IGCSE Biology / Homeostasis and excretion
Stomata and guard cells revision notes
Edexcel International GCSE Biology (4BI1), chapter 8: Homeostasis and excretion, specification point 2.41B, 2.43B, 2.44B, 2.70. Free lesson.
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Stomata are pores in a leaf that allow gases to move in and out. Guard cells control when they open, balancing the need for carbon dioxide with the need to limit water loss.
How guard cells open a stoma
A stoma is the pore between two guard cells. Guard cells contain chloroplasts, unlike the surrounding epidermal cells. In the light, they photosynthesise and make sugars, which draw water into the guard cells by osmosis. They also take in potassium ions by active transport, lowering their water potential. Water enters from neighbouring cells, making the guard cells turgid. Their thicker inner walls stretch less than their thinner outer walls, so the cells curve apart and open the pore. In the dark, potassium ions leave the guard cells. Water follows by osmosis, the cells become…
Why the guard cells curve
The difference between the two walls makes the guard cells open the pore when they swell. The thick inner wall cannot stretch as much as the thin outer wall, so each turgid cell curves away from its partner. The surrounding epidermal cells lack chloroplasts and do not control the pore in this way. The guard cells are sausage-shaped and contain chloroplasts. The stoma is the gap between the two guard cells.
Which gases move in daylight and darkness
Gas movement depends on the balance between photosynthesis and respiration. Photosynthesis is fast in bright daylight and does not happen in darkness. Respiration happens in both conditions at about the same rate. In daylight, photosynthesis uses carbon dioxide faster than respiration makes it. There is a net movement of carbon dioxide into the leaf. In daylight, photosynthesis produces oxygen, so there is a net movement of oxygen out of the leaf. In darkness, respiration continues without photosynthesis. Carbon dioxide moves out of the leaf, and oxygen moves in. At dawn and dusk, the…
How stomata balance gas exchange and water loss
Opening stomata allows carbon dioxide to enter for photosynthesis, but it also allows water to escape. Plants balance these needs by having most stomata on the underside of the leaf, where it is shaded and less water is lost. Day: carbon dioxide moves in and oxygen moves out. Darkness: oxygen moves in and carbon dioxide moves out. Stomata are mostly on the underside to reduce water loss.