Biology, 4b.

Water.

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Osmosis.

Water moves in and out of plant cells through the cell wall and membrane.

Osmosis is the net movement of water molecules from an area of high water concentration, to an area of low water concentration, across a partially, permeable mambrane, and is a consequence of the random movement of individual particles. It is a type of diffusion of water.

A partially, permeable membrane will allow certain molecule, (small), to pass through by diffusion. It acts as a barrier to others.

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Turgor Pressure.

The plant cell wall provides support. Both the inelastic cell wall and water are essential for support of plants. Plants are supported by the turgor pressue in cells.

Turgor pressure helps support the plant tissue. Water pressure acts against the cell wall.

Too much water being drawn in by osmosis can make the cells turgid. A turgid cell has a vacuole full of water. It is firm but doesn't burst due to cell wall.

However, if the plant cell is lacking in turgor pressure, it is said to be flaccid. It is limp, floppy and has lost water to osmosis. In extreme cases the plant cell can become plasmolysed, this is when the membrane pulls off the cell wall. A healthy plant must balance water loss with water uptake. 

Animal cells don't have an elastic cell, which is why plants are more tolerant in periods of drought. If animals take in too much water, the cell membrane will break down, and it will burst - known as lysis. Too little water and it will shrivell up - known as crenation. This is why water intake in animals has to be kept constant.

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Water Flow Through Plants.

Root hairs take in water by osmosis. It is then transported through the plant to the leaves. Then its evaporated out from the leaves, (transpiration). The roots have root hairs, which increase the surface area, so that more water can be taken into the roots by osmosis. 

Transpiration provides plants with water to; keep the plant cool. Provide it with a constant supply of water for photosynthesis. Create turgor pressure to support the plant. And helps with the movement of molecules. This along with water loss is a way in which leaves, are adapted for photosynthesis. 

The loss of water in leaves is by the diffusion of water molecules. The structure of a leaf is adapted to reduce excessive water loss. 1. The waxy cuticle layer, makes the upper surface of the leaf, waterproof. 2. most of the stomata are found on the lower surface, to slow the rate of diffusion. 

The cellular structure of a leaf is adapted to reduce water loss. The stomata opens and closes, as the guard cells go flaccid, and turgid. They are sensitive to light, so they close at night, to conserve water. Plants in hot climates have fewer, and smaller stomata, and non on the upper surface. 

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