Mass transport in plants: Water transport

Describe the movement of water through a plant

Water enters plants through their roots

Water enters a plant's root hair cell via osmosis

It moves through the cell cytoplasm or cell walls towards the xylem

The xylem transports water from the roots up to the leaves

Water is used for photosynthesis

Some water evaporates from leaf cells by transpiration and diffuses out of the plant

What does the cohesion-tension theory explain

The cohesion-tension theory explains how water move upwards through the xylem against gravity

What is cohesion

Hydrogen bonding causes water molecules to stick together and move as one continuous column

What is adhesion?

Hydrogen bonding between polar water molecules and non-polar cellulose in xylem vessel water pulls water upwards through the xylem

What is transpiration pull/tension?

Evaporation of water at leaves creates the transpiration pull, and this tension is transmitted down the whole water column due to cohesion, causes water to be pulled up through the xylem vessels

Explain the cohesion-tension theory

The water is removed from the top of the xylem vessels into the mesophyll cells down the water potential gradient and evaporates (or transpires) through the leaf stomata

This reduces the water potential in the cell

Water is drawn out of the xylem, creating tension

The attractive force between water molecules is known as cohesion. This allows water to be drawn up the xylem as a column

Water molecules also adhere to the walls of the xylem (adhesion)

As water is pulled up the xylem, the walls of the xylem are pulled in by tension

What are the factors affecting the rate of transpiration?

Number of leaves

number/size or position of stomata

Presence of waxy cuticle

Amount of light present

Temperature

Humidity of air

Wind speed

Water availability

How can the amount of light present affect transpiration

At high light intensities, stomata open for maximum CO2 absorption for photosynthesis, increasing the transpiration rate.

How can temperature affect the rate of transpiration?

At high temperatures, evaporation of water molecules is faster due to higher kinetic energy, increasing the transpiration rate.

How can the humidity of the air affect transpiration?

Low humidity increases the water vapour gradient between the leaf and atmosphere, increasing the transpiration rate

How can the wind speed affect the rate of transpiration?

High wind speeds increase the water vapour gradient between the leaf and atmosphere, increasing the transpiration rate.

What do you use to investigate the rate of transpiration

The rate of transpiration can be investigated using a potometer where water lost by the leaf is replaced by water in the capillary tube.

What needs to be ensured before using the potometer

When setting up a potometer it is important to make sure all the joints and seals are airtight

Explain how to set up of a potometer

Cut the shoot underwater to prevent air from entering the xylem and to maintain a continuous water column

Cut the shoot at a slant to increase the surface area available for water uptake

Assemble the potometer in water and insert the shoot underwater, so no air/bubbles can enter the xylem

Keep the end of the capillary tube submerged in a beaker of water

Dry the leaves and give the shoot time to acclimatise and shut the tap

Remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put the end of the tube back into the water

Record the starting position of the air bubble

Measure the distance the air bubble moves and the time taken

The rate of air bubble movement is an estimate of the transpiration rate

Only change one variable at a time. All other conditions must be kept constan

How do you calculate the rate of transpiration with a potometer?

Calculate the cross-sectional area of the capillary tube (this is the area of the circle that forms a cross section of the tube):

Surface area of a circle = r squared

Calculate the volume of water uptake by multiplying the value from step 1 by the distance the air bubble travelled, 'd' (that is, calculate the area of the cylinder of water that has been taken up by the plant):

Volume of water uptake = r squared x d

How do you calculate water uptake and then the rate of transpiration

Calculate the rate of water uptake (an estimate for the rate of transpiration) by dividing the value calculated in step 2 by the time taken:

Rate of transpiration = volume of water uptake/ time taken