Transpiration 3.3.4

Question 1

What is transpiration?

Answer 1

The loss of water from a plant by evaporation, via the stomata (or sometimes through the top surface of the leaf)

Question 2

What limits transpiration from the top of the leaf?

Answer 2

The waxy cuticle

Question 3

State the sequence of water movement across the leaf

Answer 3

• Water is pulled along the xylem
• Water moves into spongy mesophyll cells down the water potential gradient by osmosis from the xylem
• Water vapour diffuses into the air down the water potential gradient from the spongy mesophyll cells lowering the water potential.

Question 4

What are the requirements for transpiration to occur?

Answer 4

There must be a less negative water vapour potential inside the leaf than the outside.

Question 5

State 4 reasons of why transpiration is important.

Answer 5

• Provides support for stems/leaves through turgor pressure
• Cooling as water evaporates
• Mineral ions are transported in aqueous solutions
• Water is needed for photosynthesis, growth, and cell elongation

Question 6

State and explain 3 factors affecting transpiration

Answer 6

• Humidity -> the higher the humidity, the slower the rate of transpiration as the water potential gradient is lower.
• Air movement -> increased air movement increases the rate of transpiration as it moves water away from the leaf quickly.
• Light intensity -> greater light intensity increases the rate of transpiration because it causes the stomata to open (gas exchange) so transpiration increases.

Question 7

How does wilting occur?

Answer 7

If more water evaporates from the leaves of a plant than is available in the soil to move into the root by osmosis, then wilting will occur.
- It occurs when all the cells of the plant are not full of water, so the strength of the cell walls cannot support the plant and it starts to collapse.

Question 8

What measures the rate of transpiration?

Answer 8

Potometer

Question 9

How does a potometer work?

Answer 9

As the leaf loses water, the air bubble in the potometer moves and the distance moved over a time period is measured.

Question 10

Name 3 limitations of a potometer

Answer 10

• measures water uptake
• cutting plant shoot may damage plant
• plant has no roots so no resistance to water being pulled up

Question 11

State the formula for the rate of transpiration

Answer 11

Rate = volume/time

Question 12

What is the formula

Answer 12

V = (pi) x r(squared) x h
(where r is the radius of the capillary tube and h is the length of the capillary tube)