OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

When a substrate does not fit with an enzyme, the correct outcome is that a chemical reaction will not occur. Enzymes are highly specific biological catalysts that work on specific substrates, fitting together much like a key fits into a lock. This specific interaction is crucial for facilitating the biochemical reactions they catalyze. If a substrate is not compatible, the enzyme cannot bind effectively, and therefore cannot catalyze the reaction. This specificity ensures that enzymes efficiently participate in metabolic processes without unwanted reactions. Other scenarios, such as the modification of the substrate to fit or the activation of an enzyme, do not accurately represent what happens in this context because such alterations or activations would suggest that the enzyme's specificity is not being maintained, which contradicts the fundamental principles of enzyme-substrate interactions.