Cells → tissues → organs → systems

Living things are organised at increasing levels of complexity:

• Cells are the basic building blocks of all living organisms.
• A tissue is a group of cells with a similar structure and function (e.g. muscle tissue).
• An organ is an aggregation of tissues performing a specific function (e.g. the stomach).
• An organ system is several organs working together (e.g. the digestive system) — and organ systems work together to form the whole organism.

Watch out: the order is cells → tissues → organs → organ systems → organism. An organ is made of several tissues, not the other way round.

Levels of organisation

An organ system for food

The digestive system is an organ system — several organs work together to digest and absorb food, turning large insoluble molecules into small soluble ones that can be absorbed into the bloodstream.

Word equations only — AQA does not ask for chemical symbol equations here.

Enzymes are biological catalysts

Enzymes are large protein molecules. They catalyse specific reactions because of the shape of their active site — only a substrate with a matching shape fits, like a key in a lock.

Misconception alert: enzymes don't "die". Too much heat or the wrong pH changes the shape of the active site — the enzyme is denatured, so the substrate no longer fits.

Temperature & pH change the rate

Each enzyme has an optimum temperature and pH where it works fastest:

Required practical 5: investigate the effect of pH on amylase. Use a continuous sampling technique — drop iodine every 30 s — to time how long the amylase takes to fully digest starch at each pH. Keep temperature constant with a water bath.

Above the optimum

Carbohydrases, proteases, lipases

You must recall the sites of production and the action of these enzymes:

• Carbohydrases break carbohydrates → simple sugars. Amylase is a carbohydrase that breaks starch → glucose (made in salivary glands, pancreas, small intestine).
• Proteases break proteins → amino acids (made in the stomach, pancreas, small intestine).
• Lipases break lipids (fats) → glycerol + fatty acids (made in the pancreas, small intestine).

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

Enzyme → product

Tap an enzyme on the left, then tap the molecules it produces on the right.

Testing food for nutrients

Use these qualitative reagents — a colour change shows a positive result:

• Iodine test for starch: orange-brown → blue-black.
• Benedict's test for sugars (reducing): heat in a water bath; blue → brick-red / orange.
• Biuret reagent for protein: blue → purple/lilac.

Beyond AQA: the Sudan III (emulsion) test for lipids — red layer on top — is a common extension but is not named in the 8461 specification (the spec lists only Benedict's, iodine and Biuret).

Reading a food test

The heart & double circulation

The heart is an organ that pumps blood in a double circulatory system — two loops:

• The right ventricle pumps blood to the lungs (gas exchange).
• The left ventricle pumps blood to the rest of the body.

Pacemaker: a group of cells in the right atrium set the natural resting heart rate. An artificial pacemaker corrects an irregular rate.

Arteries, veins & capillaries

Three vessels, each adapted to its job:

• Arteries carry blood away from the heart — thick, elastic, muscular walls to take high pressure.
• Veins carry blood back to the heart — thinner walls, wide lumen, valves stop backflow at low pressure.
• Capillaries are one cell thick — a short diffusion distance for exchange of substances with tissues.

Misconception alert: arteries carry blood away from the heart and veins carry it back — this is about direction, not whether the blood is oxygenated (the pulmonary artery carries deoxygenated blood!).

Name the vessel

Blood is a tissue

Blood is a tissue made of plasma with three things suspended in it:

• Red blood cells — carry oxygen. Biconcave, packed with haemoglobin, no nucleus (more room for oxygen).
• White blood cells — part of the immune system; engulf pathogens or make antibodies. They have a nucleus.
• Platelets — small cell fragments that help the blood clot at a wound.
• Plasma — the straw-coloured liquid that transports CO₂, dissolved food, urea, hormones and the cells.

Which component?

Coronary heart disease

In coronary heart disease (CHD), layers of fatty material build up inside the coronary arteries, narrowing them. Less blood flows to the heart muscle, so it gets less oxygen.

• Stents — wire mesh tubes that keep a narrowed coronary artery open so blood can flow.
• Statins — drugs that reduce blood cholesterol, slowing the rate of fatty deposits.

Faulty heart valves can be replaced (biological or mechanical valves). In heart failure, a donor heart (or heart and lungs) can be transplanted; artificial hearts can keep a patient alive while waiting.

Evaluate each treatment by weighing benefits against risks (e.g. surgery risk, clots, drug side-effects, donor shortage).

Health, disease & lifestyle

Health is the state of physical and mental well-being. Disease is a major cause of ill health, and different diseases can interact (e.g. a weakened immune system → more infections; some viruses trigger cancers).

• Communicable diseases can spread (caused by pathogens).
• Non-communicable diseases cannot spread (e.g. CHD, type 2 diabetes, cancer).

Risk factors are linked to a higher rate of a disease — lifestyle aspects or substances in the body/environment:

• Diet, smoking & exercise → cardiovascular disease.
• Obesity → type 2 diabetes.
• Alcohol → liver and brain damage (and harm to unborn babies).
• Smoking → lung disease and lung cancer (and harm to unborn babies).
• Carcinogens, including ionising radiation → cancer.

A correlation does not always prove cause — a causal mechanism has been proven for some risk factors but not all.

Benign vs malignant tumours

Cancer is the result of changes in cells leading to uncontrolled growth and division:

• Benign tumours — abnormal cells contained in one area, usually within a membrane. They do not invade other parts of the body.
• Malignant tumour cells are cancers — they invade neighbouring tissues and spread in the blood to form secondary tumours.

There are lifestyle risk factors (e.g. smoking, UV) and genetic risk factors for some cancers.

Tumour type

Plant tissues & leaf structure

Plant tissues include epidermal tissue, palisade mesophyll, spongy mesophyll, xylem & phloem, and meristem (at growing tips). The leaf is an organ:

Transpiration & translocation

The roots, stem and leaves form an organ system that moves substances around the plant:

• Xylem — hollow tubes strengthened by lignin; carries water & mineral ions from roots → leaves in the transpiration stream (one way, upwards).
• Phloem — tubes of elongated cells with pores in the end walls; carries dissolved sugars from leaves → rest of the plant. This is translocation (both directions).
• Root hair cells — adapted for efficient uptake of water by osmosis and mineral ions by active transport.

Transpiration is the loss of water vapour from the leaves. Stomata (opened/closed by guard cells) control gas exchange and water loss. Transpiration speeds up with:

• Higher temperature, more air movement (wind), higher light intensity (stomata open wider), and lower humidity.

Misconception alert: transpiration = water loss through the leaves (xylem); translocation = sugar transport (phloem). Don't mix the two T's up.

Speeding up transpiration

Xylem or phloem?

Tap a statement, then tap the tissue it belongs to.

The key facts to know

You've covered the whole of AQA 4.2 — animal and plant organisation. Press Finish to see your score.