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 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:
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:
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:
|Tissue||Process||What is moved||Structure|
|Xylem||Transpiration||Moves water and minerals from roots to leaves||Columns of hollow, dead reinforced cells|
|Phloem||Translocation||Moves food substances from leaves to rest of plant and from stores such as in the roots||Columns of living cells|