Transpiration and Cohesion-Tension theory

Question 1

What is transpiration?

Answer 1

The loss of water vapour from the stomata by evaporation

Question 2

Where are stomata found on plants?

Answer 2

Stomata are found on leaves, specifically on their underbelly

Question 3

What are the four key factors that affect the rate of transpiration?

Answer 3

Light intensity
Temperature
Humidity
Wind

Question 4

What are the three factors that have positive correlations? What is the one factor that has a negative correlation?

Answer 4

Negative correlations: Humidity

Positive correlations: Temperature, Wind and Light intensity

Question 5

LIGHT INTENSITY:
Does it have a positive/negative correlation?
How does it affect transpiration?
Explain why it has an affect on transpiration?

Answer 5

Light intensity has a positive correlation. The more light there is, the more stomata that will open, increasing the rate of transpiration.

This is because, the more stomata that are open allow for a larger surface area for water vapour to evaporate out of.

Question 6

TEMPERATURE:
Does it have a positive/negative correlation?
How does it affect transpiration?
Explain why it has an affect on transpiration?

Answer 6

There is a positive correlation. The higher the temperature, the faster the rate of transpiration.

This is because the more heat applied, increase the kinetic energy of the water molecules. This leads to the water molecules moving faster and the eventual increase in the rate of transpiration

Question 7

HUMIDITY:
Does it have a positive/negative correlation?
How does it affect transpiration?
Explain why it has an affect on transpiration?

Answer 7

There is a negative correlation. The more humid air surrounding the leaf, the lower the rate of transpiration will be.

This is because the more humid air containing water vapour will make the water potential more outside the leaf. Therefore, reducing the water potential gradient and stopping the water from evaporating outwards

Question 8

WIND:
Does it have a positive/negative correlation?
How does it affect transpiration?
Explain why it has an affect on transpiration?

Answer 8

There is a positive correlation. The more air movement, the faster the rate of transpiration.

This is because the air movement will carry away the humid air containing water vapour. Therefore reducing the levels of saturated air and making it possible to maintain the water potential gradient

Question 9

What are the three factors that allow the cohesion -tension theory to work?

Answer 9

The cohesion - tension theory is possible because of cohesion, capillarity (adhesion) and root pressure

Question 10

What is cohesion? Why does it work with water molecules?

Answer 10

Cohesion is the attraction of molecules with each other

Water molecules have good cohesion due to their hydrogen bonds between their hydrogen and oxygen molecules of different water molecules

Question 11

What does the cohesion (sticking together) of water allow it to do in the xylem?

Answer 11

Water cohesion allows water to travel up the xylem as continuous water column.

Instead of water moving one by one as droplets. Water molecules move up the xylem as a continuous water column in cohesion due to the hydrogen bonds between the oxygen and hydrogen molecules.

Question 12

What is adhesion (capillarity)? How does water use adhesion to move up the xylem?

Answer 12

Adhesion is when different molecules are able to bond with each other. Water bonds with the walls of the xylem to move upwards

Question 13

How does the width of the xylem affect the capillarity of water?

Answer 13

When the xylem is narrower, the more capillarity the water has

Question 14

How does root pressure occur? What does root pressure result in?

Answer 14

When water moves up the roots by osmosis, there is an increase in the volume of liquid inside the root. This leads to an increase pressure in the root, known as root pressure.

This increase in pressure in the roots forces water above it upwards, up the root. Positive pressure.

Question 15

What three elements together form the cohesion - tension theory?

Answer 15

Root pressure + Cohesion + Capillarity (adhesion)

Question 16

How does water move up the xylem, with all three components involved?

Answer 16

1. Water evaporates out of the stomata on the leaves. This loss in water volume creates a lower pressure
2. As a result, the water lose through transpiration is replaced. More water is pulled up to xylem to replace the water lost. This movement is due to negative pressure
3. Due to the hydrogen bonds between the water molecules, there is cohesion that occurs that creates a column of water within the xylem, which moves up
4. Water molecules also adhere (stick) to the walls of the xylem, which helps pull the water column upwards
5. As this water is pulled the xylem, there is tension created which pulls the walls of the xylem. Therefore, making it become narrower and increasing the impact of capillarity further

Question 17

What is the process involved in the movement of water up the xylem?

Answer 17

1. Water evaporates from the leaves at the top of the xylem. This process is called transpiration
2. This creates tension (suction), which pulls more water into the leaf
3. Water molecules are cohesive (they stick together) so when some are pulled into the leaves, others follow. This means the whole column of water in the xylem, from the leaves down to the roots, moves upwards
4. Water then enters the stem though the roots