Explanation:
The correct answer is: A. substrate

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What is a substrate in enzyme reactions?
A substrate is the molecule that an enzyme acts upon. It is transformed during the reaction to form a product.

Enzyme Reaction: Starch → Sugar

- In the reaction where starch is broken down into sugar by the enzyme amylase:
- Starch is the substrate.
- Amylase is the enzyme that facilitates the breakdown.
- The sugar (often maltose or glucose) is the product.

Let’s look at the options:

#A. Substrate
- Correct.
- Starch is the substrate in this reaction because it is the substance the enzyme amylase acts upon to produce sugar.

#B. Product
- The product is the sugar that results from the breakdown of starch, not the starch itself.

#C. Enzyme
- Amylase is the enzyme, which facilitates the conversion of starch into sugar but is not the substrate.

#D. Enzyme-substrate complex
- The enzyme-substrate complex is a temporary structure formed when the enzyme (amylase) binds to the substrate (starch) before the reaction occurs, but the starch itself is the substrate, not the complex.

#E. Reaction mixture
- The reaction mixture refers to the collection of all components in the reaction (enzyme, substrate, and product), but starch is specifically the substrate.

Summary:
In the reaction where amylase breaks down starch into sugar, starch is the substrate that the enzyme acts on.