IGCSE Biology Biomolecules & enzymes notes

The pattern is the same for all three groups: large molecules are made from smaller molecules. Glucose units join into long chains of starch (the energy store in plants), glycogen (the energy store in animals, stored in the liver and muscles) or cellulose (the structural fibre of plant cell walls). Amino acids join end to end into proteins — folded into enzymes, antibodies and many other working molecules. One glycerol molecule joined to fatty acid chains makes a fat or oil. Digestion (Topic 6) simply runs this in reverse: large insoluble molecules are broken back down into the small soluble ones that can be absorbed.

Details that earn marks: Benedict’s solution must be heated (usually in a hot water bath) — at room temperature it stays blue even with sugar present. The biuret reagent is added to the food solution and the colour changes slowly from blue through purple. In the ethanol emulsion test, the food is shaken with ethanol to dissolve any fat, then poured into water — fat droplets scatter light and make the cloudy white layer. For vitamin C, add the food solution to DCPIP drop by drop; the volume needed tells you how much vitamin C is present — less juice needed means more vitamin C.

Supplement: a DNA molecule has two strands coiled around each other to form a double helix — like a ladder twisted into a spiral. Each strand is a backbone carrying chemicals called bases. Across the middle of the “ladder”, bonds between paired bases hold the two strands together, and the pairing follows one strict rule: A pairs only with T, and C pairs only with G.

A labelled diagram to picture: a ladder shape twisted at both ends, with the two backbones labelled “strand 1” and “strand 2”, and the rungs labelled in pairs — A-T, C-G, T-A — with small dots between the letters to show the bonds. You do not need the chemical names of the bases for 0610, just the letters and the pairing rule.

A catalyst increases the rate of a chemical reaction without being changed itself. Enzymes are proteins that catalyse all the metabolic reactions in living organisms. Without them, the reactions of life would run thousands of times too slowly to sustain life — cells need reactions to happen at a rate that keeps them alive, and only enzymes deliver that rate at body temperature.

How? Every enzyme has an active site — a shaped hollow whose shape is complementary to its substrate (the molecule it acts on). The substrate slots into the active site, an enzyme-substrate complex forms, the reaction happens, and products are released, leaving the enzyme unchanged and ready to work again (Supplement wording). Because the fit is so exact, each enzyme works on only one kind of reaction: this is specificity (Supplement) — amylase fits starch, not protein.

1. 1Collision

   The substrate collides with the enzyme’s active site.

2. 2Fit

   The substrate binds because its shape is complementary — lock and key.

3. 3Complex

   The enzyme-substrate complex forms and the reaction proceeds.

4. 4Release

   Products are released; the enzyme is unchanged and reusable.

A labelled diagram to picture: a pac-man-shaped enzyme with a wedge cut out (the active site), a matching wedge-shaped substrate slotting in, and then two smaller shapes (products) floating away while the enzyme stays exactly as it started. Draw the enzyme unchanged at the end — that is what makes it a catalyst.

Enzyme activity rises with temperature at first — warmer molecules have more kinetic energy, so enzyme and substrate collide more often, and more collisions are effective. But above a certain temperature the enzyme’s protein structure shakes apart: the active site changes shape, the substrate no longer fits, and the enzyme is denatured — permanently. The temperature of fastest activity is the optimum temperature. Human enzymes typically have optima around 37 °C; plant and microbial enzymes vary with their habitat.

pH matters the same way. Each enzyme has an optimum pH: above or below it, the charges holding the protein’s shape together are disturbed, the active site deforms and the enzyme denatures. Pepsin in the stomach works best around pH 2; amylase works best around neutral; trypsin in the small intestine prefers mildly alkaline conditions (Topic 6). The wrong pH changes shape and fit — exactly like overheating, but the trigger is ionic, not kinetic.

A graph worth being able to sketch: activity on the y-axis, temperature on the x-axis — a rising curve peaking at the optimum, then a steep fall where denaturation bites. The equivalent pH graph is a bell shape centred on the optimum pH. Label both axes with units and mark the optimum — the labels carry marks.