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

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What happens to guard cells when they are flaccid?

Stoma opens
Stoma closes
They become rigid
They divide rapidly

The correct answer is: Stoma closes

When guard cells are flaccid, they lose water, which causes them to become less turgid and causes the stoma to close. This is a crucial aspect of plant physiology, as the closure of the stomata helps to reduce water loss during periods when the plant may be under water stress or when conditions are not conducive for photosynthesis. When the guard cells are flaccid, the surrounding low water potential causes them to lose their shape, making it impossible for them to pull apart properly, which would be necessary for the stoma to remain open. This mechanism plays a vital role in the regulation of gas exchange and evaporation of water in plants, ensuring that they maintain homeostasis and conserve water.