8.3 Transpiration

Question 1

what is transpiration

Answer 1

the loss of water vapour from the leaves

Question 2

describe transpiration

Answer 2

water evaporates from the surfaces of the mesophyll cells into the air spaces and then diffuses out of the leaves through the stomata as water vapour

Question 3

what is water vapour loss related to

Answer 3

• the large internal surface area provided by the interconnecting air spaces between mesophyll cells
• size of stomata
• number of stomata

Question 4

identify the positions of tissue (xylem phloem cortex)

Question 5

explain a transpiration pull

Answer 5

-water molecules are held together by forces of attraction between them called cohesion
-therefore a transpiration draws up a column of water molecules up the xylem vessels
-as the water evaporates at the leaf and diffuses out of the stomata, more water is drawn up the plant from the roots

Question 6

what is used to measure transpiration rate

Answer 6

-potometer

Question 7

precautions of using a potometer

Answer 7

-entire apparatus must be set up underwater to avoid any air bubbles entering the xylem vessels which may block transpiration stream
-all joints must be sealed properly

Question 8

method to measure transpiration rate

Answer 8

1. leafy shoot is cut at the end and immersed in water
2. cut 1cm of bottom of stem using scalpel underwater
3. potometer is put underwater and the tap of reservoir is turned on to fill the graduated capillary with water until it is full
4. stem is inserted into rubber bung underwater and smeared with petroleum jelly
   5.end of capillary tube immersed in a beaker of water
   6.a single air bubble is introduced into the capillary tube by lifting the end out of water for a short while
5. the tap on the reservoir is opened to add water to push the air bubble back to zero on the scale
6. a timer is started and a set time is measured
7. the distance the air bubble travels along the scale is recorded
8. the experiment can be repeated with different environmental conditions

Question 9

what is the conclusion for a potometer

Answer 9

the faster the bubble moves, the greater the rate of water uptake and so the greater rate of transpiration

Question 10

how to test for factors affecting the rate of transpiration

Answer 10

• temperature - the room temperature may be changed with a heater
• wind speed - this can be tested using a fan at different speeds

Question 11

factors affecting transpiration rate

Answer 11

• temperature
  -wind speed
• humidity

Question 12

effect of temperature

Answer 12

increases - kinetic energy of water molecules increase so they evaporate and diffuse faster from the mesophyll cells

Question 13

effect of wind speed

Answer 13

increases - the wind removes away the water vapour surrounding the leaf faster

Question 14

effect of humidity

Answer 14

decreases - if the air surrounding the leaf has more water vapour, there will be a weak concentration gradient for diffusion

Question 15

how does wilting occur

Answer 15

if the amount of water a plant loses from its leaves is > than amount of water available in the soil to travel into the plant through its roots then wilting will occur

Question 16

effect of wilting

Answer 16

water in plant keeps it turgid to support it but when wilting occurs the plant will become soft and droop because the cell walls become flaccid and will not be able to support the plant

Question 17

why does wilting happen to plant

Answer 17

more water has evaporated from the leaves and there’s not enough water in the soil