Biological Molecules and Enzymes
What you must understand
You must know the structure, building blocks, tests and roles of carbohydrates, lipids, proteins and water. Starch and glycogen are polymers of glucose used for storage; cellulose is a glucose polymer giving plant cell walls strength; proteins are polymers of amino acids joined by peptide bonds; lipids are glycerol plus fatty acids. Learn the food tests: Benedict's for reducing sugars (blue to brick-red on heating), iodine for starch (yellow-brown to blue-black), Biuret for protein (blue to purple), ethanol emulsion for lipids (milky white), and DCPIP for vitamin C (blue to colourless).
Enzymes are biological catalysts, usually proteins, that lower activation energy. Their activity depends on temperature and pH. Above an optimum, denaturation changes the active site shape so the substrate no longer fits - this is the lock-and-key or induced-fit idea. Enzymes are specific and reusable.
Traps examiners set
- Saying an enzyme is "killed" by heat - it is denatured, not alive.
- Confusing the Benedict's colour change with the iodine change.
- Forgetting that enzyme rate graphs flatten because substrate runs out, not because the enzyme stops working.
- Describing denaturation as the enzyme "breaking down" - the active site changes shape.
Worked rule of thumb
When explaining any enzyme graph, name the limiting factor at each stage: initially substrate concentration or enzyme concentration; at the peak, temperature or pH optimum; after the peak, denaturation. A good answer links shape of active site to ability to form enzyme–substrate complexes.