Describe how water and mineral salts are transported in the xylem and how food is transported in the phloem, and explain transpiration

What this dot point is asking

This outcome wants you to describe the two transport tissues in a plant, xylem and phloem, and say what each one carries and in which direction. You should be able to explain transpiration, the loss of water from the leaves, and how it pulls water up the plant. You should also know the factors that change how fast a plant loses water. The marks reward clear comparison and linking the process to the structure.

The answer

Two transport tissues

A plant moves substances around using two kinds of tube-like tissue running through the roots, stem and leaves:

• Xylem carries water and dissolved mineral salts. It moves them in one direction only: upwards, from the roots to the leaves.
• Phloem carries dissolved food, mainly sugar made in the leaves by photosynthesis. It can move food in both directions, up and down the plant, to wherever it is needed or stored.

How water gets in and moves up

Water enters the plant through the roots. The roots have tiny root hair cells with a large surface area, and water moves into them by osmosis (from the soil, where there is plenty of water, into the root cell). The water then travels up the xylem to the leaves.

Transpiration

At the leaves, water evaporates and escapes as water vapour through tiny holes called stomata (mostly on the underside of the leaf). This loss of water from the leaves is called transpiration. As water leaves the top of the plant, it pulls more water up the xylem behind it, like sucking through a straw. This continuous flow of water up the plant is the transpiration stream.

Factors that affect the rate

A plant loses water faster when:

• the temperature is higher (water evaporates faster);
• it is windier (wind carries water vapour away, keeping the air outside the leaf dry);
• the air is drier (a bigger difference in moisture between inside and outside the leaf);
• the light is brighter (the stomata open wider in light, so more vapour escapes).

Examples in context

Example 1. Why cut flowers last longer in water. A cut flower has lost its roots but still loses water by transpiration from its leaves and petals. Standing it in water lets it draw water up the xylem to replace the loss, so it stays firm and fresh for longer. Without water it wilts as the cells lose their firmness.

Example 2. Why desert plants have few, small leaves. Plants in dry places often have very small leaves or spines and few stomata. This reduces the surface area for transpiration, so they lose less precious water. It is the same idea as a wilting plant, solved by cutting down water loss.

Try this

Q1. Name the tissue that carries water in a plant. [1 mark]

• Cue. The xylem.

Q2. State what transpiration is. [2 marks]

• Cue. The loss of water from a plant by evaporation from the leaves, escaping as water vapour through the stomata.

Q3. Explain why a plant loses water faster on a windy day. [2 marks]

• Cue. Wind blows water vapour away from the leaf, keeping the air outside dry, so the gradient stays steep and evaporation continues quickly.