9.3 transpiration

Question 1

What are the stages of transpiration

Answer 1

Water moves by osmosis from the xylem to mesophyll cell in the leaf
Water evaporates from the surfaces of the spongy mesophyll into air spaces
Water vapour diffuses out of the leaf from the air spaces through the stomata down the conc. gradient
The stomata remain open during the day to allow for gas exchange enabling photosynthesis
Since the stomata are open, water vapour is lost so transpiration is a consequence of gaseous exchange

Question 2

Define transpiration

Answer 2

Loss of water vapour
Occurs through evaporation from leaf surface and through stomata

Question 3

Components of the transpiration stream

Answer 3

Transpiration stream: movement of water from roots to leaves
Capillary action
Transpiration pull

Question 4

What’s capillary action

Answer 4

Water rising through narrow tubing against gravity

Needs cohesion + adhesion

Adhesion: Formation of H-bonds between water molecules and carbs in xylem walls

Cohesion: Formation of H-bonds between water molecules

Question 5

What’s transpiration pull

Answer 5

Water moving out of xylem creating a pull to replace loss of water vapour from the leaves
Since water is cohesive it moves as on body under tension, which pulls the water up the stem
known as cohesion-tension theory

Question 6

Factors affecting transpiration

Answer 6

Light intensity - light needed for photosynthesis - stoma open, at night, no photosynthesis, stoma close
Relative humidity - amount of water vapour in air
High humidity = low rate of transpiration due to decreased water vapour gradient vice versa
Temperature - increases evaporation so there will be a higher water potential in leaf
Temp rise - also increases amount of water external air can hold before saturation, decreasing humidity and increasing water vapour gradient
Air movement - if there is more wind this blows water vapour (trapped in the hairs at the surface of the leaf) increasing the water vapour gradient
Soil-water availability - if there is not enough water in soil, plant will be under water stress reducing rate of transpiration

Question 7

What happens when the stomata are turgid

Answer 7

When conditions are favourable guard cells actively pump solutes in increasing turgor
Cellulose hoop stop cells from swelling widthways and they extend lengthways
Inner cell wall less flexible than outer making a bean shape
If water becomes scare - hormonal signals from roots trigger guard cell water loss closing the aperture.

Question 8

What happens when the stomata are flaccid

Answer 8

Turgor pressure is low (guard cells are flaccid)

Pore closes

Question 9

Evidence for cohesion tension theory

Answer 9

Change in tree diameter - rate of transpiration is highest during the day, xylem vessel tension is highest - tree diameter shrinks
At night transpiration rate is low and the tension on xylem is low, increasing tree diameter
Xylem vessel damage - air is drawn into xylem, plant can’t transpire, continuous water stream is broken