Plant transport structures and processes

Water enters by osmosis. This is the net movement of water through a partially permeable membrane from a dilute solution to a more concentrated solution. When the soil solution is more dilute than the cell sap, water moves into the root hair cell through its membrane.

Mineral ions, such as nitrate ions, are taken up by active transport. This allows ions to enter even when their concentration is lower in the soil than inside the root cell: they move against their concentration gradient. Active transport requires energy released by respiration. Root hair cells contain many mitochondria, helping to supply this energy.

Xylem forms a continuous route upwards

After absorption, water and mineral ions pass into the root’s xylem. Xylem vessels form hollow tubes extending through the roots and stem into the leaves.

Their hollow interiors contain no cell contents obstructing the flow, and the absence of end walls allows water to move continuously along the vessels. The walls are strengthened by lignin, which helps prevent the tubes from collapsing as water is pulled through them.

The continuous upward movement of water from roots to stems and leaves is the transpiration stream. Mineral ions travel dissolved in this water.

Water loss from leaves maintains the transpiration stream

Transpiration is the loss of water vapour from leaves through their stomata. It involves a sequence of movements:

1. Water leaves the xylem and supplies the leaf cells.
2. Water evaporates from the internal surfaces of the leaf into its air spaces.
3. Water vapour diffuses out through the stomata into the surrounding air.

As water is lost from the leaves, more water is drawn upwards through the xylem to replace it. This links water loss at the leaves to water uptake at the roots.

Stomata balance gas exchange and water loss

A stoma is a small pore in the leaf surface; the plural is stomata. Stomata are mainly found on the underside of leaves. Each pore is surrounded by a pair of guard cells, which control whether it is open or closed.

Open stomata allow carbon dioxide to diffuse into the leaf for photosynthesis and oxygen to diffuse out. However, they also provide an escape route for water vapour. Guard cells therefore regulate both gas exchange and water loss.

Stomata usually open during the day, allowing gas exchange for photosynthesis. In dry conditions, plants may close their stomata to conserve water. Closing the pores reduces water loss, but also restricts carbon dioxide entry.

Phloem distributes dissolved sugars

Photosynthesis produces sugars in the leaves, but cells throughout the plant need them. Translocation is the movement of dissolved sugars through phloem tissue to other parts of the plant.

The sugars may be used immediately, for example in respiration, or stored for later use. Sugars made in a leaf can move downwards to roots or upwards to growing shoots. Phloem transport is therefore not restricted to the upward direction of the transpiration stream.

Phloem consists of tubes of elongated cells joined end to end. Unlike xylem vessels, these cells have end walls, but the walls contain pores. Cell sap containing dissolved sugars passes through these pores from one cell to the next.

Transpiration and stomata

Water leaves xylem → evaporates from internal leaf surfaces → water vapour diffuses out through stomata. Water lost from leaves is replaced by water drawn up the xylem.

Guard cells control stomatal opening: open pores allow gas exchange but also water loss; closing them conserves water but limits carbon dioxide entry.

Roots, stem and leaves work together as the plant’s transport organ system.