (breathing out)
External intercostal muscles	Contract → ribs move up and out	Relax → ribs move down and in
Diaphragm	Contracts → flattens	Relaxes → domes upwards
Volume of thorax	Increases	Decreases
Pressure in thorax	Decreases below atmospheric	Increases above atmospheric
Air	Moves in	Moves out

Inspired vs expired air: expired air contains less oxygen (about 16% vs 21%), more carbon dioxide (about 4% vs 0.04%), more water vapour, and is warmer. Test using limewater — it turns milky faster with expired air.

Effects of smoking: tar is a carcinogen causing lung cancer, and it damages cilia so mucus accumulates (chronic bronchitis); nicotine is addictive and raises heart rate and blood pressure; carbon monoxide binds to haemoglobin, reducing oxygen transport; emphysema destroys alveoli walls, reducing surface area for gas exchange.

Skill check: Explain why breathing rate and depth increase during exercise.

Solution: Muscles respire faster and need more oxygen to release energy, and produce more carbon dioxide. The rise in CO₂ in the blood is detected by the brain, which increases breathing rate and depth so more oxygen is taken in and carbon dioxide removed faster.

Topic 10 · 3 units

Respiration

10.1Respiration 10.2Aerobic 10.3Anaerobic

DefinitionRespiration is the chemical reactions in cells that break down nutrient molecules to release energy for metabolism. It happens in all living cells, all the time.

Energy released is used for: muscle contraction, protein synthesis, cell division, active transport, growth, passing nerve impulses, and maintaining a constant body temperature.

	Aerobic respiration	Anaerobic respiration
Oxygen	Required	Not required
In muscle	glucose + oxygen → carbon dioxide + water	glucose → lactic acid
In yeast	As above	glucose → alcohol (ethanol) + carbon dioxide
Energy released	Much more	Much less (glucose only partly broken down)
Site	Mitochondria	Cytoplasm

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (aerobic)

Oxygen debtDuring hard exercise, muscles respire anaerobically and lactic acid builds up, causing fatigue and cramp. The oxygen debt is the extra oxygen needed afterwards to break the lactic acid down — which is why you continue breathing deeply and quickly after stopping.

Fermentation — anaerobic respiration in yeast — is used in brewing (the alcohol) and bread-making (the carbon dioxide makes the dough rise).

To show respiration produces carbon dioxide, use hydrogencarbonate indicator: it is red at normal atmospheric CO₂, turns yellow when CO₂ increases (respiration exceeding photosynthesis), and purple when CO₂ falls (photosynthesis exceeding respiration). Always run a control tube without organisms, and cover tubes with foil where you need to exclude light.

Skill check: A runner keeps breathing hard for minutes after a sprint. Explain why.

Solution: During the sprint, oxygen could not be supplied fast enough, so muscles respired anaerobically, producing lactic acid. This created an oxygen debt. Continued deep, rapid breathing supplies the extra oxygen needed to break down the lactic acid, so it is repaid gradually after exercise stops.

Topic 11 · 4 units

Transport in humans

11.1Circulatory system 11.2Heart

Humans have a double circulation: blood passes through the heart twice for each complete circuit — once to the lungs (pulmonary) and once to the body (systemic). The advantage is that blood can be re-pressurised before going to the body, so it flows faster and delivers oxygen and nutrients more efficiently.

The left ventricle has the thickest muscular wall because it pumps blood at high pressure all around the body; the right ventricle only pumps to the nearby lungs.

Coronary arteries supply the heart muscle itself with oxygen and glucose. If they become blocked by fatty deposits, the muscle is starved of oxygen — a coronary heart attack. Risk factors: diet high in saturated fat and salt, smoking, lack of exercise, stress, obesity, age and genetics.

Heart rate increases during exercise so oxygen and glucose reach the muscles faster and carbon dioxide is removed faster.

11.3Blood vessels 11.4Blood

	Artery	Vein	Capillary
Direction	Away from the heart	Towards the heart	Links arteries to veins in tissues
Wall	Thick, muscular and elastic	Thinner wall	One cell thick
Lumen	Narrow	Wide	Very narrow
Valves	No	Yes — prevent backflow	No
Pressure	High	Low	Falling
Function	Withstand high pressure surges	Return blood at low pressure	Exchange of substances with cells

Blood component	Structure	Function
Red blood cells	Biconcave, no nucleus, contain haemoglobin	Transport oxygen as oxyhaemoglobin
White blood cells — phagocytes	Lobed nucleus, flexible	Engulf and digest pathogens
White blood cells — lymphocytes	Large round nucleus	Produce antibodies
Platelets	Cell fragments	Cause blood clotting, sealing wounds and preventing entry of pathogens
Plasma	Straw-coloured liquid	Transports blood cells, nutrients, carbon dioxide, urea, hormones and heat

Skill check: Explain two ways a red blood cell is adapted to its function.

Solution: It has no nucleus, leaving more room for haemoglobin so it can carry more oxygen; and it is a biconcave disc, giving a larger surface area to volume ratio for faster diffusion of oxygen in and out. (Also creditable: it is small and flexible, so it can squeeze through capillaries.)

Topic 12 · 3 units

Disease and immunity

12.1Disease 12.2Antibiotics 12.3Immunity

DefinitionsA pathogen is a disease-causing organism. A transmissible disease is one in which the pathogen can be passed from one host to another.
A vector is an organism that carries a pathogen from one host to another without getting the disease itself (e.g. mosquitoes carrying the malarial parasite, houseflies carrying cholera bacteria).

Body defences:

Mechanical barriers — skin, hairs in the nose.
Chemical barriers — mucus traps pathogens, stomach acid kills them.
Cells — phagocytes engulf and digest pathogens; lymphocytes make antibodies.

Antibodies and immunityAntibodies are proteins made by lymphocytes that lock onto antigens on a pathogen's surface, marking it for destruction or causing pathogens to clump together. Antibodies are specific to one antigen.
Active immunity — defence by your own antibody production, triggered by infection or vaccination; long-lasting because memory cells remain.
Passive immunity — short-term defence from antibodies received from another individual, e.g. across the placenta or in breast milk; no memory cells, so it wears off.

How vaccination works — the four-step answer1. A weakened or dead pathogen (or its antigens) is put into the body. 2. Lymphocytes recognise the antigens and produce specific antibodies. 3. Memory cells are made and remain in the blood. 4. If the real pathogen later infects, memory cells produce antibodies faster and in greater quantity, destroying it before symptoms develop.

Antibiotics kill or stop the growth of bacteria — they have no effect on viruses, because viruses live and reproduce inside host cells and lack the structures antibiotics target.

Antibiotic resistanceOveruse and misuse of antibiotics (including not finishing a course) select for resistant bacteria. By natural selection: random mutation makes a few bacteria resistant → the antibiotic kills the non-resistant ones → the resistant survivors reproduce → the population becomes resistant. MRSA is the standard example.

Saying antibiotics "kill viruses" or that vaccines "give you antibodies". A vaccine makes your own lymphocytes produce antibodies — that is exactly what makes the immunity active and long-lasting.

Skill check: Why does a person vaccinated against measles not become ill when later exposed to the measles virus?

Solution: The vaccine caused lymphocytes to produce antibodies against the measles antigens and to form memory cells. On later exposure, these memory cells recognise the antigens and produce the specific antibodies much faster and in greater quantity, destroying the virus before it can multiply enough to cause symptoms.

Topic 13 · 2 units

Excretion

13.1Excretion 13.2Urinary system

DefinitionExcretion is the removal from organisms of the waste products of metabolism, toxic materials and substances in excess of requirements.

Organ	Excretes
Lungs	Carbon dioxide (and water vapour)
Kidneys	Urea, excess water and excess salts (as urine)
Skin	Some water, salts and urea in sweat

The liver carries out deamination: excess amino acids cannot be stored, so the liver removes the nitrogen-containing part and converts it into urea, which the blood carries to the kidneys for excretion. The liver also breaks down alcohol and other toxins.

Urinary system pathway: renal artery → kidney → ureter → bladder → urethra.

How the kidney works — two stages

Filtration (in the glomerulus/Bowman's capsule): high blood pressure forces small molecules — water, glucose, urea and salts — out of the blood into the kidney tubule. Blood cells and proteins are too large to pass through and stay in the blood.
Selective reabsorption (along the tubule): all the glucose, most of the water and some salts are reabsorbed back into the blood. Urea is not reabsorbed — it passes on as urine.

Dialysis is used when kidneys fail. Blood flows on one side of a partially permeable membrane, with dialysis fluid on the other. The fluid contains the correct concentrations of glucose and salts (so these do not diffuse out) but no urea, so urea diffuses out of the blood down its concentration gradient.

Skill check: Glucose is found in the filtrate but not in normal urine. Explain why.

Solution: Glucose is a small molecule, so it is filtered out of the blood in the glomerulus. However, it is a useful nutrient, so all of it is reabsorbed back into the blood by selective reabsorption further along the tubule — leaving none in the urine.

Topic 14 · 6 units

Coordination and control

14.1Nervous system 14.2Sense organs

The nervous system consists of the central nervous system (CNS) — brain and spinal cord — and the peripheral nervous system — the nerves.

The reflex arc — learn this sequence exactly

stimulus → receptor → sensory neurone → relay neurone (in the CNS) → motor neurone → effector → response

A reflex action is a means of automatically and rapidly integrating and coordinating stimuli with responses. It is involuntary — it does not involve conscious thought — which makes it fast and protective, e.g. pulling a hand off a hot object.

A synapse is a junction between two neurones. The impulse arrives, causing a neurotransmitter to be released into the gap; it diffuses across and binds to receptors on the next neurone, starting a new impulse. Synapses make impulses travel in one direction only.

The eye: the cornea refracts light, the iris controls how much light enters, the lens focuses light onto the retina, which contains light receptors, and the optic nerve carries impulses to the brain.

Condition	Iris muscles	Pupil
Bright light	Circular muscles contract, radial muscles relax	Constricts (gets smaller) — protects the retina
Dim light	Radial muscles contract, circular muscles relax	Dilates (gets larger) — lets in more light

Accommodation (focusing): for a near object the ciliary muscles contract, the suspensory ligaments slacken and the lens becomes fatter/more convex, refracting light more. For a distant object the ciliary muscles relax, the ligaments tighten and the lens becomes thinner.

14.3Hormones 14.4Homeostasis 14.5Temperature 14.6Blood glucose

	Nervous control	Hormonal control
Transmitted by	Electrical impulses along neurones	Chemicals in the blood
Speed	Very fast	Slower
Duration	Short-lived	Longer-lasting
Target	Precise — a specific effector	Widespread — any cell with the right receptors

AdrenalineSecreted by the adrenal glands in situations of fear, stress or excitement — the "fight or flight" response. Effects: increased heart rate and breathing rate, increased blood glucose concentration, pupils dilate, blood diverted to muscles — all preparing the body for vigorous activity.

HomeostasisHomeostasis is the maintenance of a constant internal environment.
It works by negative feedback: a change away from the set point is detected, and the response acts to reverse that change and restore the norm.

When too hot	When too cold
Vasodilation — arterioles supplying skin capillaries widen, so more blood flows near the surface and more heat is lost by radiation	Vasoconstriction — arterioles narrow, so less blood flows near the surface and less heat is lost
Sweating increases — water evaporates from the skin, taking heat with it	Sweating decreases
Hairs lie flat	Hairs stand erect, trapping an insulating layer of air
	Shivering — muscles contract rapidly, and respiration releases heat

Writing that blood vessels "move up and down" in the skin to control heat loss. They do not move. It is the arterioles that widen or narrow (vasodilation/vasoconstriction), changing how much blood flows through the capillaries near the surface.

Blood glucose too high	Blood glucose too low
The pancreas secretes insulin	The pancreas secretes glucagon
The liver converts glucose → glycogen for storage; cells take up more glucose	The liver converts glycogen → glucose, released into the blood
Blood glucose falls back to normal	Blood glucose rises back to normal

Type 1 diabetes occurs when the pancreas does not produce enough insulin, so blood glucose stays dangerously high. It is treated with insulin injections, along with a controlled diet and exercise.

Skill check: Explain how the body responds when core temperature rises above 37 °C, naming the process.

Solution: The rise is detected by the brain (hypothalamus). Vasodilation occurs — arterioles supplying the skin capillaries widen, so more blood flows near the surface and more heat is lost by radiation. Sweating increases, and as sweat evaporates it takes heat energy from the body. Hairs lie flat, so less air is trapped. These responses reverse the rise — an example of negative feedback in homeostasis.

Topic 15 · 1 unit

Coordination and response in plants

15.1Coordination and response in plants

DefinitionsA tropism is a growth response of a plant in which the direction of the response is determined by the direction of the stimulus.
Phototropism — response to light. Gravitropism (geotropism) — response to gravity.
Shoots are positively phototropic and negatively gravitropic; roots are negatively phototropic and positively gravitropic.

The auxin explanationAuxin is a plant hormone made at the shoot tip. When light shines from one side, auxin moves to the shaded side. There it causes cells to elongate more, so the shaded side grows longer and the shoot bends towards the light.

Why the responses are advantageous: shoots growing towards light get more light for photosynthesis; roots growing downwards reach water and mineral ions and anchor the plant.

A standard tropism practical: grow seedlings in a box with a single hole (unidirectional light) versus in all-round light, with a control in darkness. For gravitropism, use a clinostat — a slowly rotating drum that gives an even gravitational stimulus on all sides, acting as the control that shows gravity, not something else, causes the bending.

Skill check: A shoot is lit from the left and bends left. Explain in terms of auxin.

Solution: Auxin is produced at the shoot tip and moves away from the light to the shaded (right) side. There it causes the cells to elongate more than those on the lit side, so the right side grows longer than the left, and the shoot bends towards the light on the left.

Topic 16 · 4 units

Development of organisms and continuity of life

16.1Nuclear division 16.2Asexual and sexual reproduction

	Mitosis	Meiosis
Number of divisions	One	Two
Daughter cells	2	4
Chromosome number	Same as parent (diploid)	Halved (haploid)
Genetically	Identical to the parent cell	Different — a source of variation
Used for	Growth, repair, replacement, asexual reproduction	Production of gametes

	Asexual reproduction	Sexual reproduction
Definition	A process resulting in genetically identical offspring from one parent	The joining of gamete nuclei from two parents (fertilisation) forming a zygote
Advantages	Fast; only one parent needed; good traits preserved exactly	Variation — populations can adapt to change and to new diseases
Disadvantages	No variation, so the whole population is vulnerable to one disease or change	Slower; requires two parents and often a pollinator/mate

16.3Sexual reproduction in plants

Flower part	Function
Anther (part of stamen)	Produces pollen grains (male gametes)
Filament	Supports the anther
Stigma	Receives pollen
Style	Supports the stigma; the pollen tube grows down it
Ovary / ovule	Contains the female gamete; becomes the fruit/seed
Petals	Attract insects (in insect-pollinated flowers)

	Insect-pollinated	Wind-pollinated
Petals	Large, brightly coloured, scented, with nectar	Small, green, no scent or nectar
Anthers	Inside the flower, firm	Hanging outside, loosely attached
Stigma	Inside, sticky	Outside, large and feathery
Pollen	Sticky or spiky, smaller amounts	Smooth, light, produced in large amounts

Key distinctionPollination is the transfer of pollen from an anther to a stigma. Fertilisation is the fusion of the pollen nucleus with the ovule nucleus — it happens after a pollen tube grows down the style. Self-pollination is within the same plant; cross-pollination is between different plants of the same species, giving more variation.

Germination requires water (to activate enzymes and for transport), oxygen (for aerobic respiration to release energy) and a suitable temperature (for enzyme action). Light is not required.

Using "pollination" and "fertilisation" interchangeably. Pollination is transfer of pollen; fertilisation is the fusion of nuclei. Questions often award marks for making exactly this distinction.

16.4Sexual reproduction in humans

Structure	Function
Testes	Produce sperm and testosterone
Sperm ducts / urethra	Carry sperm
Ovaries	Produce eggs and oestrogen
Oviduct	Carries the egg to the uterus; site of fertilisation
Uterus	Where the embryo implants and develops
Cervix / vagina	Ring of muscle at the base of the uterus; receives sperm

The placenta allows exchange between the mother's blood and the fetus's blood without the two mixing. Oxygen, glucose, amino acids and antibodies pass to the fetus; carbon dioxide and urea pass back to the mother. The umbilical cord connects fetus to placenta; amniotic fluid cushions the fetus against physical damage.

Menstrual cycle (about 28 days): the uterus lining breaks down (menstruation, days 1–5), then rebuilds; ovulation — release of an egg — occurs around day 14; the lining is maintained ready for implantation, and breaks down again if no fertilisation occurs. Oestrogen repairs and thickens the lining; progesterone maintains it.

Sexually transmitted infections: HIV is transmitted by unprotected sex, infected blood/needles and from mother to child in breast milk. HIV infects and destroys lymphocytes, weakening the immune system and leading to AIDS, so the person dies of other infections. Prevention: condoms, testing, not sharing needles.

Skill check: Give two substances passing from mother to fetus across the placenta, and two passing the other way.

Solution: To the fetus: oxygen and glucose (also amino acids, antibodies, water). To the mother: carbon dioxide and urea. Note the two blood supplies come close but never mix — exchange happens by diffusion across the placenta.

Topic 17 · 4 units

Inheritance

The most calculation-like topic in biology. Genetic diagrams are marked on their layout as much as their answer — set them out fully every time and the marks are reliable.

17.1Variation 17.2DNA

	Continuous variation	Discontinuous variation
Range	A range of values with no distinct categories	Distinct, separate categories
Examples	Height, mass, leaf length	Blood group, tongue rolling, sex
Caused by	Genes and the environment	Genes alone
Graph	Histogram — a smooth, often bell-shaped curve	Bar chart with separate bars

DNA and genesDNA is a molecule made of two strands coiled into a double helix, held together by pairs of bases. The bases pair up specifically: A with T, and C with G.
A gene is a length of DNA that codes for a protein. The sequence of bases determines the sequence of amino acids, which determines the protein made.
A chromosome is a thread of DNA carrying many genes. Humans have 46 chromosomes (23 pairs) in body cells.

Skill check: One DNA strand reads A–T–G–C–C–A. Give the complementary strand.

Solution: A pairs with T and C pairs with G, so the complementary strand is T–A–C–G–G–T.

17.3Inheritance

Terms — use these preciselyInheritance is the transmission of genetic information from generation to generation.
Allele — an alternative form of a gene.
Dominant — an allele expressed if it is present. Recessive — only expressed when two copies are present.
Genotype — the genetic make-up (e.g. Tt). Phenotype — the observable features (e.g. tall).
Homozygous — two identical alleles (TT or tt). Heterozygous — two different alleles (Tt). Two identical homozygous individuals breeding together are pure-breeding.

How to set out a genetic diagram — every time

State the parents' phenotypes and genotypes.
Circle or state the gametes each parent can produce.
Draw a Punnett square and fill in the offspring genotypes.
State the offspring phenotypes and give the ratio.

Always define your symbols first (e.g. T = tall allele, t = short allele), using the same letter in capital and lower case.

Worked example — a 3 : 1 cross

In pea plants, tall (T) is dominant to short (t). Two heterozygous tall plants are crossed. Find the offspring ratio.

Parents: tall × tall, genotypes Tt × Tt. Gametes: T or t from each.

	T	t
T	TT	Tt
t	Tt	tt

Genotypes: 1 TT : 2 Tt : 1 tt. Phenotypes: 3 tall : 1 short — so a 25% chance of a short plant.

Worked example — a 1 : 1 cross

A heterozygous tall plant is crossed with a short plant: Tt × tt.

	t	t
T	Tt	Tt
t	tt	tt

Phenotypes: 1 tall : 1 short (50% each).

Codominance — the ABO blood groupsAlleles I^A and I^B are codominant (both expressed together), and both are dominant to I^o.
Group A = I^AI^A or I^AI^o · Group B = I^BI^B or I^BI^o · Group AB = I^AI^B · Group O = I^oI^o

Worked example — sex determination

Human sex is determined by XX (female) and XY (male). Show why roughly half of children are male.

	X	X
X	XX	XX
Y	XY	XY

Mother (XX) can only give X; father (XY) gives X or Y. Ratio 2 XX : 2 XY = 1 female : 1 male, so a 50% chance each time.

Why observed ratios differ from expected ones: fertilisation is random, so predicted ratios are probabilities, not guarantees. Deviations are especially large with small numbers of offspring — just as tossing a coin ten times rarely gives exactly five heads.

MutationsA gene mutation is a random change in the base sequence of DNA — the example to know is sickle cell anaemia. A chromosome mutation is a change in chromosome number or structure — the example is Down's syndrome (47 chromosomes instead of 46).
Sources of genetic variation in populations: mutation, meiosis, random mating and random fertilisation. Ionising radiation and some chemicals increase the mutation rate.

Skill check: Two brown-eyed parents have a blue-eyed child. Blue (b) is recessive. Give both parents' genotypes and explain.

Solution: The child is blue-eyed, so its genotype is bb — it must have received one b allele from each parent. Since both parents are brown-eyed, each must carry one B and one b: both are Bb (heterozygous). A Bb × Bb cross gives a 3 : 1 ratio, so a 25% chance of a blue-eyed child.

17.4Selection

Natural selection — the five-step answer

There is variation within the population.
Organisms produce many offspring.
There is a struggle for survival, including competition for resources.
Individuals better adapted to the environment are more likely to survive and reproduce.
They pass on their alleles to the next generation, so the frequency of those alleles increases.

Over many generations, the inherited features of the population evolve.

Worked application — antibiotic resistance (MRSA): in a bacterial population, random mutation makes a few individuals resistant. When the antibiotic is used, the non-resistant bacteria are killed, but the resistant ones survive and reproduce, passing on the resistance allele. Over time the population becomes largely resistant — natural selection in action, and fast, because bacteria reproduce so quickly.

Natural selection	Artificial selection (selective breeding)
The environment determines which individuals survive and reproduce	Humans choose which individuals breed
Leads to adaptation to the environment	Leads to desirable features for humans (higher yield, disease resistance)

Writing that organisms "adapt" or "try to change" to survive. Individuals do not change during their lifetime — the variation already exists, and selection acts on it. Say "individuals with the allele for X were more likely to survive", never "they developed X because they needed it".

Skill check: A farmer sprays insecticide each year, and it becomes less effective over time. Explain using natural selection.

Solution: There is variation among the insects, and a few have an allele (from a random mutation) giving resistance to the insecticide. When sprayed, the non-resistant insects die but the resistant ones survive and reproduce, passing on the resistance allele. Each generation the proportion of resistant insects increases, so the insecticide becomes less effective.

Topic 18 · 2 units

Biotechnology and genetic modification

18.1Biotechnology 18.2Genetic modification

Why bacteria are useful in biotechnology: they reproduce very rapidly, they have plasmids that can readily take up genes, they have few ethical concerns compared with animals, and they can make complex molecules and be grown easily in a fermenter.

Uses: yeast in bread-making (CO₂ raises the dough) and brewing (ethanol); bacteria in yoghurt production; pectinase to increase fruit juice yield; biological washing powders containing protease and lipase to digest stains; and lactase to produce lactose-free milk.

Genetic modificationGenetic modification is changing the genetic material of an organism by removing, changing or inserting individual genes.

Making human insulin — the standard example

Restriction enzymes cut the human insulin gene out of human DNA, leaving "sticky ends".
The same restriction enzyme cuts open a bacterial plasmid, leaving complementary sticky ends.
DNA ligase joins the human gene into the plasmid, forming recombinant DNA.
The plasmid is inserted into a bacterium.
The bacteria are grown in a fermenter, reproducing rapidly and producing human insulin, which is extracted and purified.

Advantages of GM	Concerns
Large quantities of pure human protein (insulin) — no risk of animal disease and suitable for those with religious/dietary objections to animal products	Modified genes may spread to wild populations
Crops with herbicide or pest resistance give higher yields	Possible long-term effects on health or ecosystems not fully known
Crops enriched with vitamins can reduce deficiency diseases	Ethical concerns; reduced biodiversity; cost and dependence on seed suppliers

Skill check: Why is insulin from genetically modified bacteria often preferred to insulin extracted from animals?

Solution: It is identical to human insulin, so it works better and is less likely to cause an allergic reaction or rejection; it can be produced in large quantities cheaply and continuously; there is no risk of transferring animal diseases; and it is acceptable to people whose beliefs or diet exclude animal products.

Topic 19 · 5 units

Relationships of organisms with one another and with the environment

19.1Energy flow 19.3Ecosystems and biodiversity

Key termsProducer — an organism that makes its own organic nutrients, usually by photosynthesis. Consumer — an organism that gets energy by feeding on other organisms. Decomposer — an organism that gets energy from dead or waste organic matter.
Food chain — shows the transfer of energy from one organism to the next, beginning with a producer. Food web — a network of interconnected food chains.
Trophic level — the position of an organism in a food chain. Ecosystem — a unit containing the community of organisms and their environment, interacting together.

The Sun is the principal source of energy input to biological systems. Energy flows from producers through consumers, but the arrows in a food chain show the direction of energy transfer, i.e. "is eaten by".

Why food chains are short — and pyramids narrowOnly about 10% of energy passes to the next trophic level. Energy is lost as: heat from respiration, undigested material in faeces, and excretory products such as urea. After four or five levels there is not enough energy left to support another population.
Consequence: feeding humans on crops rather than on animals supports more people from the same land, because it removes one energy-losing trophic level.

Pyramids: a pyramid of numbers counts organisms (and can be an odd shape — one oak tree supports thousands of insects); a pyramid of biomass measures the mass of living material and is almost always a true pyramid.

19.2Nutrient cycles

The carbon cycle: carbon dioxide is removed from the atmosphere by photosynthesis, and returned by respiration (of plants, animals and decomposers), combustion of fossil fuels and wood, and decomposition. Carbon passes along food chains as organic molecules when organisms feed.

Decomposers (bacteria and fungi) break down dead organisms and waste, releasing carbon dioxide and returning mineral ions such as nitrates to the soil, where plants can absorb them again. Without decomposers, nutrients would remain locked in dead material.

19.4Effects of humans on ecosystems 19.5Conservation

Human activity	Effect
Deforestation	Loss of habitat and biodiversity (extinction); less photosynthesis so more atmospheric CO₂; soil erosion and flooding as roots no longer bind the soil
Burning fossil fuels	Increased CO₂ → enhanced greenhouse effect and climate change; sulfur dioxide → acid rain
Fertiliser run-off	Eutrophication: algal bloom → light blocked → plants die → decomposing bacteria multiply and use up dissolved oxygen → fish and aquatic organisms die
Non-biodegradable plastic waste	Persists in the environment; harms and kills wildlife; accumulates in food chains
Overfishing / overhunting	Populations fall below recovery levels; species may become extinct

Sustainable resources and conservationA sustainable resource is one produced as rapidly as it is removed, so it does not run out. Sustainable development means providing for today's population without harming the environment or reducing resources for future generations.

Conservation methods: education, protected areas and national parks, controlled/monitored harvesting and fishing quotas, captive breeding and reintroduction programmes, seed banks, replanting forests (with replacement of felled trees), and recycling to reduce resource use and waste.

Why maintain biodiversity: species may have future medical or agricultural value, ecosystems remain stable and resilient, and there are ethical and aesthetic reasons for preserving them.

Sampling with a quadrat: place quadrats randomly (using random number coordinates) to avoid bias, count the organisms or estimate percentage cover in each, take many samples and calculate a mean, then scale up to the whole area. Use a transect instead when you are investigating how distribution changes along an environmental gradient, such as from the shade of a wall out into open ground.

Skill check: Explain step by step how fertiliser run-off can kill fish in a river.

Solution: Fertiliser washes into the river, adding nitrates. Algae grow rapidly (an algal bloom) and block the light, so submerged plants cannot photosynthesise and die. Decomposing bacteria feed on the dead plants and algae and multiply rapidly, respiring aerobically and using up the dissolved oxygen. With too little oxygen, fish and other aquatic organisms suffocate and die. This process is eutrophication.

Worth 20% of your grade

Practical skills — Paper 3 and Paper 4

Everyone sits one practical paper, testing AO3 alone. Papers 3 and 4 examine identical skills — the only difference is whether you handle the apparatus.

The experimental contexts you must know

Quantitative measurement of volumes, masses, temperatures, times and lengths
Diffusion; osmosis; food tests; rates of enzyme-catalysed reactions
pH and the use of hydrogencarbonate indicator, litmus and universal indicator
Photosynthesis (rate and limiting factors); effect of mineral ions on plant growth; transpiration
Heart rate and breathing rate; respiration; tropic responses; nervous responses
Observation and dissection of seeds and flowers; germination
Continuous and discontinuous variation; sampling techniques
Use of a microscope to examine biological specimens; calculating magnification

Planning and controlling variables

Name the variables explicitlyIndependent — what you change. Dependent — what you measure. Control variables — what you keep the same, and why: "so that any change in the dependent variable is caused only by the independent variable."
Biology-specific controls that examiners look for: same volume and concentration of solutions, same temperature (water bath), same pH (buffer), same species/age/size of organism, same time allowed, same light intensity.

The control experimentA control is a parallel setup with the one factor being tested removed or inactivated — for example boiled (denatured) enzyme, a tube with no organism, or a plant kept in the dark. It shows the result is caused by the factor under test and nothing else. Do not confuse a control experiment with a control variable.

Recording, graphs and evaluation

Column headings carry the quantity and unit separated by a solidus: time / s, mass / g. Units never appear beside the numbers.
All raw readings in a column have the same number of decimal places.
Use percentage change when starting values differ (as in the potato osmosis experiment), so comparisons are fair.
Independent variable on the x-axis; scales chosen so points fill more than half the grid; plot as small crosses; draw a single thin best-fit line or smooth curve.
Ignore clearly anomalous results when drawing the line, but circle them and say you have.

Common source of error	Improvement to suggest
Timing a colour change by eye	Use a colorimeter, or a white tile and a fixed judging point; repeat and average
Temperature not constant	Use a thermostatically controlled water bath
Blotting potato cylinders inconsistently	Blot each for the same time in the same way
Only one repeat	Repeat at least three times and calculate a mean
Small sample size in sampling	Take more quadrat samples, placed randomly

ConclusionsQuote the data, then judge: "As temperature rose from 20 °C to 40 °C the volume of oxygen produced increased from 8 cm³ to 21 cm³, so the results support the prediction that rate increases with temperature up to the optimum." Naming the comparison is what earns the mark.

Safety: wash hands after handling biological material; treat all body fluids as potentially infectious; use eye protection with iodine, Benedict's solution and sodium hydroxide; never heat ethanol directly — use a water bath, as it is highly flammable; take care with sharp scalpels when dissecting, cutting away from yourself on a tile.

Reference

Definitions bank

Biology marks hinge on exact wording. These are the definitions asked most often, phrased as examiners expect them.

Term	Definition
Diffusion	The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of random movement
Osmosis	The net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane
Active transport	Movement of particles through a cell membrane from a lower to a higher concentration, using energy from respiration
Enzyme	A protein that functions as a biological catalyst
Denatured	The enzyme's active site has changed shape, so the substrate no longer fits
Photosynthesis	The process by which plants make carbohydrates from raw materials using energy from light absorbed by chlorophyll
Limiting factor	The factor present in the shortest supply, which limits the rate of a process
Transpiration	The loss of water vapour from leaves by evaporation and diffusion through the stomata
Translocation	The transport of sucrose and amino acids in phloem, from source to sink
Digestion	The breakdown of large insoluble food molecules into small soluble molecules
Absorption	The movement of small food molecules through the wall of the intestine into the blood
Assimilation	The movement of digested food molecules into cells where they are used
Respiration	The chemical reactions in cells that break down nutrient molecules to release energy for metabolism
Excretion	The removal of the waste products of metabolism, toxic materials and substances in excess of requirements
Deamination	The removal of the nitrogen-containing part of amino acids in the liver, forming urea
Homeostasis	The maintenance of a constant internal environment
Reflex action	A means of automatically and rapidly integrating and coordinating stimuli with responses
Synapse	A junction between two neurones
Tropism	A growth response in which the direction of the response is determined by the direction of the stimulus
Pathogen	A disease-causing organism
Vector	An organism that carries a pathogen from one host to another
Active immunity	Defence against a pathogen by the production of your own antibodies
Passive immunity	Short-term defence from antibodies acquired from another individual
Species	A group of organisms with similar features that can breed to produce fertile offspring
Gene	A length of DNA that codes for a protein
Allele	An alternative form of a gene
Genotype / phenotype	Genotype is the genetic make-up; phenotype is the observable features
Homozygous / heterozygous	Two identical alleles / two different alleles
Mitosis / meiosis	Mitosis gives two genetically identical diploid cells; meiosis gives four genetically different haploid gametes
Asexual reproduction	A process resulting in genetically identical offspring from one parent
Sexual reproduction	The joining of gamete nuclei from two parents to form a zygote
Pollination / fertilisation	Transfer of pollen from anther to stigma / fusion of gamete nuclei
Mutation	A random change in the base sequence of DNA (gene) or in chromosome number or structure
Producer / consumer / decomposer	Makes its own nutrients by photosynthesis / feeds on other organisms / gets energy from dead or waste organic matter
Ecosystem	A unit containing the community of organisms and their environment, interacting together
Sustainable resource	One produced as rapidly as it is removed, so it does not run out