Xylem

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Structure

The xylem are plant vessels that are responsible for transporting water and mineral ions. The structure of the xylem is specialised for this role.

Vessel elements

The xylem vessels are long, tubes of cells that run up the stem of plants.
The cells of the xylem are called vessel elements.
The vessel elements in the xylem are dead.

End walls

The vessel elements are stacked on top one another.
There are no cell walls at the ends of each vessel element.
This creates a continuous tube for water to flow through.

Lignin

The walls of the xylem are lined with a waterproof polymer called lignin.
Lignin reinforces the walls of the vessel elements to provide structural support.

Cohesion-Tension Theory

Water and inorganic ions travel up the xylem through cohesion and tension. The steps involved in this process are:

1) Transpiration

Some of the water in the leaves is used in photosynthesis.
Most of the water in the leaves evaporates in a process called transpiration.

2) Tension

The loss of water from the leaves creates tension in the xylem.
Tension is the formation of hydrogen bonds between water molecules and the sides of the xylem vessel elements.
Water in the xylem is pulled upwards by this tension towards the leaves.

3) Cohesion

Individual water molecules also form hydrogen bonds with each other. This process is called cohesion.
When water molecules are pulled up the xylem, other molecules of water are also pulled upwards due to cohesion.
The combination of cohesion and tension together continuously pull water upwards to replace water that has been lost in the leaves by transpiration.

4) Diffusion in the roots

When water is pulled up the stem, the water potential at the bottom of the plant decreases.
Water diffuses into the roots via osmosis down its water potential gradient.

Investigating Transpiration Rate

The rate of transpiration can be influenced by a number of factors. An experiment can be set up using a potometer to estimate transpiration rate. The steps involved are:

Apparatus

1) Assemble the potometer

A potometer is a piece of equipment that is used to estimate the volume of water taken up by a plant in a given time.
The potometer is filled with water and a cutting of a shoot is placed inside.
The shoot must be cut and placed into the potometer while underwater to ensure no air enters the xylem.

2) Form the air bubble

Remove the end of the capillary tube from the water beaker.
Wait for a bubble of air to form in the capillary tube.
Place the capillary tube back into the water.
The air bubble is used to record the volume of water used by the shoot.

3) Record bubble movement

Mark the starting position of the air bubble.
Use a stopwatch to record the distance moved by the air bubble in a given time period.

4) Calculate transpiration rate

Calculate the rate of movement of the bubble per hour.
The rate of bubble movement is equal to the transpiration rate.

5) Change the variable

The experiment can be repeated by changing a different variable each time (e.g. temperature or light).
It is important to only change one variable at one time and to keep all other conditions constant.
Changing one variable allows the effects of an environmental factor on transpiration rate to be compared.