OCR gateway science B4b- Higher

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  • Created by: Nicole
  • Created on: 22-04-11 14:56

Predicting Osmosis

You can predict which way water molecules will move if you know the concentrations of the solutions on either side of a partially permeable membrane.

Water is important to plants because it helps to support them.

- The contents of the turgid cells push against the inelastic cell walls.

- This turgor pressure helps to support the cell.

- When there is little or no turgor pressure, the plant wilts.

When plant cells are full of water the cells are turgid. When the plant wilts and the cells are short of water they are flaccid.

If a plant loses too much water, the cell membrane may pull away from the cell wall. We call this plasmolysis. A plasmolysed cell cannot return to being turgid and usually dies

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Water Movement and transpiration

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The stomata have 2 guard cells on either side. these can change their shape and open or close the pore (stomata) between them.

They work like this:

- When there is a lot of light, and plenty of water, the guard cells take in water and become turgid. The inner walls of the guard cells bend and the stomata opens.

- In low light, or with water in short supply, the opposite happens and the stomata closes. 

Plants that live in the desert have very few stomata on both leaf surfaces, to minimise water loss in the dry desert conditions. The size of the stomata is usually small when compared to plants that live where water is readily available.

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