Biological molecules and enzymes
The chemicals that make up living things, how to test for them, and how enzymes work and are affected by temperature and pH.
📌 Key points
- Carbohydrates, fats and proteins all contain carbon, hydrogen and oxygen; proteins also contain nitrogen (and sometimes sulfur).
- Starch, glycogen and cellulose are made from glucose; proteins are made from amino acids; fats and oils are made from fatty acids and glycerol.
- Food tests: starch – iodine solution turns from orange-brown to blue-black; reducing sugars – Benedict's solution, heated, turns from blue to green, yellow, orange or brick-red; protein – biuret solution turns from blue to purple.
- Fats and oils – the ethanol emulsion test gives a cloudy white emulsion; vitamin C – DCPIP turns from blue to colourless.
- DNA is made of two strands coiled into a double helix; the bases pair up: A with T and C with G.
- Water is important as a solvent, e.g. in digestion, excretion and transport.
- Enzymes are proteins that function as biological catalysts: they speed up reactions and are not changed by them.
- The active site of an enzyme has a shape complementary to its substrate. The substrate fits to form an enzyme–substrate complex and products are released (lock and key).
- As temperature rises to the optimum, enzyme activity increases because molecules have more kinetic energy and collide more often. Above the optimum the enzyme is denatured: the active site changes shape and the substrate no longer fits.
- Each enzyme has an optimum pH; extremes of pH also denature it. Pepsin in the stomach works best at about pH 2; amylase works best at about pH 7.
📖 Key terms
- Enzyme
- A protein that functions as a biological catalyst.
- Catalyst
- A substance that increases the rate of a chemical reaction and is not changed by the reaction.
- Active site
- The part of an enzyme where the substrate binds.
- Substrate
- The molecule that an enzyme acts on.
- Denatured
- Permanently changed in shape, so the active site no longer fits the substrate.
- Optimum
- The temperature or pH at which an enzyme works fastest.
✏️ Worked example
Amylase and starch were mixed at different temperatures and tested with iodine every 30 s. Time for starch to disappear: 20 °C – 8 min, 40 °C – 2 min, 60 °C – still present after 20 min. Explain the results.
- When starch has gone, iodine stays orange-brown instead of turning blue-black.
- From 20 °C to 40 °C the time falls, so the rate increases: enzyme and substrate molecules have more kinetic energy and collide more often.
- At 60 °C starch is still present, so amylase has stopped working.
- The high temperature has denatured the enzyme: the active site changed shape and starch no longer fits.
- Answer: the rate is fastest near 40 °C (close to the optimum) and the enzyme is denatured at 60 °C.
Never say an enzyme is "killed" – enzymes are not alive. Say it is denatured and explain that the active site has changed shape.
Test yourself8 questions from a bank of 21. A new mix every time.
Start the quiz →