Plant organisation
Plant leaves are adapted for photosynthesis and gas exchange. Roots absorb water and mineral ions through root hair cells and are transported up the plant by the xylem.
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.
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.
A summary of water uptake, water transport and transpiration:
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:
| Factor | Change in factor that increases transpiration rate | Explanation |
|---|---|---|
| Temperature | Increase | Increases molecular movement so that: more water molecules evaporate from cell surfaces; the rate of diffusion of water molecules from the leaf is increased |
| Humidity | Decrease | Reduces the concentration of water molecules outside the leaf; diffusion of water from the leaf increases |
| Air movement | Increase | Removes water vapour from leaf surfaces; more water diffuses from the leaf because a high concentration gradient is maintained |
| Light intensity | Increase | Increases 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.