Transpiration

When the plant opens its stomata to let in carbon dioxide, water on the surface of the cells of the spongy mesophyll and palisade mesophyllevaporates and diffuses out of the leaf. This process is called transpiration.

Cross-section of a leaf showing water travelling through xylem cells to the spongy mesophyll cells, where it evaporates and exits through the stoma as water vaper

Water is drawn from the cells in the xylem to replace that which has been lost from the leaves.

Water molecules inside the xylem cells are strongly attracted to each other. There is strong cohesion between the molecules because of hydrogen bonding. A continuous column of water is therefore pulled up the stem in the transpiration stream by evaporation from the leaves.

As water travels through the xylem in the stem and leaf, it is being replaced by water taken up by the roots.

Transpiration is an unavoidable consequence of photosynthesis – only five per cent of the water taken up by the plant is used for photosynthesis – but does have its purposes:

  • provides the water for photosynthesis
  • transports mineral ions
  • cools the leaf as water evaporates
  • provides water that keeps the cells turgid, which supports herbaceous (non-woody) plants

Water uptake and transport across the root

Root hairs are single-celled extensions of epidermal cells in the root. They grow between soil particles and absorb water and minerals from the soil.

Water enters the root hair cells by osmosis. This happens because soil water has a higher water potential than the cytoplasm of the root hair cell. Minerals enter by active transport.

Diagram of water entering the root hair cells by osmosis

A summary of water uptake, water transport and transpiration:

Diagram illustrating how water enters the plant and is transported across the root

Translocation

Photosynthesis produces glucose in the green parts of plants, which are often leaves. This is then converted into sucrose. The sucrose is transported around the plant in phloem vessels. It needs to be able to reach all cells in the plant so that the sucrose can be converted back into glucose for respiration.

The movement of sucrose and other substances like amino acids around a plant is called translocation. In general, this happens between where these substances are made (the sources) and where they are used or stored (the sinks).

This means, for example, that sucrose is transported:

  • from sources in the root to sinks in the leaves in early spring time
  • from sources in the leaves to sinks in the root in the summer

Comparison of xylem and phloem

TissueProcessWhat is movedStructure
XylemTranspirationMoves water and minerals from roots to leavesColumns of hollow, dead reinforced cells
PhloemTranslocationMoves food substances from leaves to rest of plant and from stores such as in the rootsColumns of living cells