Organisation

• Cells are the basic building blocks of every living organism.
• A tissue contains cells with similar structures and functions; several tissues working together form an organ.
• Organs cooperate in organ systems, and the organ systems work together to form an organism.

The punchline: At every level, connect the structure to the job it performs.

• Enzymes are proteins that catalyse specific reactions without being used up; specificity depends on active-site shape.
• In the simplified lock-and-key model, a substrate binds, products form and leave, and the enzyme can work again.
• Warming increases kinetic energy and successful collisions up to an optimum; higher temperatures can denature the enzyme and alter its active site.
• An unsuitable pH can change the active site's shape, so fewer substrates bind and the reaction rate falls.

The punchline: Explain enzyme rate as collision first, active-site shape second.

• Amylase is made in the salivary glands, pancreas and small intestine; it acts in the mouth and small intestine, converting starch to sugars.
• Proteases are made in the stomach, pancreas and small intestine; they act in the stomach and small intestine, producing amino acids.
• Lipases are made in the pancreas and small intestine; they act in the small intestine, producing glycerol and fatty acids.
• These small, soluble products can cross the small-intestine wall and enter the bloodstream.

The punchline: Digestion makes food molecules small and soluble enough to absorb.

• Bile is made in the liver, stored in the gall bladder and released into the small intestine.
• Its alkalinity helps neutralise stomach acid; emulsification disperses fat into droplets with a larger surface area.
• Digestion products build new carbohydrates, lipids and proteins, while some glucose is used in respiration.
• Read word equations as substrate → products: starch → sugars; protein → amino acids; lipid → glycerol + fatty acids.

The punchline: Bile changes conditions and surface area; enzymes break chemical bonds.

• The vena cava brings deoxygenated blood to the right atrium; the right ventricle sends it through the pulmonary artery to the lungs.
• The pulmonary vein returns oxygenated blood to the left atrium; the left ventricle pumps it through the aorta to the body.
• Air travels down the trachea and through the bronchi to alveoli with thin walls, a large surface area and a surrounding capillary network.
• Valves prevent backflow. A natural pacemaker in the right atrium sets the resting rhythm; an artificial pacemaker can correct irregularities.

The punchline: In one complete circulation, blood passes through the heart twice.

• Arteries carry blood away at high pressure; thick muscular, elastic walls withstand and maintain it, while the lumen is relatively narrow.
• Veins carry blood towards the heart at lower pressure; a wide lumen reduces resistance and valves prevent backflow.
• Capillary walls are one cell thick and their narrow lumen keeps blood close to tissues, creating a short exchange distance.

The punchline: Artery and vein name direction—not oxygen content.

• Plasma suspends the cells and platelets and carries dissolved substances such as glucose, amino acids, carbon dioxide and urea.
• Red blood cells have haemoglobin, a biconcave shape and no nucleus, giving more room and surface area for oxygen transport.
• Different white blood cells engulf pathogens or produce antibodies and antitoxins as part of the body's defence.
• Platelets are cell fragments that trigger clotting, reducing blood loss and entry of microorganisms through a wound.

The punchline: For each blood component, pair its adaptation with its function.

• An immune-system defect can make infectious disease more likely, and some viral infections can trigger cancer.
• Immune reactions first triggered by a pathogen can cause allergies, while severe physical illness can contribute to depression.
• Treating one condition may therefore improve more than one part of a person's health.

The punchline: Explain the biological link between conditions rather than merely listing them.

• Non-communicable diseases impose human and financial costs on individuals, local communities, nations and the world.
• Translate incidence data between numbers, frequency tables, bar charts and histograms; use scatter graphs to identify correlations.
• Judge whether sampling is representative, the sample is large enough and other variables could explain the pattern.
• A repeated association and plausible mechanism strengthen a causal claim, but correlation alone does not prove cause.

The punchline: Interrogate the sample before trusting the pattern.

• Diet, smoking and exercise affect cardiovascular-disease risk; obesity is a risk factor for Type 2 diabetes.
• Smoking raises risks of lung disease and lung cancer; alcohol can damage the liver and brain function. Either can harm an unborn baby during pregnancy.
• Carcinogens, including ionising radiation, increase cancer risk, and several risk factors can interact in one disease.
• Compare incidence at local, national and global scales, remembering that lifestyle, environment and other factors may vary together.

The punchline: Use ‘increases the risk of’, not ‘always causes’.

• In coronary heart disease, fatty material narrows coronary arteries and reduces oxygen delivery to heart muscle.
• Stents restore blood flow quickly by holding an artery open, but insertion has procedural risks and the artery may narrow again.
• Statins lower blood cholesterol and slow fatty deposits, but require long-term use and can cause side effects.

The punchline: Compare treatments through mechanism, benefit, risk and timescale.

• A valve that will not open fully restricts flow; a leaking valve allows backflow, so the heart pumps less effectively.
• Mechanical valves are durable but usually require anticoagulants; biological valves may wear out sooner but usually need less anticoagulation.
• For severe heart failure, a donor heart or heart-and-lung transplant may help, but donors are scarce and rejection requires immunosuppressant drugs.
• An artificial heart can keep someone alive while awaiting a transplant or let the heart rest, but clots, bleeding and infection are risks.

The punchline: Match each treatment to the fault, then weigh benefit against risk.

• Cancer results from cell changes that lead to uncontrolled growth and division.
• A benign tumour remains contained in one area, usually within a membrane, and does not invade other tissues.
• Malignant cells invade neighbouring tissues and can travel in blood to form secondary tumours elsewhere.
• Lifestyle factors raise the risk of some cancers, and inherited genetic factors raise the risk of others.

The punchline: Distinguish tumour behaviour, then discuss risk without promising an outcome.

• Transparent epidermal tissue lets light reach palisade mesophyll cells, which contain many chloroplasts for photosynthesis.
• Spongy mesophyll has air spaces that shorten the diffusion route for carbon dioxide and oxygen.
• Guard cells change the size of stomata, controlling gas exchange and water loss.
• Meristem tissue at root and shoot tips produces new cells for plant growth.

The punchline: For every plant tissue, pair a structural feature with its function.

• Root hair cells provide a large surface area for water uptake by osmosis and mineral-ion uptake by active transport.
• Xylem forms hollow tubes strengthened by lignin, carrying water and mineral ions from roots to stems and leaves.
• Phloem has tubes of elongated living cells with porous end walls; translocation moves dissolved sugars to where they are used or stored.

The punchline: Keep xylem's transpiration stream separate from phloem translocation.

• Water evaporates from leaf cells and diffuses through stomata, helping pull more water upwards.
• Greater light intensity, higher temperature and faster air movement usually increase transpiration when other factors stay unchanged.
• High humidity usually reduces the rate because the water-vapour concentration difference between leaf and air is smaller.

The punchline: Explain each factor by linking it to evaporation or the diffusion gradient.