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 xylem to replace the water that has been lost from the leaves.

Water molecules inside the xylem cells are strongly attracted to other water molecules. 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 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

Factors affecting transpiration

The rate of transpiration is affected by several factors. These include:

  • temperature
  • humidity
  • air movement
  • light intensity

The table below explains how factors increase the rate of transpiration:

FactorChange in factor that increases transpiration rateExplanation
TemperatureIncreaseIncreases molecular movement so that: more water molecules evaporate from cell surfaces; the rate of diffusion of water molecules from the leaf is increased
HumidityDecreaseReduces the concentration of water molecules outside the leaf; diffusion of water from the leaf increases
Air movementIncreaseRemoves water vapour from leaf surfaces; more water diffuses from the leaf because a high concentration gradient is maintained
Light intensityIncreaseIncreases the rate of photosynthesis; stomata open so that water diffuses out of the leaf
Question

What conditions will decrease the rate of transpiration?

Decrease in temperature, increase in humidity, no or reduced air movement and low light intensity.