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

What occurs during stage three of the lock and key model?

1. A chemical reaction where amylase breaks down starch

2. A chemical reaction where glucose is converted into starch

3. A chemical reaction where proteins are formed from lipids

4. A physical reaction that involves only starch

The correct answer is: A chemical reaction where amylase breaks down starch

In the lock and key model, stage three represents the part of the enzyme activity where the enzyme-substrate complex has formed and the actual chemical reaction takes place. For example, in the case of amylase, which is an enzyme that specifically works on starch, the substrate (starch) fits into the active site of the enzyme (amylase) much like a key fits into a lock. During this stage, the enzyme catalyzes the breakdown of starch into simpler sugars like glucose, which is a crucial process in digestion. This reaction involves the breaking of glycosidic bonds in starch, facilitating its conversion into smaller molecules that can be easily absorbed by the body. The other options provided involve different processes that do not accurately describe what occurs during stage three of the lock and key model. For instance, the conversion of glucose into starch does not involve an enzyme breaking down an existing substrate, and the formation of proteins from lipids does not relate to the lock and key mechanism specifically associated with starch breakdown by amylase. Additionally, describing a physical reaction involving only starch does not account for the enzymatic activity necessary for the chemical transformation that occurs when amylase acts on starch.