Substrate

In the syllabus and the exam

Substrate is defined in direct relation to enzyme and active site, and the syllabus expects candidates to correctly name the substrate of common enzymes discussed at SPM level, such as starch for amylase, protein for pepsin or trypsin, and hydrogen peroxide for catalase, since exam questions frequently ask candidates to identify the substrate, the enzyme and the product together in one structured answer. A key idea tested repeatedly is specificity: because an enzyme's active site has a particular shape, it can normally only bind and act on one substrate, or a small group of very similarly shaped substrates, which is why, for instance, amylase cannot break down protein even though both are biomolecules. A common error is confusing the substrate with the product, especially in reactions with multiple steps, or naming the general food group (such as 'carbohydrate') instead of the specific molecule involved (such as 'starch') when the question expects precise terminology. Candidates are also frequently asked to explain graphs of reaction rate against substrate concentration, where the rate increases with more substrate up to a point and then levels off because all the enzyme's active sites are already occupied at any one time, a saturation point that examiners specifically test candidates' ability to identify and explain. When a question describes an unfamiliar enzyme, candidates should look for the substrate named or implied in the scenario rather than guessing from memorised examples, since SPM questions increasingly test application to novel enzyme-substrate pairs rather than only the handful covered directly in class.

Example

In the digestion of starch by amylase, starch is the substrate, and it is converted into maltose as the enzyme's product.

Related terms

EnzymeActive site

Source:SRC-DSKP-EN