Uses of Water

Plants need water for a variety of reasons:

• For photosynthesis
• To transport dissolved chemicals around the plant
• To cool the plant by evaporating from leaves
• To keep the cells firm and support the plant

When water moves into plant cells by osmosis, it increases the pressure inside the cell as the vacuole pushes against the cell wall. 

It is this pressure which keeps the cells rigid (maintains their turgor) providing support.

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Plant Transport Systems

Water and nutrients are transported in two separate systems inside plant stems.

Water and dissolved mineral salts are transported inside xylem vessels.

Water is absorbed from the soil through root hair cells by osmosis.

Water moves upwards due to evaporation of water from the leaves at the top of the plant.  This is called transpiration.

This flow of water is called the transpiration stream.

Nutrients, such as sugar made in the leaves, are transported both up and down the plant in phloem tissue.

Nutrients are used by cells for growth and excess nutrients are stored in storage organs, such as the roots.

Water travels upwards in the xylem.

Nutrients travel up and down in the phloem.

You can remember which is which if you remember that food is transported in the phloem.

Transpiration

Transpiration is the loss of water vapour from the surface of the leaves.

The rate of transpiration is faster in hot, dry and windy conditions.

A potometer can be used to investigate the rate of water uptake by a shoot.

A shoot is cut from a plant and inserted into the apparatus under water, to prevent any air entering the system.

All the joints are sealed with vaseline, so are airtight.

The distance the water moves along the capillary tube in a given time is measured.

The further it moves, the faster the shoot is taking in water, so the faster its rate of transpiration.

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They contain many chloroplasts.

Water enters the leaf from xylem vessels in veins.

The water diffuses into the air spaces to the palisade cells.

Some water vapour evaporates from the leaf through tiny holes called stomata. 

These are found mainly on the lower surface of the leaf, where it is cooler.

Carbon dioxide enters the leaf through the stomata from the air.

The size of the stomata is controlled by the guard cells.

If the plant has plenty of water, the guard cells are firm (turgid) and open the stoma to let carbon dioxide in.

If the plant is short of water, the guard cells become soft (flaccid) and close the stoma to reduce further water loss.  This helps to prevent wilting.

Water loss is also reduced by the waxy cuticle, which is waterproof.

Plants that live in dry conditions have thicker cuticles, e.g. cacti.

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