GCSE Biology Enzymes & digestion revision

The exam loves asking you to *name* the levels in order and then apply them: the stomach is an organ because it contains muscle tissue (to churn), glandular tissue (to release acid and enzymes) and epithelial tissue (to line and protect). One sentence per level, in order, gets full marks.

Enzymes are proteins that act as biological catalysts: they speed up reactions without being used up. Every enzyme has an active site — a groove with a very specific shape. Only a substrate whose shape fits can bind, which is why enzymes are specific: amylase will not touch protein. The traditional explanation is the lock-and-key model: the substrate is the key, the active site is the lock, and the products are released once the reaction is done. After the reaction the enzyme is unchanged and ready for the next substrate — a single amylase molecule can digest thousands of starch molecules.

pH works the same way, only sideways: each enzyme has an optimum pH (stomach protease works best around pH 2 in acid; amylase around neutral). On the wrong side of the optimum, bonds in the protein are disturbed, the active site changes shape, and the enzyme denatures. In graph answers, remember the enzyme is not killed — “denatured” is the word that earns the mark, and the shape change of the active site is the reason.

Digestive enzymes convert food into small soluble molecules that can be absorbed into the bloodstream. Big insoluble polymers cannot cross the gut wall; their small soluble monomers can. The conversion is a breakdown (catabolic) job split across three enzyme families:

Word equations keep it tidy: starch →(amylase)→ simple sugars; protein →(protease)→ amino acids; lipids →(lipase)→ glycerol + fatty acids. The products are not waste — they are building material. Cells use the absorbed sugars to build new carbohydrates, glycerol and fatty acids to build new lipids, and amino acids to build new proteins; some glucose is used straight away in respiration to power everything else.

1. 1Set up the amylase practical

   Place separate tubes of amylase solution, starch solution and buffer solutions of different pH in a water bath at a fixed temperature (for example 30 °C) so temperature is controlled.

2. 2Mix and start the clock

   Mix the amylase with the starch for each pH and start timing immediately.

3. 3Sample every 30 seconds

   Every 30 seconds take a drop of the mixture and spot it into a dimple tile with iodine; record whether the iodine still turns blue-black.

4. 4Find the end point

   When iodine stays orange-brown, the starch is fully digested; record the time taken for each pH.

5. 5Calculate and compare

   Rate = 1 ÷ time for each pH, then plot or compare rates; the pH with the shortest time has the highest rate.

Variables (amylase practical): the independent variable is pH; the dependent variable is the time taken (or rate = 1/time) for starch to disappear; controls are temperature (water bath), amylase and starch concentrations, and volumes. Risks: iodine stains and irritates; hot water baths scald; handle glass tubes carefully. Typical results: the fastest digestion near neutral pH, with much longer times at pH 2 and pH 9 — a hump-shaped graph of rate against pH.

Digestion only matters because of absorption: small soluble products cross the gut wall into the bloodstream through the villi (large surface area, thin walls, rich capillary network — chapter 2’s exchange-surface rules in action). Once aboard, the body does two things with them: builds new carbohydrates, lipids and proteins for growth and repair, and burns some glucose in respiration to release energy for everything else. The same enzymes that dismantle food in the gut have counterparts inside cells, where they assemble and disassemble molecules — this metabolic to-and-fro is what later chapters call metabolism.