As a well-watered plant is exposed to daylight in the morning, its stomata open, allowing gas exchange for photosynthesis to increase. (a) Describe the arrangement and structure of a pair of guard cells around a stoma, and explain how an increase in guard cell turgor causes the…

Answer: ((a)) A pair of guard cells surrounds each stomatal pore, one on each side, in the leaf epidermis. Their walls are unevenly thickened, with a thicker wall on the side next to the pore and a thinner, more elastic wall on the outer side away from the pore. When the guard cells take up w… ((b)) In the light, guard cells actively pump hydrogen ions (H+) out across their cell-surface membrane, using ATP. This active transport lowers the hydrogen ion concentration inside the cell and creates an electrochemical gradient that drives potassium ions (K+) into the guard cell t… ((c)) Opening stomata during daylight hours allows carbon dioxide to diffuse into the leaf for use in the light-independent reactions of photosynthesis, which can only occur while light is available to drive the light-dependent reactions. Closing the stomata at night, when photosynthe…. Part (a): Guard cell structure and how turgor opens the pore A stoma is flanked by a pair of guard cells, one on either side of the pore. Guard cell walls are unevenly thickened: the wall next to the pore is thicker, while the outer wall (…

A2 Biology 9700: How Guard Cells Open Stomata in the Light

Part (a): Guard cell structure and how turgor opens the pore

A stoma is flanked by a pair of guard cells, one on either side of the pore. Guard cell walls are unevenly thickened: the wall next to the pore is thicker, while the outer wall (away from the pore) is thinner and more elastic.

When a guard cell takes up water and becomes turgid, the thinner outer wall stretches more readily than the thicker inner wall. Because the two guard cells are joined at their tips, this uneven stretching makes each cell bow outward into a curved shape, which pulls the two cells apart along the middle and opens the stomatal pore between them.

Part (b): How light triggers stomatal opening

Light stimulates guard cells to actively pump hydrogen ions (H⁺) out of the cell across the cell-surface membrane, using ATP (from photosynthesis and/or respiration in the guard cell). Removing positive charge from inside the cell creates an electrochemical gradient that drives potassium ions (K⁺) into the guard cell through specific channels; anions such as chloride ions (or organic acids such as malate produced within the cell) also accumulate inside to balance this incoming positive charge.

The build-up of these solutes inside the guard cell lowers its water potential (makes it more negative) relative to the surrounding epidermal cells. Water then moves into the guard cell by osmosis, down this water-potential gradient, increasing its turgor and opening the pore, as described in part (a).

Part (c): Advantage of opening stomata by day and closing them at night

Photosynthesis can only proceed while there is light available to drive the light-dependent reactions, which supply the ATP and reduced NADP needed by the light-independent reactions that fix carbon dioxide. Opening the stomata during daylight hours therefore allows carbon dioxide to diffuse into the leaf when it can actually be used.

At night, no light is available, so photosynthesis (and therefore the plant’s need for carbon dioxide) effectively stops. Keeping the stomata closed at night avoids losing water vapour by transpiration through open pores for no photosynthetic benefit, conserving water without reducing the rate of photosynthesis during the day.

Final answers

(a) Uneven wall thickening (thicker near the pore) means turgid guard cells bow outward, pulling apart at the middle and opening the pore.
(b) Light-driven H⁺ pumping out of the guard cell drives K⁺ (and accompanying anions) in, lowering water potential inside the cell, so water enters by osmosis and turgor rises, opening the stoma.
(c) Opening by day lets in CO2 when photosynthesis can use it; closing at night, when photosynthesis cannot occur, prevents wasteful water loss by transpiration.