Water and mineral transport in plants

In a nutshell

Water and mineral transport is crucial for the survival of plants and requires many key parts to ensure that the plant has enough water to survive. They must balance the water they lose and the water they uptake through their transpiration stream. Their stomata and waxy cuticle are adaptations of the leaves that regulate this balance. The rate of transpiration can be measured using a potometer and impacts of different factors such as light intensity can also be investigated.

Root hairs

Root hairs grow out from the surface of plant roots and into the soil. Each root can be covered in millions of the microscopic hairs. The root hairs increase the surface area for absorption of water and mineral ions from the soil. 

Mineral ions are often transported by active transport as the concentration of ions is higher in the root hair cells than in the soil. The concentration of water is higher in the soil than in the root hair cells so it can be transported by osmosis.

Phloem

The phloem are tubes that transport food substances, such as sucrose, that are made in the leaves to the rest of the plant for immediate use or storage. Immediate use of these substances might be needed in growing parts of the plant. This transport is bidirectional (happens in both directions) and it is called translocation. Translocation requires energy from respiration. 

The phloem are elongated living cells that have end walls between the cells. These have small pores to allow the substances to pass through. 

Xylem

Xylem are tubes that transport water and mineral ions from the roots to the stem and the leaves where they will be used in processes such as photosynthesis. Unlike the phloem, these are made up of dead cells that have no end walls between the cells. The cells are strengthened with a strong material called lignin.

The xylem form part of the transpiration stream, which is the movement of water from the roots, up the xylem and out of the leaves.

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.

Stomata

Stomata is the plural of stoma. These are small pores found mainly on the underside of the leaf that allow carbon dioxide and oxygen to diffuse in and out. During transpiration, water vapour will also leave the leaf through the stomata.

Stomata have guard cells (1.) that control the size of the pore (2.) by changing their shape. When they are turgid (as they are swollen with water) they cause the stomata to open (A), allowing water vapour to diffuse out of the leaf. However, when they are flaccid (as they are limp and have little water) they cause the stomata to close (B).

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 the stomata are not open, there is 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.

Air

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. 

Procedure

Example

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

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

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

Balancing 

Plants need to balance their water loss and water uptake. They have adaptations to regulate the amount of water that is lost. 

​