Past paper questions

Roles of potassium

guard cell that controls the size of the stoma through which water vapour diffuses out. It does this by taking in K+ ions to lower the water potential inside, so water enters by osmosis and the cells become turgid. When K+ move out, the guard cell becomes flaccid and the stoma

H+ are pumped out of the guard cells using ATP. This makes the guard cell more negative so
potassium ions enter the cell. This lowers the water potential of the cell so water moves into
the guard cell by osmosis. As the guard cell becomes turgid the stoma opens, as a result of
the uneven thickening of the guard cell wall

adoption of xylem

No cell contents so nothing stopping water flow;

No end walls;

Strong walls made of lignin;

Pits in walls allow water to flow out sideways

plant cells

Stoma and gurad cell

Guard cells control the size of the stoma. When the guard cell is turgid, the stoma is open and
when it is flaccid the stoma is closed. The stoma opens for gas exchange, allowing carbon dioxide
into the leaf for photosynthesis and oxygen out. Water vapour is also lost from the stomata