The opening of the guard cells of the epidermis in plants is by the process of

Step 1: Analyze the question The question asks which process causes the guard cells (which surround the stomata) to open.

Step 2: Explain the concept Guard cells control the opening and closing of stomata. The sequence goes like this:

• In light, guard cells photosynthesize and/or actively accumulate solutes (such as potassium ions and sugars). Their cell sap becomes more concentrated.
• Water then moves in from neighbouring epidermal cells, from a region of higher water potential to one of lower water potential, through a partially permeable membrane. This is osmosis.
• The guard cells become turgid. Their inner walls are thicker and less elastic than the outer walls, so they bend outward and the stomatal pore opens.
• In darkness or during water stress, water leaves the guard cells by osmosis. They become flaccid, and the stomata close.

Step 3: Evaluate the options

• A. Osmosis: Correct. The movement of water into the guard cells makes them turgid, and this directly opens the stoma.
• B. Diffusion: This is the movement of particles (such as gases) down a concentration gradient. It is not the process that swells the guard cells.
• C. Active transport: Potassium ions may be pumped into the guard cells using energy. This is a contributing step, but the opening itself results from water entering by osmosis. At WASSCE level, the expected answer is osmosis.
• D. Transpiration: This is the loss of water vapour through the stomata. It is a result of the stomata being open, not the cause.

Step 4: Answer The correct option is A. osmosis.

Exam tip: Remember "Turgid = Open, Flaccid = Closed". Whenever a question is about guard cells swelling or shrinking, think of water movement, which means osmosis.