9.3 Transpiration

Question 1

what is an important adaptation that prevents the leaf cells losing water rapidly?

Answer 1

their surfaces are covered with a waxy cuticle that makes them waterproof

Question 2

why do leaves have a large surface area?

Answer 2

for capturing sunlight and carrying out photosynthesis

Question 3

how does carbon dioxide enter the leaf

Answer 3

from the air

Question 4

how does oxygen leave the leaf?

Answer 4

by diffusion down concentration gradients through microscopic pores in the leaf

Question 5

what is stomata?

Answer 5

microscopic pores in a leaf that oxygen leaves the leaf from

Question 6

how can stomata be opened and closed?

Answer 6

by guard cells

Question 7

what happens when the stomata opens?

Answer 7

allows an exchange of carbon dioxide and oxygen
water vapour also moves out by diffusion and is lost

Question 8

what is the loss of water vapour from leaves called?

Answer 8

transpiration

Question 9

what is the definition of transpiration?

Answer 9

an inevitable consequence of gaseous exchange

Question 10

why do stomata open and close?

Answer 10

control the amount of water lost by a plantw

Question 11

why does some stomata need to be open at all times?

Answer 11

during thr day a plant needs to take in carbon dioxide for photosynthesis and at night when no oxygen is being produced by photosynthesis it needs to take in oxygen for cellular respiration

Question 12

what is the transpiration stream?

Answer 12

moves water up from the roots of a plant to the highest leaves.

Question 13

how does transpiration stream work?

Answer 13

water molecules evaporate from the surface of mesophyll cells into air spaces in the leaf and move out of the stomata into the surrounding air by diffusion down a concentration gradient
the loss of water by evaporation from a mesophyll cell lowers the water potential of the cell, so water moves into the cell from an adjacent cell by osmosis, along both apoplast and symplast pathways
this is repeated across the leaf to the xylem. water moves out of the xylem by osmosis into the cells of the leaf
water molecules form hydorgen bonds with the carbohydrates in the walls of the narrow xylem veseels (adehsion)

Question 14

what is the cohesion-tension theory?

Answer 14

a model of water moving from the soil in a continuous stream up the xylem and across the leaf

Question 15

what evidence is there that support the cohesion-tension theory?

Answer 15

changes in the diameter of trees
when a xylem vessel is broken

Question 16

how is transpiration a problem for the plant?

Answer 16

the amount of water available is often limited
for example : in intense sunlight when the plant is photosynthesising fast there will be a high rate of gaseous exchange, stomata will all be open and the plant may lose so much water through transpiration

Question 17

what process is the stomata opening and closing?

Answer 17

turgor-driven process

Question 18

what happens when turgor is low?

Answer 18

the asymmetric configuration of the guard cell walls close the pores

Question 19

what happens when the environment is favourable

Answer 19

guard cells pump in solutes by active transport increasing their turgory

Question 20

why do stomata become bean shaped

Answer 20

because the inner wall of the guard cell is less flexible than the outer wall

Question 21

how do stomata close?

Answer 21

when water becomes scarce, hormonal signals from the roots can trigger turgor loss from the guard cells

Question 22

what factors affect transpiration

Answer 22

increasing light intensity increases the rate of transpiration
a very high relative humidity will lower the rate of transpiration because of the reduced water vapour potential gradient between the inside of the leaf and air
temperature can affect is two ways -increase in temp increases rate of evaporation
- increasein temp increases concentration of water vapour (both increase diffusion gradient between the air inside and outside the leaf)
air movement
soil-water availabilty

Question 23

what is the difference between transpiration and transpiration stream?

Answer 23

transpiration - evaporation of water from surface of a leaf
transpiration stream - flow of water moved up from soil into root hair and through root cortex by osmosis into xylem and up through stem by cohesion of water molecules cross leaf cells by osmosis and out leaf by evaporation and diffusion

Question 24

compare root pressure and transpiration pull

Answer 24

root pressure - active movement of solutes followed by passive movement of water by osmosis, whcih gives a positive pressure forcing water up the xylem
transpiration pull - pulling of a constant stream of water molecules up xylem held togethe by cohesive forces as a result of evaporation of water from surface of spongy mesophyll cells in leaf

Question 25

how to use a potometer to investigate the effect of air movement on transpiartion rates

Answer 25

all conditions except air movement kept the same, use fan for set time, fan placed at different distances from potometer, control readings in still air allow plant recovery time between different distances