Form 4 · Common mistakes

Metabolism and Enzymes, common mistakes

The mistakes SPM students make on Metabolism and Enzymes, why each one loses marks, and the correct version.

One-hour paid trial · Same-day reply

The mistakes, why they lose marks, and the fix

Common mistake	Why it loses marks	What earns the mark
Saying an enzyme is 'killed' at high temperature.	An enzyme is not alive, so it cannot be killed; heat denatures it by changing the shape of its active site.	State that the enzyme is denatured at high temperature, so its active site no longer fits the substrate.
Writing that a low temperature denatures an enzyme.	Low temperature only slows enzyme activity; the activity returns when the enzyme is warmed again.	Only high temperature and extreme pH denature an enzyme; a low temperature merely slows it down.
Treating the enzyme as used up during the reaction.	An enzyme is a catalyst, so it is unchanged at the end and can be reused.	State that one enzyme molecule can catalyse the same reaction repeatedly because it is not used up.
Saying an enzyme can act on any substrate.	Only a substrate complementary to the active site can bind, so each enzyme is specific.	State that the enzyme is specific and its active site fits only one substrate or type of substrate.
Claiming the rate keeps rising as substrate concentration increases without limit.	Once every active site is occupied, the enzyme becomes the limiting factor and the rate cannot rise further.	State that the rate levels off into a plateau once all active sites are occupied.
Assuming adding more enzyme always speeds up the reaction.	If substrate is the limiting factor, extra active sites have nothing to bind.	Adding more enzyme raises the rate only while substrate is in excess.
Writing that lipase breaks down protein.	Lipase acts on lipids; protease is the enzyme that breaks down protein.	State that lipase breaks lipids into fatty acids and glycerol, while protease breaks down protein.
Assuming every enzyme in the body has the same optimum pH.	Different enzymes are suited to different conditions, such as pepsin in acid and trypsin in mild alkali.	State that each enzyme has its own optimum pH matching where it works.
Describing an enzyme merely as something that 'helps' a reaction.	This misses the mechanism: an enzyme lowers the activation energy and is a catalyst.	Define an enzyme as a biological catalyst that lowers activation energy without being used up.
Reading the rate off a graph as the final value the reaction reaches.	The reaction slows as substrate is used up, so the final value is not the rate.	Measure the initial rate from the steepest part of the graph near the start.
Using 'metabolism' and 'digestion' as if they mean the same thing.	Digestion is only one catabolic process; metabolism covers every reaction in the cell.	State that metabolism is the broad term for all reactions, while digestion is one catabolic example.

How to avoid these mistakes

Learn the lock-and-key model and draw the enzyme-substrate complex.
Sketch the temperature and pH graphs and explain each part.
Practise a Paper-3 enzyme experiment with variables and expected results.
Sketch the rate-against-substrate-concentration graph and explain why it plateaus.
List five common '-ase' enzymes with their substrate and where each is used in daily life.
Compare the effect of temperature, pH, substrate concentration and enzyme concentration in one summary table.
Compare the optimum pH of pepsin and trypsin and explain why each suits its location in the digestive system.
Research one industrial use of immobilised enzymes and note why immobilisation is an advantage.
Write one-sentence definitions of metabolism, anabolism and catabolism, then check each against the syllabus wording.

Frequently asked questions

Why does enzyme activity fall at high temperature?

As temperature rises, activity increases up to an optimum. Beyond that, the heat changes the three-dimensional shape of the enzyme's active site, so the substrate no longer fits. The enzyme is denatured and the reaction rate falls sharply, usually permanently.

What is the lock-and-key model?

It describes how an enzyme is specific. The substrate (key) has a shape complementary to the enzyme's active site (lock), so only the correct substrate can bind to form an enzyme-substrate complex, which is why each enzyme catalyses one type of reaction.

Are enzymes used up when they work?

No. Enzymes are catalysts, so they are not used up or permanently changed by the reaction they speed up. A single enzyme molecule can catalyse the same reaction repeatedly, which is why cells need only small amounts.