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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Xylem and Water Transport
Plants need to move water and nutrients throughout their bodies, just like animals do. However, plants don't have hearts to pump fluids around. Instead, they've evolved clever transport systems that work without muscles or pumps. One of these transport systems is the xylem, which carries water and minerals from the roots up to the leaves.
Key Definitions:
Xylem tissue is made up of several types of cells, but the most important for water transport are the xylem vessels. These are long, hollow tubes formed from dead cells placed end to end. The cells have lost their end walls to create continuous pipes and their side walls are strengthened with a substance called lignin. This makes the xylem vessels both strong and waterproof, perfect for transporting water efficiently.
The main job of xylem is to transport water and dissolved minerals from the roots up to the leaves. This might not sound impressive until you consider that some trees move water up more than 100 metres against gravity! Xylem also provides structural support to the plant because of the strong lignin in its cell walls. This helps plants stand upright and resist damage from wind and rain.
Water's journey through a plant is fascinating and involves several physical processes working together. Let's follow a water molecule from soil to air:
Water enters the plant through the roots, specifically through tiny root hair cells that increase the surface area for absorption. These root hairs grow between soil particles and absorb water by osmosis. Water moves into the root hair cells because they contain a higher concentration of dissolved substances (like minerals and sugars) than the surrounding soil water.
Once inside the root, water can take two pathways to reach the xylem:
Once water reaches the xylem vessels in the roots, it begins its journey upward. But how does it move against gravity without a pump? The answer lies in a combination of forces:
Minerals are actively transported into the xylem in the roots, creating a more concentrated solution. Water follows by osmosis, creating a pushing force from below. This helps start the water moving upward but isn't strong enough to push water to the top of tall plants.
Water molecules stick to each other (cohesion) due to hydrogen bonding. This creates a continuous water column in the xylem vessels that can be pulled upward without breaking, like a chain being pulled from the top.
Water molecules also stick to the walls of the xylem vessels (adhesion). This helps support the water column and prevents it from falling back down due to gravity.
The main force driving water movement in plants is transpiration pull. This is how it works:
This mechanism is sometimes called the Cohesion-Tension Theory and explains how plants can move water to great heights without using energy directly for transport.
The tallest trees in the world, the coastal redwoods (Sequoia sempervirens), can reach heights of over 100 metres. Water must be transported from the roots all the way to the topmost leaves against gravity. Scientists have calculated that the tension created by transpiration pull in these trees can be as strong as -15 atmospheres of pressure!
In a large tree on a warm, sunny day, more than 100 litres of water can move through the xylem and be lost through transpiration.
The rate at which plants lose water through transpiration can vary greatly depending on environmental conditions:
Plants have evolved various features to control water loss:
Scientists can measure the rate of transpiration using a simple piece of equipment called a potometer. This device measures how quickly a plant takes up water, which is approximately equal to the rate of transpiration (assuming the plant isn't growing rapidly or storing extra water).
A potometer consists of:
As the plant transpires, it draws water through the capillary tube. By measuring how far the water meniscus moves in a given time, we can calculate the rate of water uptake.
Xerophytes are plants adapted to survive in extremely dry conditions. Cacti are classic examples, with numerous adaptations for conserving water:
These adaptations allow cacti to survive in deserts where other plants would quickly die from water loss.
Efficient water transport is crucial for plant survival for several reasons:
Without the remarkable xylem system, plants couldn't grow tall, compete for sunlight, or colonise dry habitats. The evolution of efficient water transport systems was one of the key adaptations that allowed plants to move from aquatic environments onto land and eventually dominate terrestrial ecosystems.
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