Transpiration

Question 1

Define transpiration

Answer 1

The process by which water is lost as water vapour from the aerial parts of the plant (leaves) through the stomata

Question 2

Explain why transpiration is a consequence of gas exchange

Answer 2

When the stomata are open to allow for the gas exhchange of oxygen and carbon dioxide for photosynthesis, water vapour inside the leaf is moves out by diffusion and is lost

Question 3

Name the five environmental factors which affect the rate of transpiration

Answer 3

1. Soil water availability
2. Temperature
3. Air movement
4. Relative humidity
5. Light

remember: STAR Light

Question 4

How does soil-water availability affecct the rate of transpiration?

Answer 4

1. When there is a low soil-water availability this puts the plant under water stress
2. This leads to some of the stomata closing which reduces the rate of transpiration

Question 5

How does temperature affect transpiration rate?

Answer 5

1. An increase in temperature increases the kinetic energy of the water molecules
2. This increases the rate of evaporation of water from the spongy mesophyll to the air spaces in the leaf which increases the rate of transpiration

Question 6

Explain how air movement affects the rate of transpiration

Answer 6

1. Water vapour that diffuses out of the leaf is trapped by a thin layer of still air around the leaf
2. This increases the water vapour potential gradient but reduces the diffusion gradient
3. Air movement moves the layer of still air which increases the diffusion gradient and thus the rate of transpiration

Question 7

Explain how relative humidity affects the rate of transpiration

Answer 7

Increasing the relatvie humidity will decrease the rate of transpiration as it decreases the water vapour potential gradient between the air on the inside and the outside of the leaf.
-Very dry air has the opposite effect and increases transpiration rate

Question 8

What is the definition of relative humidity?

Answer 8

The amount of water vapour in the air compared to the total concentration of water the air can hold

Question 8

Explain how light intensity affects the rate of transpiration

Answer 8

Increasing light intensity increases the number of open stomata which increases the rate of water vapour diffusion out of the plant which increases the evaporation from the surfaces of the leaf which increases the rate of transpiration

Question 9

What piece of apparatus can be used to measure the rate of transpiration?

Answer 9

A potometer

Question 10

How does a potometer work?

Answer 10

The distance travelled by the air bubble can be recored every minute to give the rate of water uptake in the plant

Question 11

Give two disadvantages of using a potometer to measure transpiration rate

Answer 11

1. It measure the rate of water uptake which is not the same as transpiration rate. Some of the water taken in will be used as a reactant for photosynthesis
2. Not a whole plant is used, there are no roots and so there is no resistance to water uptake

Question 12

Give four ways to improve validity when using a potometer

Answer 12

1. Trim the shoot under water to prevent air bubbles
2. Keep the leaves out of water and dry them off if they get wet
3. Put the stem in the bung and grease the joint to prevent air from entering
4. Leave the apparatus for 10 minutes to allow it to acclimatise