Homeostasis in plants

Biology A2 · Homeostasis. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A plant must take in carbon dioxide for photosynthesis, but the same open stomata let water vapour out. By opening and closing its stomata, the plant balances carbon dioxide uptake by diffusion against water loss by transpiration.

Each stoma lies between two guard cells. When the guard cells take in water they become turgid and bend apart, so the stoma opens. When they lose water they become flaccid and the stoma closes.

Stomata also follow a daily rhythm: they usually open in the day and close at night. During water stress the hormone abscisic acid (ABA) makes them close.

Rules to remember

Stomata tend to open in light and when carbon dioxide in the leaf is low; they tend to close in darkness, high carbon dioxide, low humidity, high temperature and water stress
Opening: proton pumps use ATP to move H⁺ out of guard cells → K⁺ channels open and K⁺ enters → water potential falls → water enters by osmosis → cells turgid → stoma opens
Guard cell walls are thicker on the side next to the pore, and cellulose microfibrils are arranged in hoops, so a turgid cell gets longer and curves
Guard cells have chloroplasts and many mitochondria; other cells of the lower epidermis have no chloroplasts
ABA binds to receptors on guard cell membranes → Ca²⁺ enters the cytoplasm as a second messenger → K⁺ leaves → water potential rises → water leaves by osmosis → cells flaccid → stoma closes
The daily rhythm continues for some days in constant light or constant dark, so it is controlled from inside the plant

Common mistake

Students mix up the direction of water potential. When K⁺ enters the guard cells their water potential falls (more negative) and water enters; when K⁺ leaves, their water potential rises and water leaves.

Worked example

A fungal toxin keeps the proton pumps of guard cells working all the time, even in the dark. What happens to a plant treated with it in dry soil?

H⁺ is pumped out of the guard cells all the time, so K⁺ keeps entering.
The water potential of the guard cells stays low, water enters by osmosis and the cells stay turgid.
The stomata stay open day and night, so transpiration continues when water cannot be replaced from the soil.

Answer: The stomata cannot close, the plant loses too much water and wilts.

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