Plant Transport Tissues: Xylem, Phloem and Root Hair Cells

Transport across a whole plant

A plant needs a transport system because its resources are obtained or made in different places. Roots take up water and mineral ions from the soil, while leaves make sugars by photosynthesis. The roots, stem and leaves therefore work together as a plant organ system.

Three specialised structures make this possible. Root hair cells absorb substances from the soil. Xylem tissue carries water and mineral ions from the roots to the stems and leaves. Phloem tissue distributes dissolved sugars from leaves to parts of the plant where they are used immediately or stored.

An adaptation is useful only if it can be linked to a function. For example, it is not enough to state that a root hair cell has a long extension: the important link is that the extension provides a large surface area, allowing faster absorption.

Root hair cells provide a large absorbing surface

Root hair cells are specialised cells on the outside of young roots. Each has a long, narrow projection that extends between soil particles. This greatly increases the cell's surface area in contact with soil water. A larger area of cell membrane allows more water and mineral ions to be absorbed at the same time.

The wall of the root hair is thin, so substances have only a short distance to travel before reaching the cell membrane and entering the cell. Together, the large surface area and short transport distance make absorption efficient.

Water enters a root hair cell by osmosis. The cell membrane is partially permeable, and water moves into the more concentrated contents of the cell from the more dilute soil solution. Osmosis is passive, so it does not require energy from respiration.

Mineral ions such as nitrate ions may be present at a lower concentration in the soil than inside the root hair cell. Moving them into the cell can therefore be against the concentration gradient. This requires active transport, using transport proteins in the cell membrane and energy released by respiration. Root hair cells contain many mitochondria, providing sites for the respiration that releases this energy.

These adaptations work together: the projection supplies a large membrane area, the thin wall gives a short transport distance, and the mitochondria support the active uptake of mineral ions. Root hair cells are not absorbing food; their role is to absorb water and mineral ions.

Xylem forms strong, hollow water-carrying tubes

After entering the root, water and mineral ions must reach the stem and leaves. They are carried in xylem tissue as part of the transpiration stream.

Xylem is arranged as long, hollow tubes. The hollow interior provides a continuous route through which water containing dissolved mineral ions can travel, with little material obstructing the flow. This tube-like arrangement is suited to moving water over the long distance from the roots to the aerial parts of the plant.

The walls are strengthened with lignin. This reinforcement helps the tubes resist collapse as water is pulled through them and also contributes support to the plant. Xylem therefore combines a transport function with a strengthening function: its hollow space allows movement, while its lignified walls keep the pathway open and strong.

Phloem forms a route for dissolved sugars

Sugars made by photosynthesis must be carried from the leaves to other parts of the plant. Some are needed immediately, for example by growing or respiring cells, while others are transported to storage organs. Phloem tissue performs this transport function.

Phloem consists of tubes made from elongated cells. The end walls between neighbouring cells contain pores, so cell sap containing dissolved sugars can pass from one phloem cell to the next. Joining many elongated cells in this way creates a transport route extending through the plant.

The movement of dissolved food molecules through phloem is called translocation. At this stage, the key structural explanation is straightforward: elongated cells form tubes, and pores in their end walls allow sap to move between cells. A detailed account of phloem structure or the mechanism driving transport is not required.

Comparing the three specialised structures

Root hair cells are individual absorbing cells, whereas xylem and phloem are tissues made from specialised cells working together.

• Root hair cells take up water by osmosis and mineral ions by active transport.
• Xylem carries water and mineral ions from roots towards stems and leaves through hollow, lignin-strengthened tubes.
• Phloem carries dissolved sugars from leaves to places where they are used or stored through tubes of elongated cells with porous end walls.

This gives one connected transport system: absorption begins at the root surface, xylem supplies the upper plant with water and ions, and phloem distributes sugars made in the leaves.