transpiration

Question 1

what is transpiration?

Answer 1

loss of water vapour by evaporation from the aerial parts of the plants (leaves) via stomata

Question 2

why do stomata open during the day?

Answer 2

stomata open during the day to allow gaseous exchange for photosynthesis to occur

Question 3

how do guard cells open?

Answer 3

for guard cells to open:

• potassium ions are pumped into the guard cells
• this lowers the water potential inside the guard cells so water moves into the cell and vacuole via osmosis
• the cell becomes turgid

Question 4

what part of the guard cells structure allows the stomata to be opened?

Answer 4

As the wall beside the pore (inner wall) is thicker than the outer wall, the outer wall bends more opening the stomata

Question 5

how do guard cells close?

Answer 5

when the guard cells lose water the cells become flaccid and the stomatal opening closes. this may occur when the plant has lost an excessive amount of water and loses turgidity. in addition, it generally occurs daily as light levels drop and the use of carbon dioxide in photosynthesis decreases

Question 6

describe the inner wall of guard cells?

Answer 6

thick and inelastic inner wall

Question 7

describe the outer wall of guard cells?

Answer 7

thin and elastic outer wall

Question 8

how are stomata adapted to their role?

Answer 8

stomata have chloroplasts to provide ATP for active transport
have transport proteins in the plasma membrane
have a unevenly thickened inner cell wall

Question 9

in low light what happens to guard cells?

Answer 9

in low light guard cells lose water and become flaccid, causing the stomata to close

Question 10

why do stomata close at night?

Answer 10

stomata close in the dark when no carbon dioxide is needed for photosynthesis and therefore prevent unnecessary water loss

Question 11

describe the transpiration stream?`

Answer 11

Water moves up the xylem vessels
water leaves a xylem vessel through a pit. it may enter the cytoplasm or cell wall of a mesophyll cell
water evaporates from the cell wall into an air space
water vapour diffuses from the air space through an open stomata
water vapour is carried away from the lead surface by air movements

Question 12

how many factors affect the rate of water loss?

Answer 12

6 factors

Question 13

how does number of leaves affect the rate of water loss?

Answer 13

the more leaves the more surface area in which water vapour can be lost

Question 14

how does number and position of stomata affect the rate of water loss?

Answer 14

more stomata means more water vapour loss

if the stomata are on the lower surface, water vapour loss is slower

Question 15

how does temperature affect the rate of water loss?

Answer 15

a higher temperature will increase the rate of water vapour loss because:

• more evaporation from the cell surfaces so the water vapour potential in leaf rises
• increase rate of diffusion through the stomata as water molecules have more kinetic energy

Question 16

how does light intensity affect the rate of water loss?

Answer 16

an increase in light intensity causes more stomata to open

so more water vapour is released

Question 17

how does wind affect the rate of water loss?

Answer 17

air moving outside;

• removes saturated air (water vapour that has just diffused out of the leaf)
• this maintains a high water potential gradient

Question 18

how does humidity affect the rate of water loss?

Answer 18

low humidity when air is dry - there is a steep water potential gradient between leaf and air
so more water vapour is released

Question 19

what happens if the plant loses too much water?

Answer 19

non woody plants will wilt and eventually die

the leaves of wood plants will also wilt and the plant will eventually die

Question 20

what apparatus is used to estimate the rate of the water loss?

Answer 20

the potometer

Question 21

why is the potometer not an exact measure of photosynthesis?

Answer 21

the potometer is not an exact measure of photosynthesis as some of the absorbed water is retained by the plant or is used in biochemical processes such as photosynthesis and making cells turgid

Question 22

however why does the potometer give a reasonably accurate estimate of water loss?

Answer 22

because about 99% of the water taken up is lost in transpiration, it does give a reasonable estimate of water loss

Question 23

what in the potometer is used as a measure of the transpiration rate?

Answer 23

A bubble of air is introduced into the tube and its rate of movement across a scale is used as a measure of the transpiration rate

Question 24

how does the potometer work?

Answer 24

as water evaporates from the leaves, more water is drawn into the xylem vessels that are exposed at the cut end of the stem
water is drawn along the capillary tubing
if you record the position of the meniscus at set time intervals you can plot a graph of distance moved against time
if you expose the plant to different conditions you can compare the rate of water uptake

Question 25

what must be done to ensure the apparatus works properly?

Answer 25

to ensure the potometer works properly you must set it up correctly

Question 26

how do you set up the potometer correctly?

Answer 26

check the shoot is healthy
cut the shoot under water to prevent air bubbles
cut last 2-3cm off cut end and cut at an angle
check there are no air bubbles
apparatus must be watertight
leaves must be dry