Investigating transport in plants

What is a potometer used for?

‣ To estimate the rate of transpiration.

What assumption is made when using a potometer?

Water loss by transpiration is equal to water uptake.

Why might the volume of water lost by transpiration be different to the volume of water taken up?

‣ Water used in photosynthesis.

‣ Water used to maintain turgidity.

‣ Water produced by respiration.

Why would the rate of

water movement through

a shoot in a potometer be

different to the rate of

water movement through

a whole plant?

A whole plant also has roots.

‣ The xylem in a shoot are narrow(er).

How would you determine the rate of water loss per mm2 of a leaf?

• Draw around leaf on graph paper.

‣ Count the number of squares to determine the area.

‣ Multiply by two to account for the upper and lower surface of the leaf.

‣ Divide the rate of water uptake by the total

surface area of the leaf.

Precautions that must be made when setting up a potometer

• Cut stem underwater, assemble apparatus underwater & insert stem underwater → so no air can enter the xylem (air bubble could block xylem) → need to obtain a continuous column of water.

‣ Dry leaves before starting the investigation → wet leaves would reduce the transpiration due to decreased concentration gradient between leaves and the air.

‣ Remove end of capillary tube from beaker of water until one air bubble (only) has formed, then replace end of tube in water to prevent more than one air bubble forming.

‣ Ensure joints are watertight and airtight.

‣ Don’t allow air bubble to move too far → so that air bubble doesn’t enter xylem. Use the syringe filled with water to move the air bubble back to the start of the scale

‣ Shoot must be supported to avoid breaking the stem → could damage the walls of the xylem, allowing air to enter the xylem and interrupt the continuous column of water.