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 mesophyll evaporates 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.

Xylem

The xylem transports water and minerals from the roots up the plant stem and into the leaves.

In a mature flowering plant or tree, most of the cells that make up the xylem are specialised cells called vessels.

Vessels

  • Lose their end walls so the xylem forms a continuous, hollow tube.
  • Become strengthened by a chemical called lignin. The cells are no longer alive. Lignin gives strength and support to the plant. We call lignified cells wood.

Transport in the xylem is a physical process. It does not require energy.

Diagram showing how the xylem transports water to the rest of the plant

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 about 5% of the water taken up by the plant is used for photosynthesis - but transpiration does have its purposes as it:

  • provides the water for photosynthesis
  • transports mineral ions
  • cools the leaf as water evaporates
  • provides water that keeps the cells turgid, which supports herbaceous 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 osmosisSoil water moves into the root hair cell by osmosis and across the root cortex to the xylem

A summary of water uptake, water transport and transpiration:

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