AQA GCSE Biology Paper 1 2025 – 400 Free Practice Questions to Pass the Exam

The correct answer highlights the unique structure of guard cells that facilitates their primary function of regulating stomata. Guard cells are specialized cells located on the leaf surface, and their structural adaptation includes having thin outer walls combined with thicker inner walls. This configuration allows the guard cells to change shape effectively.

When the cells become turgid (swollen with water), the thicker inner walls of the guard cells cause them to bow outwards, which results in the stomata opening. Conversely, when the cells lose water and become flaccid, they straighten, causing the stomata to close. This mechanism is essential for maintaining water balance within the plant by controlling gas exchange, as it enables the plant to regulate the intake of carbon dioxide and the release of oxygen, while also minimizing water loss.

The other options do not accurately reflect the functional morphology of guard cells and their roles in stomatal regulation.