In testing for starch in a leaf, the leaf is first boiled in water for about a minute so that the

Analyze the question: The first step in the starch test is dipping the leaf in boiling water for about a minute. We need to know why this is done.

Concept: The starch test has these steps:

1. Boil the leaf in water (about 1 minute): this kills the cells, stops all enzyme and metabolic activity, and breaks down the cell membranes. It also softens the leaf and makes it permeable, so iodine can get in later.
2. Boil in ethanol (water bath): this dissolves the chlorophyll out, decolorising the leaf.
3. Rinse in warm water: this softens the leaf again.
4. Add iodine solution: a blue-black color shows that starch is present.

Evaluating the options:

• A. Cell walls are hardened. Wrong. Boiling softens the tissues, it does not harden them.
• B. Cells are killed. Correct. This is the main purpose of boiling in water. Killing the cells stops further chemical reactions, such as starch being converted or used, so the leaf shows the starch content at the time it was picked. It also makes the cells permeable.
• C. Chlorophyll is dissolved out. Wrong. Chlorophyll does not dissolve in water. It dissolves in hot ethanol, which is the next step.
• D. Iodine will penetrate. Partly true, since boiling does make the cells permeable. But this is a result of the cells being killed, and WASSCE marking treats "cells are killed" as the direct purpose.

Answer: B. The cells are killed.

Exam tip: Remember the sequence "Water kills, alcohol bleaches, iodine tests." If a question asks about boiling in water, think "kills the cells." If it asks about boiling in ethanol, think "removes chlorophyll."