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Plants have a system that transports nutrients, minerals and water between different parts of the plant.
- Xylem transports water and minerals from the roots to other parts of the plant.
- Phloem transports sucrose and amino acids between the leaves and other parts of the plant.
👇 Xylem and phloem are distributed throughout the root, stem and leaves of plants. 👇
Structure of Xylem and Phloem
Xylem
- The xylem is made of dead cells which become hollow, making a continuous tube for water and minerals to flow through.
- The walls of the xylem are stiffened with lignin, which is a woody material.
- Lignin is impermeable to water, so water will not be absorbed to other parts of the plant, making it possible for xylem to transport water.
- The transport of water and minerals in the xylem is one-way only; it moves upwards from the root to the leaves. Water and mineral ions will never be transported downwards. Transpiration pull enables water to be transported upwards against gravity. (We will learn about transpiration later in this article 😄)
Phloem
- Unlike the xylem, the phloem consists of living cells connected end-to-end.
- Cells have end walls with holes in it so that materials can move through the cells.
- Sucrose and amino acids are transported from where they are made to where they are needed. They can be transported in both directions. This transport process is called translocation.
A Closer Look Into the Movement of Water
- Transpiration is the evaporation of water from stomata on the surface of leaves.
- Because water molecules are cohesive, the water molecules leaving the plant pulls water molecules at the stem upwards. This is called the transpiration pull.
- As more water leaves through transpiration, more water is absorbed from the soil through the roots.
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Most of the water absorbed by plants is absorbed by root hair cells.
- Root hair cells are specialized cells in plant root that have a long and thin extension that increases surface area for water absorption.
- Water is absorbed by root hair cells by osmosis. Soil has a higher water potential than the root hair cell cytoplasm, so water moves from the soil into the root hair cell.
