Topic 8: Transport in plants

Plants have no heart and no beating pump, yet a tall tree can lift water more than a hundred metres from its roots to its highest leaves. How? Plants use two specialised transport tissues — xylem and phloem — together with the sun’s energy and the simple physics of evaporation. In this topic you’ll learn how water and minerals travel up, how sugars are shared around the plant, and why a wilting plant is really just running short of water.

Two transport tissues: xylem and phloem

Plants contain two transport tissues arranged together in vascular bundles.

• Xylem carries water and dissolved mineral ions from the roots upwards to the stem and leaves. Xylem vessels are dead, hollow tubes strengthened with lignin, so they also help support the plant.
• Phloem carries dissolved food (mainly sucrose and amino acids) from the leaves to other parts of the plant. Phloem is made of living cells.

In a root the vascular bundle sits in the centre; in a leaf the bundles form the veins; in a stem they are arranged near the outside in a ring.

Water uptake and movement

Water enters the plant through the root hair cells. These long, thin extensions hugely increase the surface area of the root for absorbing water and mineral ions.

• Water enters by osmosis, moving from the soil (a dilute solution) into the cell (a more concentrated solution) down a water potential gradient.
• Mineral ions such as nitrate and magnesium enter by active transport, which needs energy because they move against their concentration gradient.

From the root, water crosses to the xylem and is then pulled up the plant in a continuous column — the transpiration stream.

Transpiration

Transpiration is the loss of water vapour from the leaves, mainly through tiny pores called stomata. As water evaporates from the surfaces of mesophyll cells and diffuses out, it pulls more water up the xylem to replace it — this pull is what lifts water up the whole plant.

The rate of transpiration increases when:

• temperature rises (faster evaporation);
• humidity falls (steeper concentration gradient out of the leaf);
• wind increases (water vapour is carried away, keeping the gradient steep);
• light intensity rises (stomata open for photosynthesis).

If a plant loses water faster than it can absorb it, the cells lose turgor and the plant wilts.

Translocation

Translocation is the movement of sucrose and amino acids in the phloem from sources to sinks.

• A source is a part that makes or releases sugar, such as a photosynthesising leaf or a storage organ in spring.
• A sink is a part that uses or stores sugar, such as a growing root, a fruit, or a developing bud.

Because sources and sinks change with the seasons, phloem can carry food in different directions at different times — unlike xylem, which only ever moves water upwards.