module 3 - 9.3 transpiration

Question 1

what is transpiration?

Answer 1

loss of water vapour from leaves through evaporation

Question 2

what happens during transpiration?

Answer 2

• water enters leaves & goes into mesophyll cells by osmosis
• water evaporates from leaves to form water vapour
• large air spaces between mesophyll cells allow water vapour to collect & diffuse through leaves (makes WP ↑)

Question 3

how do plants control their water intake?

Answer 3

opening/ closing stomata caused by water levels changing in guard cells

Question 4

how is water pulled up the plant?

Answer 4

• water evaporates from leaves & moves into air spaces from adjacent cells
• water moves out of xylem into cells in leaves
• water hydrogen bonds to itself & walls of xylem vessel causing capillary action

Question 5

why is it called the ‘cohesion tension theory’?

Answer 5

water hydrogen bonds with itself (cohesion) and bonds to walls of xylem vessel (tension)

Question 6

where does water move (to do with hydrostatic pressure)?

Answer 6

area of high hydrostatic pressure to area of low hydrostatic pressure

Question 7

what is the cohesion tension theory?

Answer 7

• water is polar so in xylem, water molecules spontaneously arrange, oppositely charged poles next to each other
• causes molecules to cohere (stick together) so some leave plant by transpiration, some are pulled up behind them

Question 8

what evidence is there for the cohesion tension theory relating to changes in tree diameter/ time of day?

Answer 8

• at high transpiration rates (in daytime) diameter decreases
• low transpiration rates (at night) diameter increases
  (both due to tension)

Question 9

what evidence is there for the cohesion tension theory relating to cut flowers?

Answer 9

often they draw air in as water moves up cut stem (rather than leaking water out)

Question 10

what evidence is there for the cohesion tension theory relating to broken xylems?

Answer 10

broken/cut xylems stop drawing up water as air drawn in breaks transpiration stream - cohesion between water molecules

Question 11

what is the transpiration stream made up of?

Answer 11

cohesion between water molecules

Question 12

what is root pressure?

Answer 12

when a plant can’t transpire, water can be transported by positive pressure (root pressure) from below

Question 13

what happens when the stomata are turgid/open?

Answer 13

• water moves into vacuoles by osmosis
• outer wall more flexible than inner wall - cell bends and opens/ closes stomata

Question 14

what happens when the stomata are flaccid/ closed?

Answer 14

• water moves out of the vacuoles by osmosis
• outer wall more flexible than inner walls - cell bends back and closes stomata

Question 15

what has to happen to the guard cells for the stomata to close?

Answer 15

they must lose water and become flaccid

Question 16

what has to happen to the guard cells for the stomata to open?

Answer 16

they must gain water by osmosis and become turgid to open the pore

Question 17

during daylight, what does blue light activate?

Answer 17

activates enzyme ATPase within guard cells

Question 18

what does ATPase do?

Answer 18

catalyses hydrolysis of ATP, generating energy required to operate a proton pump

Question 19

what happens if hydrogen ions are actively pumped out of the guard cells?

Answer 19

the protons re enter the cells by carrier-assisted mechanism that simultaneously transports chloride ions into guard cells

Question 20

what does the blue light, ATPase and hydrogen (proton) pumps result in?

Answer 20

the entry of potassium ions

Question 21

what does increased ion concentration within guard cells create (when opening stomata)?

Answer 21

gradient of water potential that draws water into guard cell - swells and stomatal pore opens

Question 22

what happens in the absence of light when opening/ closing stomata?

Answer 22

proton gradient can’t be maintained: ions & water flow in reverse direction & flaccid guard cells cause stomata to close