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Science

Photosynthesis

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Root hair cells and osmosis

Roots

Root hair cell

Plants absorb water from the soil by osmosis. Root hair cells are adapted for this by having a large surface area to speed up osmosis.

The absorbed water is transported through the roots to the rest of the plant where it is used for different purposes:

  • It is a reactant used in photosynthesis
  • It supports leaves and shoots by keeping the cells rigid
  • It cools the leaves by evaporation
  • It transports dissolved minerals around the plant

Leaves

Leaves are adapted for photosynthesis by having a large surface area, and contain openings, called stomata to allow carbon dioxide into the leaf. Although these design features are good for photosynthesis, they can result in the leaf losing a lot of water. The cells inside the leaf have water on their surface. Some of this water evaporates, and the water vapour can then escape from inside the leaf by diffusion.

To reduce loss the leaf is coated in a wax cuticle to stop the water vapour escaping through the epidermis. Leaves usually have fewer stomata on their top surface to reduce this water loss.

Shows the waxy cuticle on top of the upper epidermis.Under this is the palisade mesophyll layer and spongy mesophyll layer, which has air spaces in it. At the bottom, is the lower epidermis and wax cuticle. Gases are exchanged through the stoma. On each side of the stoma there is a guard cell with chloroplasts.

Reducing water loss

Stomata

Plants growing in drier conditions tend to have small numbers of tiny stomata and only on their lower leaf surface, to save water loss. Most plants regulate the size of stomata with guard cells. Each stoma is surrounded by a pair of sausage-shaped guard cells. In low light the guard cells lose water and become flaccid, causing the stomata to close. They would normally only close in the dark when no carbon dioxide is needed for photosynthesis.

Turgidity

Most plant cells are turgid at all times. This supports the weight of the plant, which is especially important where there is no woody tissue, such as leaves, shoot and root tip. If the plant loses water faster than it can be absorbed the cells lose turgor pressure and become flaccid. This causes the plant to wilt.

You should be able to explain why most plants will wilt if they get flooded by sea water. (Hint: sea water contains many chemicals in solution, such as salt. Osmosis will move water across the plant cell membrane, from the weaker to the stronger solution.)

Osmosis

Osmosis is the movement of water molecules from an area of high concentration of water to an area of lower concentration of water through a partially permeable membrane. This can be the cell membrane. An example is the flooding of plants by sea water. Sea water contains many chemicals in solution, such as salt. Osmosis will move water across the plant cell membrane, from the weaker to the stronger solution.

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