Edexcel A Level Biology: Molecules, Transport and Health — Revision Notes

Condensed for the final weeks. For the full explanation, use the Molecules, Transport and Health study guide.

Biological molecules

Condensation joins monomers and releases water; hydrolysis breaks bonds using water.

Monomer	Bond	Polymer
Monosaccharide	Glycosidic	Polysaccharide
Amino acid	Peptide	Polypeptide
Nucleotide	Phosphodiester	Nucleic acid

α-glucose forms starch and glycogen — coiled or branched, for storage. β-glucose forms cellulose: alternate molecules are inverted, giving straight chains that hydrogen-bond into microfibrils of high tensile strength.

Lipids are not polymers. A triglyceride is glycerol plus three fatty acids joined by ester bonds. Saturated fatty acids have no C=C; unsaturated have one or more, producing kinks that prevent close packing — hence oils are liquid at room temperature.

Proteins: primary (sequence) → secondary (α-helix, β-pleated sheet, hydrogen bonds) → tertiary (3-D shape; hydrogen, ionic, disulfide, hydrophobic interactions) → quaternary (multiple polypeptides). The primary structure determines all higher levels.

Water

• Polar, so it is an excellent solvent for ions and polar molecules — the medium for all metabolic reactions and for transport.
• High specific heat capacity — buffers temperature change, keeping enzymes near their optimum.
• High latent heat of vaporisation — evaporation gives effective cooling.
• Cohesion between molecules — produces surface tension and allows a continuous water column in the xylem.
• Less dense as ice, so ice floats and insulates the water beneath, allowing aquatic life to survive winter.

Every one of these traces back to hydrogen bonding, which is what the mark scheme wants stated.

The circulatory system

Mass transport is necessary in large organisms because their surface area to volume ratio is too small for diffusion alone to supply their needs quickly enough over the distances involved.

Double circulation: blood passes through the heart twice per circuit, keeping oxygenated and deoxygenated blood separate and allowing the systemic circuit to run at higher pressure.

Vessel	Structure	Reason
Artery	Thick muscular wall, elastic fibres, narrow lumen	Withstands and maintains high pressure; elastic recoil smooths flow
Vein	Thin wall, wide lumen, valves	Low pressure; valves prevent backflow
Capillary	One cell thick, very narrow	Short diffusion distance, large surface area

The cardiac cycle

1. Atrial systole — atria contract, forcing blood into the ventricles.
2. Ventricular systole — ventricles contract; atrioventricular valves close (first heart sound), semilunar valves open.
3. Diastole — all chambers relax; semilunar valves close (second sound), atria refill.

Valves are opened and closed by pressure differences alone — they are passive flaps, not muscles. A valve opens when the pressure behind it exceeds the pressure in front, and closes when the reverse is true. Reading a pressure–time graph is simply applying that rule.

Atherosclerosis and CVD

The sequence, which must be given in order:

1. Damage to the endothelium of an artery, often from high blood pressure or smoking.
2. Inflammatory response — white blood cells accumulate at the site.
3. Cholesterol and lipids deposit, forming a fatty streak, then an atheroma (plaque).
4. Calcium salts and fibrous tissue harden it, narrowing the lumen and reducing elasticity.
5. Narrowing raises blood pressure further, causing more damage — a positive feedback loop.
6. A plaque may rupture, triggering a thrombus (blood clot), which can block the artery entirely.

Blockage of a coronary artery causes a myocardial infarction; blockage in the brain causes a stroke.

Risk factors and correlation

Modifiable: diet high in saturated fat and salt, smoking, inactivity, obesity, high alcohol intake. Non-modifiable: age, sex, genetics.

Correlation does not establish causation. A good evaluation of a study asks about sample size, whether it was representative, whether the study was controlled, how long it ran, whether other variables were accounted for, and who funded it.

HDL transports cholesterol to the liver for disposal and is protective; LDL transports cholesterol to the tissues and is associated with plaque formation.

Exam traps

• Saying cholesterol itself causes damage, rather than depositing at an already-damaged site.
• Describing valves as actively opening rather than responding to pressure differences.
• Treating a correlation as proof of cause.
• Confusing HDL and LDL directions.
• Calling lipids polymers.
• Giving cellulose’s strength without mentioning inverted β-glucose and microfibrils.

Self-test

1. Why do large organisms need a mass transport system?
2. Give three properties of water and the single feature that explains them all.
3. Describe atherosclerosis in sequence.
4. What opens and closes heart valves?
5. Distinguish HDL from LDL.

Answers: 1. Their surface area to volume ratio is too small and diffusion distances too great for diffusion alone to meet their metabolic demands quickly enough. 2. Solvent properties, high specific heat capacity, high latent heat of vaporisation, cohesion, lower density as ice — all arising from hydrogen bonding between polar water molecules. 3. Endothelial damage; inflammatory response with white blood cells; cholesterol deposition forming an atheroma; hardening with calcium salts, narrowing the lumen; raised blood pressure causing further damage in positive feedback; possible rupture and thrombus formation. 4. Pressure differences alone — a valve opens when the pressure behind exceeds that in front, and closes when the reverse holds. 5. HDL carries cholesterol to the liver for disposal and is protective; LDL carries it to the tissues and is associated with atheroma formation.