Transport in plants: absorption of water and mineral ions

Learning objectives

By the end of this lesson, you should be able to:

• explain why plants need transport systems
• describe the structure and function of root hair cells
• distinguish osmosis from active transport
• trace water from the soil into the xylem
• explain how xylem vessels are adapted for transport

Why plants need transport

Plants need water for photosynthesis, as a solvent for metabolic reactions, and to maintain turgidity in cells. They also need mineral ions, including nitrate, magnesium and phosphate ions, for growth and metabolism. Water and mineral ions are mainly absorbed by roots, while sugars are manufactured in photosynthetic tissues, especially leaves.

A small plant may exchange materials over short distances by diffusion and osmosis. A large flowering plant has long roots, stems and leaves, so it needs specialised transport tissues to move substances efficiently over greater distances.

Root hair cells and absorption

A root hair is a long extension of an epidermal cell near the growing region of a root. Root hairs greatly increase the surface area in contact with soil water. Their thin walls provide a short pathway for water to enter. They are numerous, so the total absorbing surface of a root can be large.

Soil water usually contains dissolved mineral ions. The cell sap inside a root hair cell is often more concentrated than the surrounding soil solution. Water therefore enters the cell by osmosis.

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a selectively permeable membrane. The cell surface membrane acts as the selectively permeable barrier. Water potential is affected by the concentration of dissolved substances and pressure.

Do not define osmosis as the movement of water from a dilute solution to a concentrated solution without mentioning the selectively permeable membrane. The water-potential definition is more precise.

Mineral ion uptake

Mineral ions are absorbed in solution. Sometimes an ion moves into a root hair cell down its concentration gradient; this is passive movement. In other cases, the concentration of an ion is already higher inside the root hair cell than in the soil solution. The plant can still absorb the ion by active transport.

Active transport moves substances against a concentration gradient using energy supplied by cellular respiration. Carrier proteins in the cell membrane help move selected ions. Root cells therefore need energy, and oxygen availability can affect active uptake.

Osmosis and active transport are not the same process. Osmosis is the movement of water across a selectively permeable membrane down a water-potential gradient. Active transport moves substances such as mineral ions against their concentration gradient using energy.

Movement of water across the root

After entering a root hair cell, water can move across the root towards the xylem through neighbouring cells. It may travel through the cytoplasm of cells connected by plasmodesmata, or along cell walls and spaces between cells. At the endodermis, the Casparian strip in the cell walls of endodermal cells blocks unrestricted movement through the cell-wall route. Water and dissolved ions must cross a cell membrane, allowing the plant to regulate what enters the vascular tissue.

The water then enters the xylem, the tissue that transports water and dissolved mineral ions from roots towards stems and leaves. The xylem is continuous through the plant.

Xylem structure and function

Xylem vessels are formed from cells that lose their contents at maturity, leaving a continuous hollow pathway. Their end walls are absent or perforated, reducing resistance to water flow. The walls are strengthened with lignin, which helps prevent the vessels from collapsing under the tension created as water is pulled upwards.

Xylem tissue also contains other cell types, including fibres and parenchyma. The fibres provide support, while parenchyma cells can store materials and contribute to lateral movement.

The main functions of xylem are:

• transporting water from roots to stems and leaves
• transporting dissolved mineral ions
• providing mechanical support through lignified walls

Phloem is different. It transports dissolved organic substances, including sucrose, between sources and sinks. Do not say that xylem transports manufactured food.

Factors affecting absorption

Water absorption is influenced by the availability of water in the soil, the concentration of the soil solution, root surface area, temperature and the rate at which water is lost from leaves. A dry soil has a lower water supply. A very concentrated soil solution can make water uptake more difficult because its water potential is low.

Healthy roots with many root hairs generally absorb more effectively than damaged roots. Waterlogging can reduce oxygen around roots, limiting aerobic respiration and therefore reducing active transport of mineral ions.

WASSCE examination focus

If asked to explain how a root hair cell absorbs water, mention its large surface area, thin wall, selectively permeable membrane and the water-potential gradient that causes osmosis. If asked how mineral ions are absorbed against a concentration gradient, state active transport and the need for energy.

For a xylem adaptation question, link each feature to its function: hollow vessels provide a pathway; absent or perforated end walls reduce resistance; lignin strengthens the walls and prevents collapse.

Quick check

1. What is the function of root hairs?
2. Define osmosis accurately.
3. Why can mineral ions enter root hair cells by active transport?
4. State two adaptations of xylem vessels.

Answers: 1. They increase the surface area for absorption. 2. Net movement of water from higher to lower water potential through a selectively permeable membrane. 3. Active transport uses energy to move ions against a concentration gradient. 4. They form hollow continuous vessels and have lignified walls that resist collapse.