Chapter 9.3

Question 1

What is transpiration?

Answer 1

​Transpiration is the Loss of water by evaporation from aerial parts of the plant.
Water enters the leaves through the xylem, passes through mesophyll cells by osmosis, and diffuses through air spaces in spongy layer.
As water vapour collects, water vapour potential rises. When water vapour potential is higher inside the leaf, it diffuses out.

Question 2

Why does transpiration occur?

Answer 2

Loss of water through transpiration is unavoidable. Plants exchange gases via stomata.
During the day plants must take up carbon dioxide and release oxygen.
The stomata must be open to allow this.
This provides an easy route for water loss.

Question 3

How does the number of leaves affect transpiration?

Answer 3

• More leaves means greater surface area

- Greater area over which water can be lost

Question 4

How does the number and size of stomata affect transpiration?

Answer 4

• Large stomata means water is lost more rapidly

- If stomata are on the lower surface of the leaf, water loss is slower

Question 5

How does the presence of cuticle affect transpiration?

Answer 5

Waxy cuticle reduces evaporation

Question 6

How does light affect transpiration?

Answer 6

Light causes stomata to open

Allows gaseous exchange for photosynthesis

Question 7

How does temperature affect transpiration?

Answer 7

This will increase the rate of water loss in three ways:

1. Increases rate of evaporation from surface
2. Increase rate of diffusion through the stomata
3. Decreases relative water vapour potential in the air

Question 8

How does relative humidity affect transpiration

Answer 8

• Higher humidity will decrease water loss

- This will reduce the water vapour potential gradient between the air in the leaf and outside

Question 9

How does air movement and wind affect transpiration

Answer 9

• Moving air will carry water vapour away

- This maintains a high water vapour potential gradient

Question 10

How does water availability affect transpiration

Answer 10

If there is low water availability, the plant cannot replace the lost water
Transpiration rates are therefore reduced

Question 11

What is a potometer?

Answer 11

Can be used to estimate the rate of water loss
Only an estimate as it only measures the rate of water uptake
Gives an accurate estimate

Question 12

How do you set up a potometer?

Answer 12

1. Leafy shoot is placed in a glass tube full of water
2. Make sure there are no bubbles in the apparatus
3. The tube is raised out of the beaker
4. Movement of the meniscus and the end of the water column can then be measured
5. The distance travelled by the meniscus over a certain time period will give the rate of water uptake/loss