AQA GCSE Biology · Topic 5
Enzyme action, the effect of temperature and pH, digestive enzymes, bile and food tests.
187 questions 5 subtopics Spec 4.2
Every subtopic below has its own questions, mark schemes and notes once you start.
An enzyme is a protein that speeds up one particular reaction inside a living organism, without being changed or used up itself. Everything about how an enzyme behaves comes back to the shape of its active site.
Enzymes only work well in a narrow range of conditions. Warming speeds them up until the point where heat starts to wreck the active site, and the wrong pH does the same damage. Required practical 5 measures this using amylase, starch and iodine.
Food arrives as molecules far too big to get into the blood. Three groups of digestive enzyme break them into small soluble molecules, which are absorbed and then used to build the body's own molecules or released as energy in respiration.
The digestive system is an organ system: several organs each doing a different job so that food is broken down and absorbed. Learn which organ makes which enzyme, and keep bile separate in your mind from the enzymes, it is not one.
Required practical 4: four reagents, four colour changes. The awkward details are the ones examiners ask about, which test needs heating, which sample is not filtered, and what a negative result looks like.
Key definitions, in exam-ready wording.
The mistakes that come up again and again in this topic.
An enzyme is not alive and cannot be 'killed'. Say denatured.
The enzyme is not used up. The substrate is. Writing that enzymes are used up loses the mark for the definition of a catalyst.
The active site is not the same shape as the substrate, it is the shape the substrate fits into. Say 'complementary' or 'the substrate fits into it'.
Give both halves of the catalyst definition: speeds the reaction up, and is not changed or used up.
Denaturing changes the enzyme, not the substrate. Say the active site changes shape.
Below the optimum, nothing is being damaged, the rate rises simply because molecules are moving faster.
For rate = 1000 ÷ time, divide 1000 by the time. Doing it the other way round is the commonest slip in this practical.
Convert minutes to seconds before dividing if the answer is wanted per second.
On a graph of time taken against pH, the optimum is the lowest point, not the highest.
Amino acids come from protein; fatty acids come from fat. These two are very easily swapped under pressure.
Lipids give two products, glycerol and fatty acids. Naming only one costs a mark.
Amylase digests starch specifically, not every carbohydrate.
Chewing makes the pieces smaller but leaves the molecules unchanged. Only enzymes carry out chemical digestion.
Bile is not an enzyme. It does not catalyse anything and it does not digest fat, lipase does.
Bile is made in the liver and stored in the gall bladder. Swapping these two is the most common error in this section.
Emulsifying increases the surface area of the fat. It does not make the fat molecules smaller.
Amylase is made in three places (salivary glands, pancreas, small intestine) and protease in three (stomach, pancreas, small intestine). Only lipase has two.
Benedict's and biuret both start off blue. Benedict's needs heating; biuret does not.
The lipid sample is the one you do not filter, lipids do not dissolve and would be caught in the filter paper.
A negative result is a colour that stays the same, not no colour at all. Say 'stayed blue' or 'stayed browny-orange'.
When asked what results show, give the conclusion and the evidence for each tube, not just a list of substances.
187 questions on Enzymes and the human digestive system, marked against the scheme. Anything you get stuck on will be scheduled to come back sooner.