Transpiration

Question 1

What is transpiration?

Answer 1

Transpiration is the loss of water vapour from the stomata and that is by evaporation.

Question 2

How are the stoma made?

Answer 2

When the guard cells are turgid, they become bent creating the stoma.

Question 3

What are stoma?

Answer 3

Pores between guard cells.

Question 4

What 4 factors affect the rate of transpiration?

Answer 4

Light intensity, humidity, temperature and wind.

Question 5

Does light intensity increase or decrease the rate of transpiration?

Answer 5

Increases

Question 6

Does humidity increase or decrease the rate of transpiration?

Answer 6

Decreases

Question 7

Does wind increase or decrease the rate of transpiration?

Answer 7

Increases

Question 8

Does temperature increase or decrease the rate of transpiration?

Answer 8

Increases

Question 9

Explain how light intensity increases the rate of transpiration.

Answer 9

• Stomata open in light to let in CO2 for photosynthesis
• This gives a larger surface area for evaporation allowing more water to evaporate faster
• Stomata close when it’s dark so there is a low transpiration rate.

Question 10

Explain how wind increases the rate of transpiration.

Answer 10

• Wind blows away water molecules from around stomata
• Decreasing water potential of air around stomata
• Increasing water potential gradient so water evaporates faster

Question 11

Explain how temperature increases the rate of transpiration.

Answer 11

• Water molecules gain kinetic energy as temperature increases
• So water evaporates faster

Question 12

Explain how humidity decreases the rate of transpiration.

Answer 12

• More water in air so it has a higher water potential
• Decreasing water potential gradient from leaf to air
• Water evaporates slower

Question 13

What is the function of the xylem?

Answer 13

Transports water and mineral ions through the stem, up the plant to leaves of plants

Question 14

Suggest how xylem tissue is adapted for its function.

Answer 14

• Cells joined with no end walls forming a long continuous tube → water flows as a continuous column
• Cells contain no cytoplasm / nucleus → easier water flow / no obstructions
• Thick cell walls with lignin → provides support and prevents water loss.
• Pits in side walls → allow lateral water movements

Question 15

What is the cohesion-tension theory made up of?

Answer 15

Cohesion
Adhesion
Root Pressure

Question 16

Explain the cohesion-tension theory of water transport in the xylem.

Answer 16

In the leaf:
1.Water lost from the leaf by transpiration - water evaporates from mesophyll cells into air spaces and water vapour diffuses through open stomata
2. Reducing water potential of mesophyll cells
3. So water drawn out of xylem down a water potential gradient
In the xylem
4. Creating tension (‘negative pressure’) in xylem
5. Hydrogen bonds result in cohesion between water molecules (stick together) so water is pulled up as a continuous column
6. Water also adheres (sticks to) to walls of xylem
In the root:
7. Water enters roots via osmosis

Question 17

Describe cohesion.

Answer 17

Water is a dipolar molecule.
This allows hydrogen bonds to form between the hydrogen and oxygen of different water molecules.
This creates cohesion between water molecules - they stick together. Therefore water travels up the xylem as a continuous water column.
Cohesion is the sticking together of water molecules.

Question 18

Describe adhesion.

Answer 18

Adhesion of water is when water sticks to other molecules. Water adheres to the xylem walls.
The narrower the xylem, the bigger the impact of capillarity.

Question 19

Describe root pressure.

Answer 19

As water moves into the roots by osmosis, it increases the volume of liquid inside the root and therefore the pressure inside of the root increases. This is known as root pressure.
This increase in the pressure in the roots forces water above it upwards (positive pressure).