Transport in plants and the structure of specialised plant cells

Plants do not have a heart, blood or a circulation system, but they do need a transport system to move food, water and minerals around.

They use two different systems - xylem moves water and mineral ions from the roots to the leaves - phloem moves food substances such as sucrose (sugar) and amino acids from leaves to the rest of the plant. This movement of food is called translocation. Both of these systems contain cells that make continuous tubes running the full length of the plant from the roots, up the stem and through the leaves. They are like blood vessels for the plant.

Root hair cells

Cross-section of root hair cell: a roughly rectangular shape with a long, thin tail extending to the right and a nucleus at the top left.

Plants absorb water from the soil by osmosis. They absorb mineral ions by active transport, against the concentration gradient. Root hair cells are adapted for taking up water and mineral ions by having a large surface area to increase the rate of absorption. They also contain lots of mitochondria, which release energy from glucose during respiration in order to provide the energy needed for active transport.

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

Stomata

Stomata are tiny holes found in the underside of leaves. They control water loss and gas exchange by opening and closing. They allow water vapour and oxygen out of the leaf and carbon dioxide into the leaf.

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 bright light the guard cells take in water by osmosis and become plump and turgid. 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. Guard cells are adapted to their function by allowing gas exchange and controlling water loss within the leaf.

The size of the stomatal opening is used by the plant to control the rate of transpiration and therefore limit the levels of water loss from the leaf. This helps to stop the plant from wilting.

A diagram showing the stoma open and closed.