Transport in plantsIB MYP Biology: Revision notes

Xylem vessels carry water and dissolved mineral ions from the roots up to the stem and leaves. Water only moves upwards in the xylem.

Xylem vessels are made of dead cells joined end to end:

• They are hollow, with no cytoplasm and no end walls, so water flows through a continuous tube.
• Their walls are strengthened with lignin, a hard waterproof substance, so the tubes do not collapse and they also support the plant.

Phloem carries sucrose (and other dissolved food substances) made by photosynthesis in the leaves to the parts of the plant that need it, such as roots, growing shoots, flowers and fruits. This movement is called translocation.

Sucrose can travel up or down the plant in the phloem, depending on where it is needed. Phloem tubes are living cells, unlike xylem.

Roots take in water and minerals through root hair cells. Each is a root cell with a long, thin extension that sticks out into the soil.

• The extension gives a very large surface area for absorption.
• Its wall is thin, so water can pass quickly.
• Water moves into the cell by osmosis, because the water concentration is higher in the soil than inside the cell.

The water then passes across the root into the xylem.

In a leaf, water evaporates from the surfaces of the mesophyll cells into the air spaces. The water vapour then diffuses out through tiny pores called stomata, mainly on the underside of the leaf. This loss of water vapour is transpiration.

Each stoma is surrounded by two guard cells. When the guard cells swell they open the pore; when they lose water they close it. Stomata also let carbon dioxide in for photosynthesis.

As water leaves the leaf, more is pulled up the xylem to replace it. The continuous movement of water from the roots, up the xylem and out of the leaves is the transpiration stream.

Transpiration is faster when water vapour can leave the leaf more quickly:

• Light: brighter light makes stomata open, so more water vapour escapes.
• Temperature: higher temperature makes water evaporate faster and vapour diffuse faster.
• Wind: moving air blows away the water vapour near the leaf, which keeps the concentration gradient steep.
• Humidity: in humid air the gradient is smaller, so transpiration is slower; in dry air it is faster.

Worked example: a plant loses more water on a hot, dry, windy day than on a cool, damp, still day, because evaporation is faster and the gradient of water vapour is steeper.

A potometer can compare water uptake in different conditions, as water uptake is close to the water lost.