9.3 Transpiration

Question 1

What is transpiration?

Answer 1

loss of WATER VAPOUR

through STOMATA

by DIFFUSION DOWN a CONCENTRATION GRADIENT

due to EVAPORATION from cell surfaces.

Question 2

What is a transpiration stream?

Answer 2

MOVEMENT of water through plant

from UPTAKE in ROOTS

to loss by EVAPORATION from leaves

Question 3

Describe the transpiration stream & how water moves up the plant due to physical forces:

Answer 3

1. H2O evaporates from mesophyll cell&raquo_space; DIFFUSES out through STOMATA.
2. lowers ψ of cell&raquo_space; H2O moves in by OSMOSIS from neighbouring cell.
3. Repeats all the way back to xylem. Lost H2O REPLACED by xylem.
4. ADHESION + COHESION create TENSION in xylem&raquo_space; pulled up by CAPILLARY ACTION

Question 4

What is the difference between COHESION + ADHESION?

Answer 4

Cohesion = attraction BETWEEN H2O MOLECULES

Adhesion = attraction BETWEEN H2O MOLECULES + WALLS OF CONTAINER

Question 5

What is the cohesion-tension theory?

Answer 5

MODEL stating that water moves up XYLEM

by CAPILLARY ACTION

Question 6

How does cohesion-tension come about?

Answer 6

EVAPORATION of water from leaf

TENSION in xylem

cohesion + adhesion&raquo_space; CAPILLIARITY

Question 7

What evidence is there for cohesion-tension?

Answer 7

change in diameter of trees depending on day&raquo_space; diameter is smallest when transpiration rate is highest

xylem vessel broken&raquo_space; air DRAWN IN

xylem vessel broken&raquo_space; continuous column broken, NO more UPTAKE

Question 8

Why is the change in diameter of trees evidence for cohesion-tension?

Answer 8

during day = high rate of transpiration + diameter skrinks

suggests high TENSION in xylem

during night = low rate of transpiration + diameter increases

suggests less tension in xylem

Question 9

After the xylem has been broken, air is drawn in.

Why is this evidence for cohesion-tension?

Answer 9

If no tension pulling water up, water would spill out of the cut end due to gravity.

Since instead drawn into xylem, shows capillary motion from cohesion-tension.

Question 10

After the xylem has been broken, there is no more uptake.

Why is this evidence for cohesion-tension?

Answer 10

Air bubble introduced&raquo_space; breaks CONTINUOUS COLUMN of water&raquo_space; no more uptake.

Suggests transpiration stream relies on CAPILLIARITY created by COHESION-TENSION.

Question 11

What is capillary action?

Answer 11

movement of liquid AGAINST GRAVITY

as a CONTINUOUS COLUMN

due to COHESION + TENSION

Question 12

Why is transpiration higher during the day?

Answer 12

higher temperature due to sun’s rays

more evaporation&raquo_space; more water requiring replacement&raquo_space; higher rate of transpiration

Question 13

Why does increased transpiration change the diameter of trees?

Answer 13

￪ transpiration

￪ rate of flow of water in transpiration stream

￪ cohesion-tension forces in core of tree shorten diameter

Question 14

What are the factors affecting transpiration?

Answer 14

LIGHT INTENSITY

TEMPERATURE

HUMIDITY

AIR MOVEMENT

Question 15

How does LIGHT INTENSITY affect transpiration?

Answer 15

￪ light intensity = ￪ rate of transpiration

- faster rate of PHOTOSYNTHESIS
- More O2 produced (requiring release by stomata)
- STOMATA are open more often 
- ∴ more water lost by TRANSPIRATION

Question 16

How does TEMPERATURE affect transpiration?

Answer 16

￪ temperature = ￪ rate of transpiration

- H2O molecules have more KINETIC ENERGY 
- more EVAPORATION
- ∴ more water lost by EVAPORATION

Question 17

How does HUMIDITY affect transpiration?

Answer 17

￪ humidity = ￬ rate of transpiration

- less steep WATER POTENTIAL GRADIENT
- less water lost by TRANSPIRATION

Question 18

How does AIR MOVEMENT affect transpiration?

Answer 18

Faster winds = ￪ rate of transpiration

- faster winds remove surrounding water molecules in surrounding air
- maintain STEEP WATER POTENTIAL GRADIENT 
- ∴ more water lost by transpiration