Cohesion-Tension Theory

Question 1

What is transpiration?

Answer 1

The loss of water vapour from the stomata by evaporation

Question 2

What factors affect transpiration?

Answer 2

Temperature
Light intensity
Wind
Humidity

Question 3

How does light intensity affect transpiration?

Answer 3

Positive correlation since the more light there is the more open the stomata are so the larger the surface area for evaporation

Question 4

How does temperature affect transpiration?

Answer 4

Positive correlation since more heat means more kinetic energy so molecules move quicker resulting in more evaporation

Question 5

How does humidity affect transpiration?

Answer 5

Negative correlation since more water vapour in the air means there is a higher water potential outside the leaf which reduces the water potential gradient

Question 6

How does wind affect transpiration?

Answer 6

Positive correlation since more wind will blow away humid air which therefore maintaining the water potential gradient

Question 7

What 3 characteristics make up the cohesion tension theory?

Answer 7

• cohesion
• capillarity/adhesion
• root pressure

Question 8

How does cohesion affect the cohesion-tension theory?

Answer 8

• water is dipolar so hydrogen bonds can form between hydrogens and oxygens of different water molecules
• creates cohesion between molecules so water will travel up the xylem as a continuous water column

Question 9

How does capillarity/adhesion affect the cohesion-tension theory?

Answer 9

• water adheres to the xylem walls

- the narrower the xylem the bigger the impact of capillarity

Question 10

How does root pressure affect the cohesion-tension theory?

Answer 10

• as water moves into the roots by osmosis the pressure in the roots increases due to an increase in liquid
• increase in root pressure forces water above it upwards

Question 11

How does water move up the xylem?

Answer 11

• water evaporates out of stomata lowering the pressure
• when water is lost though transpiration more replaces it though the xylem
• a column of water is created in the xylem due to cohesion
• adhesion to xylem walls helps pull water column upwards
• xylem becomes narrower due to tension created by pulling water column up