0.5 g of potato tissue releases 24 cm3 of oxygen from hydrogen peroxide in 4 minutes. Calculate the mean rate per gram of tissue.
- Rate = volume ÷ time = 24 ÷ 4 = 6 cm3 min−1.
- Per gram: 6 ÷ 0.5 = 12.
Answer: 12 cm3 min−1 g−1.
Enzymes and metabolism
Biology · C Interaction and interdependence. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The idea
Metabolism is all the chemical reactions in a cell. Almost every one needs an enzyme, a globular protein that acts as a catalyst. Enzymes speed reactions up by lowering the activation energy and are not used up.
The substrate binds to the active site, a small region whose shape and chemical properties match it. In the induced-fit model, both the active site and the substrate change shape a little as they bind. Molecules move at random, so binding needs a collision between substrate and active site.
Heat or extreme pH can change the shape of the active site so the substrate no longer fits. This is denaturation and it is usually permanent.
Rules to remember
- Anabolic reactions build larger molecules from smaller ones and need energy (protein synthesis, photosynthesis). Catabolic reactions break molecules down and release energy (digestion, respiration).
- Specificity: each enzyme acts on only one substrate or a small group, because only they fit the active site.
- Temperature: rate rises as molecules move faster and collide more, reaches an optimum, then falls quickly as the enzyme denatures.
- pH: each enzyme has an optimum pH; the rate falls on both sides of it.
- Substrate concentration: rate rises, then levels off when all active sites are occupied.
- rate of reaction = amount of product formed (or substrate used) ÷ time taken.
- Enzymes lower the activation energy only. They do not change the energy of the reactants or products.
Common mistake
Students write that low temperature denatures enzymes. Cold only slows the molecules down, and the enzyme works again when warmed. Only high temperature or extreme pH denatures it.
Worked example
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