1. Key Biological Molecules: Water, Carbohydrates, Lipids★★☆☆☆⏱ 10 min

Water's dipole nature makes it an excellent transport solvent, as polar and ionic substances dissolve easily in it for transport around the body. Carbohydrates are grouped into monosaccharides (single sugar units), disaccharides (two units joined) and polysaccharides (long chains). Storage polysaccharides (starch in plants, glycogen in animals) are compact and easily hydrolysed to release glucose for energy. Condensation reactions form glycosidic bonds between carbohydrate units, and hydrolysis reactions break these bonds.

Triglycerides are formed via condensation of one glycerol molecule and three fatty acid molecules, with ester bonds linking each fatty acid to glycerol. Saturated fatty acids have no double bonds between carbon atoms, while unsaturated fatty acids have at least one double bond.

Exam tip:

Always use exact bond names: glycosidic for carbohydrates, ester for triglycerides. Mark schemes do not award marks for generic terms like 'covalent bond' when the specific bond name is required.

2. Core Practicals 1 and 2: Food Tests★★★☆☆⏱ 10 min

Core Practical 1 is a semi-quantitative test for reducing sugars and starch. For reducing sugars: heat the sample with blue Benedict's reagent; a positive result shows a colour change from green (low concentration) → yellow → orange → red (high concentration). Colour standards of known sugar concentration are used to estimate the concentration of unknown samples. Iodine solution turns from orange-brown to blue-black in the presence of starch.

Core Practical 2 measures vitamin C content using DCPIP indicator. Vitamin C decolourises blue DCPIP, so the volume of sample needed to decolourise a fixed volume of DCPIP is inversely proportional to the vitamin C concentration of the sample.

3. Circulatory System and Gas Transport★★★☆☆⏱ 10 min

Large multicellular animals need a mass transport system (heart and circulation) because diffusion alone is too slow to transport oxygen, nutrients and waste products over long distances to all body cells. Blood vessels are adapted to their function: arteries have thick elastic walls to withstand high blood pressure, veins have valves to prevent backflow of blood, and capillaries have thin, permeable walls for efficient substance exchange with tissues.

The cardiac cycle has three ordered stages: 1) Atrial systole: atria contract, pushing blood into relaxed ventricles. 2) Ventricular systole: ventricles contract, pushing blood out of the heart into the aorta and pulmonary artery, with atrioventricular valves closed to prevent backflow into atria. 3) Cardiac diastole: all chambers relax, and the heart fills with blood from the vena cava and pulmonary vein.

Haemoglobin in red blood cells carries oxygen from the lungs to respiring tissues, and carries a small proportion of carbon dioxide back to the lungs. Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, so its dissociation curve lies to the left of the adult curve, allowing it to take up oxygen from the mother's blood across the placenta.

4. Atherosclerosis, Blood Clotting and CVD Risk★★★☆☆⏱ 8 min

Atherosclerosis develops in stages: 1) Damage to the endothelial lining of arteries triggers inflammation. 2) White blood cells and lipids accumulate under the endothelium to form a fatty plaque. 3) The plaque narrows the artery, raising blood pressure and reducing blood flow to tissues. If the plaque ruptures, the blood clotting cascade is activated.

The blood clotting cascade follows this fixed order: endothelial damage releases thromboplastin → thromboplastin converts inactive prothrombin to active thrombin → thrombin converts soluble fibrinogen to insoluble fibrin → fibrin forms a mesh that traps platelets and red blood cells to form a clot. Clots can block coronary arteries (causing a heart attack) or cerebral arteries (causing a stroke), both forms of cardiovascular disease (CVD).

• Modifiable CVD risk factors: high LDL cholesterol, smoking, high blood pressure, obesity, low physical activity, high saturated fat diet

• Non-modifiable CVD risk factors: older age, male gender, genetic predisposition

5. Epidemiological Analysis and CVD Treatments★★★★☆⏱ 7 min

When analysing health and mortality data, you must distinguish between correlation and causation: a correlation between a variable (e.g. coffee consumption) and CVD risk does not prove the variable causes CVD, as confounding variables (e.g. smoking, low exercise) may explain the link. Valid epidemiological studies use large, representative samples to ensure results are reliable and generalisable. Public risk perception often differs from actual risk: people tend to overestimate rare, dramatic risks and underestimate common, gradual risks like CVD.

• Antihypertensives: lower blood pressure to reduce CVD risk, side effects include dizziness and fatigue

• Statins: lower LDL cholesterol levels to reduce plaque formation, rare side effects include muscle pain and liver damage

• Anticoagulants: reduce blood clotting to lower stroke risk, increase risk of excessive bleeding after injury

• Platelet inhibitors (e.g. aspirin): reduce platelet aggregation to lower clot risk, can cause stomach ulcers with long-term use

Exam tip:

When asked to evaluate study design, always comment on sample size, representativeness, and control of confounding variables as key markers of validity.