Transpiration
When the plant opens its stomata to let in carbon dioxide, water on the surface of the cells of the spongy mesophyllThe plant tissue in a leaf which has loosely packed cells and air spaces between them to allow gas exchange. and palisade mesophyllPlant tissue containing closely packed cells in the upper layer of a leaf. evaporationThe process in which a liquid changes state and turns into a gas. and diffuseWhen particles spread out from a region of higher concentration to a region of lower concentration. out of the leaf. This process is called transpirationThe loss of water from leaves by evaporation through the stomata..
Water is drawn from the cells in the xylem vesselsNarrow, hollow, dead tubes with lignin, responsible for the transport of water and minerals in plants. 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 bondingHydrogen bonding is a relatively strong intermolecular attractive force that develops between a hydrogen atom and an electronegative atom. Hydrogen bonding between water molecules involves an attractive force between a hydrogen atom in one water molecule and the oxygen atom in another.. 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 photosynthesisA chemical process used by plants to make glucose and oxygen from carbon dioxide and water, using light energy. Oxygen is produced as a by-product of photosynthesis. Algae subsumed within plants and some bacteria are also photosynthetic. - 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 mineralsNaturally occurring, inorganic chemical substances. Minerals are necessary for both plant and animal health.
- cools the leaf as water evaporationThe process in which a liquid changes state and turns into a gas.
- provides water that keeps the cells turgidEnlarged and swollen with water. Having turgor. Description of a plant cell in which the vacuole has swollen due to water gain by osmosis., which supports herbaceous plants
Water uptake and transport across the root
Root hairs are single-celled extensions of epidermal cellsCells of the epidermis, the outer covering layer of an organism. 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 transportThe transport of molecules against their concentration gradient from a region of low concentration to a region of high concentration..
A summary of water uptake, water transport and transpiration: