AQA GCSE Biology (8461) · Paper 1

🫀 Organisation

Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.

Principles of organisation

The body is organised in a series of levels, each one built from the level below, and this hierarchy is often tested directly in exams.

cell → tissue → organ → organ system → organism

TissueA tissue is a group of similar cells working together to do one job, such as muscle tissue.

OrganAn organ is made of several different tissues combined to carry out one function. The stomach, for example, contains muscle, glandular and epithelial tissue.

Organ systemAn organ system is a group of organs working together on a major job. The digestive system, for example, turns food into molecules the body can use.

Animal tissues, organs and systems

Enzymes — the body's catalysts

An enzyme is a protein that acts as a biological catalyst, speeding up a reaction without being used up itself. Each enzyme has an active site, a dent whose shape exactly fits one substrate, the molecule it acts on.

This is called the lock and key model: the substrate acts as the key and the active site as the lock. It explains why enzymes are specific, since the wrong molecule simply does not fit the active site.

Every enzyme has an optimum temperature and pH at which it works fastest. If the temperature becomes too high, or the pH moves too far from this optimum, the active site changes shape and the enzyme is denatured. Once this happens the substrate no longer fits, and the change is permanent. Enzymes in the human body typically work best at around 37°C; the stomach protease pepsin works best in the acidic conditions of pH 2, while amylase works best at a neutral pH.

🎯 Examiner tip: Enzymes should never be described as dying or being killed, since they were never alive. The mark scheme requires the word denatured: the active site changes shape so the substrate no longer fits.

Digestion

Digestion breaks large, insoluble food molecules down into small, soluble ones that can be absorbed into the blood. Starch, proteins and fats are too large to cross the gut wall unchanged, which is why digestion has to happen first.

Food passes through the body in order: the mouth, oesophagus, stomach, small intestine, large intestine and finally the rectum. The pancreas and liver are supporting organs that food never actually enters; instead, they add digestive juices into the small intestine.

Amylase (a carbohydrase)Amylase, a carbohydrase, is made in the salivary glands, pancreas and small intestine, and it breaks down starch into sugars, including glucose.

ProteasesProteases are made in the stomach (as pepsin), the pancreas and the small intestine, and they break down proteins into amino acids.

LipaseLipase is made in the pancreas and small intestine, and it breaks down lipids into glycerol and fatty acids.

BileBile is made in the liver and stored in the gall bladder. It is not an enzyme, and it has two jobs: it neutralises stomach acid, and it emulsifies fat into tiny droplets, increasing the surface area available for lipase to act on.

The products of digestion are used to build new carbohydrates, lipids and proteins, and some of the glucose is used in respiration.

The four food tests

StarchThe test for starch uses iodine solution, which turns from orange to blue-black if starch is present.

SugarsThe test for sugars uses Benedict's solution with heating, which changes from blue through green and orange to brick red if sugar is present.

ProteinThe test for protein uses Biuret solution, which turns from blue to purple or lilac if protein is present.

LipidsFor the emulsion test, shake the sample with ethanol, then add the ethanol extract to water. A cloudy white emulsion indicates lipids. Ethanol is flammable; keep it away from flames.

🧪 Required practical: In the enzyme practical, amylase and starch are mixed in a water bath, and every 30 seconds a sample is dropped onto iodine in a spotting tile. While starch remains, the spots turn blue-black; once the iodine stays orange, all the starch has been digested. The experiment is repeated at different pH values, using buffer solutions, to find the optimum pH. A shorter digestion time means a faster rate of reaction.

The lungs

Air travels through the trachea, bronchi and bronchioles before reaching the alveoli. The millions of tiny alveoli form the gas exchange surface, where oxygen diffuses into the capillary blood and carbon dioxide diffuses out. This exchange is helped by a huge surface area, walls only one cell thick, and a rich blood supply that constantly maintains the concentration gradient.

The heart and a double circulation

The circulatory system is described as double, because blood passes through the heart twice during one full circuit. In the first loop, blood travels from the heart to the lungs and back, picking up oxygen. In the second loop, it travels from the heart to the body and back, delivering that oxygen. Having two separate loops allows blood travelling to the body to stay at high pressure.

The heart has four chambers. Blood arrives into the two atria and passes down into the two ventricles, which pump it back out. The left ventricle wall is the thickest, because it must force blood around the entire body, while the right ventricle only has to pump blood to the nearby lungs. Valves stop the blood flowing backwards.

Vena cavaThe vena cava is the vein that brings deoxygenated blood from the body into the right atrium.

Pulmonary arteryThe pulmonary artery carries deoxygenated blood to the lungs. It is unusual for an artery, since arteries usually carry oxygenated blood.

Pulmonary veinThe pulmonary vein brings oxygenated blood from the lungs to the left atrium.

AortaThe aorta is the body's biggest artery, carrying oxygenated blood out to the body.

Coronary arteriesThe coronary arteries branch off the aorta and supply the heart muscle itself with oxygenated blood.

Resting heart rate is set by a group of cells in the right atrium, which acts as a natural pacemaker. If the heart's rhythm goes wrong, doctors can fit an artificial pacemaker, which corrects it using electrical impulses.

Blood vessels

ArteriesArteries carry blood away from the heart at high pressure, and they have thick, muscular, elastic walls with a small lumen.

VeinsVeins return blood to the heart at low pressure. They have thin walls, a wide lumen, and valves to stop the blood flowing backwards.

CapillariesCapillaries have walls one cell thick and a tiny lumen. This is where exchange actually happens, close to every cell in the body.

rate = amount ÷ time (the same formula applies to blood flow, transpiration and reaction rates)

Blood — a liquid tissue

Plasma is the liquid part of blood, and it transports dissolved glucose, urea and carbon dioxide, as well as hormones and heat.
Red blood cells carry oxygen as oxyhaemoglobin, and they are adapted for this with no nucleus, giving more room for haemoglobin, a biconcave shape that increases surface area, and cytoplasm packed with haemoglobin.
White blood cells are the cells of the immune system, and they defend the body by engulfing pathogens and by making antibodies.
Platelets are cell fragments that help clot the blood at wounds.

Cardiovascular disease

Cardiovascular disease (CVD) is the name for diseases of the heart and blood vessels, and it is non-communicable. In coronary heart disease, fatty material builds up inside the coronary arteries and narrows them, so less blood, and therefore less oxygen, reaches the heart muscle, which can cause chest pain or a heart attack.

StentsStents are wire mesh tubes that hold a narrowed artery open. They are a quick fix that works for the lifetime of the device, but they do not treat the underlying cause.

StatinsStatins are daily drugs that reduce blood cholesterol, slowing the fatty build-up in the arteries. They are a long-term treatment and can have side effects.

Faulty valvesA leaky or stiff valve means blood flows backwards or the heart has to strain to pump it. Faulty valves can be replaced with biological valves, from an animal or human donor, or with mechanical valves.

Heart failureOptions for heart failure include a donor transplant, or an artificial heart, which is usually a temporary measure while a patient waits for a donor.

Health, disease and risk factors

Health is a state of physical and mental wellbeing, not simply the absence of disease. Diseases are split into two groups: communicable diseases, which are caused by pathogens and can spread, such as flu and measles, and non-communicable diseases, which cannot spread, such as heart disease, cancer and diabetes.

Diseases can interact with one another. A viral infection can trigger some cancers, for example HPV can lead to cervical cancer; a weakened immune system makes other infections more likely; and physical illness can trigger depression.

A risk factor raises the chance of getting a disease, but it does not guarantee it. The classic linked pairs are worth knowing: smoking is linked to lung disease and cardiovascular disease; poor diet combined with inactivity is linked to obesity and type 2 diabetes; and alcohol is linked to liver damage and impaired brain function. Non-communicable diseases also carry large costs beyond the patient, including NHS bills, lost work, and strain on families.

Cancer

A tumour is a mass of cells produced by uncontrolled cell division. Benign tumours stay in one place, enclosed within a membrane, and are usually not dangerous unless they press on something important. Malignant tumours are cancerous: they invade neighbouring tissue and can spread through the blood to form secondary tumours elsewhere in the body. Risk factors for cancer include smoking, UV exposure, alcohol, certain viruses, and inherited genes.

Plant tissues, organs and systems

Plants are organised in the same way. The leaf is an organ, built from several tissues, and it forms part of the plant's organ system for transport.

EpidermisThe epidermis is the skin of the leaf, covering both the top and bottom surfaces. It is thin and transparent, letting light through, and it is coated in a waxy cuticle.

Palisade mesophyllThe palisade mesophyll is made of tall cells packed with chloroplasts, lined up near the top of the leaf. It is the main tissue for photosynthesis.

Spongy mesophyllThe spongy mesophyll is made of loosely packed cells with air spaces between them, allowing gases to diffuse to every cell.

Xylem & phloemXylem and phloem are the transport tissues, bundled together in the leaf's veins.

MeristemMeristem is the growing-point tissue found at shoot and root tips, where plant cells continue to divide.

Stomata

Stomata are tiny pores, each controlled by a pair of guard cells that open or close them by changing shape as they gain or lose water. Stomata let carbon dioxide in and let water vapour out. Most stomata are found on the underside of the leaf, which is cooler and shadier, so less water is lost there.

Transpiration and translocation

Transpiration is the loss of water vapour from the leaves, and this loss pulls a continuous column of water, with dissolved minerals, up the plant through the xylem. Transpiration speeds up with more light, since it causes the stomata to open, with higher temperature, with more wind, and with drier air.

Translocation is different: it is the movement of dissolved sugars made in the leaves to the rest of the plant, through the living phloem, in whichever direction the plant needs.

XylemXylem consists of dead, hollow tubes strengthened with lignin. It carries water and minerals upwards only.

PhloemPhloem consists of living cells with sieve plates. It carries dissolved sugars in any direction the plant needs.

Transpiration can be measured with a potometer. As the shoot loses water, it pulls a bubble along a tube, and dividing the bubble's distance by the time taken gives the rate. Different conditions, such as a fan or a lamp, should be compared one at a time.

🎯 Examiner tip: Xylem and phloem are commonly confused. Xylem carries water in dead tubes, moving in one direction only, upwards, while phloem carries food (dissolved sugars) in living cells, moving in any direction.

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