OCR Biology Biology: Exchange in Plants

Plnats are very efficient

They DO NOT need at transport system to distribute oxygen and carbon dioxide, the reason being it has such a large surface area to volume ratio and can therefore exchange gases to and from the atmosphere quickly. 

Water will always move in through the root hair cells and into the xylem, from there it will flow up the stem and into the leaves. 

How does the water enter the roots?

Inside the root hair cells, there is a lower water potential than outside of the root, and therefore there is a water potential gradient

This means that water will move into the root by osmosis, due to the water moving passively down the water potential gradient.

Water can also enter the via the cell walls in the apoplast pathway

However once the water is in the root hair cells, mass flow takes over.

Water enters the root and travels across the cortex (the area between the centre and the surface layers) 

water potential continues to act here, as water moves from the root hairs into the xylem. Water potential is lower in the xylem and therefore water moves pasively down the water potentil gradient.

Below is the system of how water moves from section to section:

     Water

High water potential ------------------------> Low water potential 

The xylem is an extremely specialised cell, in which it is dead. It is a hollowed out cell that acts as a tube. It is strengthened with lignin to keep the tube upright. 

If you imagine the xylem vessels as a drinking straw; you need to put your mouth over the top and apply a suction force.  Sucking means reducing pressure, and this means there is a pressure gradient up the straw.

The pressure at the top is now less and the water moves up the straw 

In the xylem the pressure gradient is created by Transpiration in the leaf, and the water moves up the leaf by mass flow

However there are other forces acting to keep the porcess going.

• Cohesion - attracts each water molecule to each other and a long tube of continuous water is formed
• Adhesion - Is the attraction between the wall of the xylem vessel walls and water, and this means the water 'crawls' up the xylem walls.

The water reaches the leaf last, and enters the spongy Mesophyll layer

Here there are many air spaces and the water will saturate these spaces and creates a high water potential. 

The water vapour is kept in by the stomata and if the water potential of the air outside is lower, the water diffuses out of the stomata and into the atmosphere.

This porcess is called transpiration.

As the water leaves the plant, the pressure gradient is increased and water flows up the xylem by mass flow to fill the spaces again.

This is how the water moves from root to leaf and out and this continuous movement is known as the transpiration stream.

Anything that increases the water potential gradient between the air spaces in the leaf and the atmosphere, will increase transpiration

• When the air is dry, and not humid, there is a steep water potential gradient and therefore transpiration rates are greater.  Therefore a low humidity = High Transpiration

• An increase in temperature increase kinetic energy of water. This increases evaporation rate, and therefore a high temperature = High transpiration

• Light Intensity - when it is dark the stomata close and no water can get out