With A-level biology it is just as important to know how to answer the question and what the examiners are looking for as it is to have the knowledge to do so. There are common questions that examiners ask each year where they change the wording and the example used but are looking for the same key points. I found enzyme specificity is a common example of this, and therefore an easy way to secure 3/4 marks. 

For this question I would first discuss the specific tertiary structure of enzymes meaning a complementary active site can only bind to this substrate only (maltose in this example). The next sentence would go on to talk about how this causes an enzyme-substrate complex.

Many pupils go on to write more as they feel two sentences are not enough for 3/4 marks and that is why familiarising yourself with what the examiners are after should be a key part of revision.