Cambridge O Level Biology · Syllabus 5090 · Plant Nutrition
Destarching
What is Destarching?
Keeping a plant in darkness for a period long enough for the starch already stored in its leaves to be used up in respiration, before an investigation into the requirements for photosynthesis. Destarching means any starch detected afterwards must have been made during the investigation itself, so the result can be attributed to the treatment rather than to starch stored beforehand.
This definition is part of the Plant Nutrition chapter in Cambridge O Level Biology.
Examiner tips on Destarching
- What this test can and cannot tell you. A blue-black leaf tells you starch had accumulated by the time the leaf was taken. It does not tell you how fast photosynthesis was going, and it does not tell you when the starch was made. That is why destarching — the next section — is not optional housekeeping but the thing that makes the whole experiment mean anything.
Questions students ask about Destarching
Why must a plant be destarched before testing for photosynthesis?
So that any starch found afterwards must have been made during the investigation. Keeping the plant in darkness stops photosynthesis while respiration continues, so the stored starch is used up. Without destarching, a positive starch test could simply be starch that was already there, and the result would support no conclusion at all.