Transport in plants - xylem

In a nutshell

Water is transported through a plant by its xylem. Water must travel against the force of gravity and it achieves this through the cohesion-tension theory of water transport. This summary details the process of water transport in plants and how it can be investigated.

Xylem

Xylem are tubes that transport water and mineral ions from the roots to the stem and the leaves. The xylem are made up of dead cells with no end walls. They transport water unidirectionally and are strengthened with a material called lignin.

Cohesion and tension

Hydrogen bonds form between water molecules. This results in strong cohesive forces that creates a continuous column of water in the xylem which helps the water move upwards by mass flow. The evaporation of water from the leaves creates a tension that pulls more water into the leaf.

Transpiration

Definition

The process through which plants lose water is called transpiration. This usually occurs when water evaporates from the leaves but it can also occur from other parts of the plant.

When water is lost through transpiration (1.), there is a shortage of water so more is taken up through the roots (2.) and transported through the xylem. This process results in a constant transpiration stream through the plant which carries water and mineral ions around the plant.

Biology; Exchange and transport; KS5 Year 12; Transport in plants - xylem

Transpiration rate

The rate at which transpiration happens is affected by different factors. This will also impact the rate of water uptake of the plant.

Light intensity

When the light is brighter, the rate of transpiration is greater. This makes sense when you think of the opposite. When it is dark the stomata don't open. This is because photosynthesis requires light so the stomata don't need to open to let in carbon dioxide. As a result, there will be less water leaving the plant.

Temperature

When the temperature is higher, the rate of transpiration is greater. The water molecules have more energy to evaporate. Therefore, they will diffuse out of the stomata.

Humidity

When humidity is low, the transpiration rate will be greater. This is because if the air around the leaf is dryer, there will be an increased concentration gradient between the air and the leaf so transpiration will increase.

Wind

When air flow is low, for example when there is no wind, the water vapour will surround the leaf resulting in a high water concentration outside of the leaf so diffusion will not happen very quickly.

Whereas, if there are strong winds, the water vapour is blown away and there is a low concentration of water surrounding the leaf. Therefore, there is a higher concentration inside the leaf and diffusion will happen quickly.

Investigating transpiration rate

To investigate the rate of transpiration you can use a potometer. Potometers measure water uptake in plants and this can be used to measure transpiration rate as the uptake and loss are directly related. This method can be adapted to measure the impact of the factors mentioned above on transpiration rate.

Biology; Exchange and transport; KS5 Year 12; Transport in plants - xylem

1.	A beaker of water.
2.	The water (and bubble) moves through the tube towards the plant.
3.	Capillary tube with a scale.
4.	The tap. This is closed during the experiment.
5.	A reservoir of water.
6.	The bubble moves through the tube in this direction.
7.	The plant used in the experiment. As the plant photosynthesises, the bubble will move through the tube in the direction of the plant.

Procedure

1.	Set up the apparatus as shown in the diagram. Cut the shoot underwater to stop air entering the xylem and ensure it is cut at a slant to increase the surface area for the uptake of water.
2.	Record the starting position of the air bubble.
3.	Using a stopwatch and the capillary tube, measure how far the bubble moves in a certain amount of time.
4.	Calculate the speed that the bubble moves to give an estimation of the rate of transpiration.

Example

The rate of transpiration was measured using a potometer. Over $30\ minutes$ , the bubble moved $60\ mm$ . What is the rate of transpiration?

$\begin{aligned}rate\ of\ transpiration&= \frac{distance\ bubble\ moved}{time\ taken}\\\\&= \frac{60\ mm}{30\ min}\\\\&= \boxed{{2\ mm\ min}{^-}{^1}}\\\end{aligned}$

$\underline{Therefore,\ the\ rate\ of\ transpiration\ is\ 2\ mm\ min{^-}{^1}}$

Dissecting plants

The xylem and phloem can be viewed under the microscope.

1.	Cut a cross section of the stem with a scalpel.
2.	Place the cross sections in toluidine blue O (TBO) stain. This will stain the lignin in the xylem walls and allow you to view them as they will appear a blue-green colour under the microscope.
3.	Rinse off the cross sections and mount them onto a slide.
4.	View under the microscope and create a biological drawing to demonstrate what you have seen.