9.3 Transpiration

Question 1

Where does photosynthesis mostly take place?

Answer 1

In the leaves

Question 2

How does Carbon Dioxide enter the leaf?

Answer 2

Diffuses into the leaf cells down a concentration gradient from air spaces within the leaf

Question 3

Gas exchange in leaves

Answer 3

Carbon Dioxide diffuses into leaves from air spaces within the leaf
Oxygen also moves out of the leaf cells into the air spaces by diffusion down a concentration gradient

Question 4

Why is the waxy cuticile important?

Answer 4

Makes the leaf waterproof and reduces water loss by evaporation from the surface of the leaf

Question 5

What controls the opening and closing of stomata?

Answer 5

Guard cells

Question 6

What is transpiration?

Answer 6

The loss of water vapour from the leaves and stems of plants

Question 7

Transpiration stream

Answer 7

Water molecules evaporate from the surface of mesophyll cells into air spaces in the leaf and move out of the stomata into air by diffusion
Loss of water by evaporation from a mesophyll cell lowers the water potential of the cell so water moves into the cell from an adjacent cell by osmosis, along both apoplast and symplast pathways
This is repeated across the leaf to the xylem, water moves out of the xylem by osmosis into the cells of the leaf
Due to adhesion and cohesion properties of water, it shows capillary action, so rises up the xylem in a continuous stream to replace water lost by evaporation
This results in a tension in the xylem, which helps move water across the roots from the soil

Question 8

Adhesion of water molecules

Answer 8

Water molecules form hydrogen bonds with carbohydrates in the walls of the narrow xylem vessels

Question 9

Cohesion of water molecules

Answer 9

Water molecules form hydrogen bonds with each other to stick together

Question 10

What is capillary action?

Answer 10

Process by which water rises up a narrow tube against the force of gravity

Question 11

What is the transpiration pull?

Answer 11

Water being drawn up the xylem in a continuous stream to replace the water lost by evaporation

Question 12

Evidence for cohesion-adhesion theory

Answer 12

Changes in the diameter of trees - when transpiration is at maximum during the day, tension in xylem vessels is at its highest, so diameter shrinks. During night, low transpiration, so tension is lower and diameter increases

When xylem vessel is broken - air is drawn into the xylem than water leaking out. If air is absorbed, plant can’t move water up the stem as the continuous stream of water molecules held together by cohesive forces has been broken

Question 13

Why does water uptake give a good indication of transpiration?

Answer 13

As around 99% of the water taken up by a plant is lost by transpiration, water absorbed gives a good model

Question 14

How is rate of water uptake measured?

Answer 14

Using a potometer

Question 15

Why are all joints of a potometer sealed with waterproof jelly?

Answer 15

Makes sure any water loss measured is as a result of transpiration from the stem and leaves

Question 16

Rate of water uptake formula

Answer 16

Distance moved by the air bubble/time taken for air bubble to move that distance
Unit = cm/s

Question 17

Why is the stem cut under water?

Answer 17

Avoid air bubbles being introduced into the stem

Question 18

What happens to guard cells when environmental conditions are favourable?

Answer 18

Guard cells pump in solutes by active transport, which increases turgor
Cellulose hoops prevent the cells from swelling in width, so they extend lengthways
Stomata are opened

Question 19

How are the guard cells able to change shape according to turgor?

Answer 19

Inner wall of the guard cell is less flexible and thicker than the outer wall
The cells become bean shaped and open the pore

Question 20

Factors affecting transpiration

Answer 20

Light intensity 
Humidity 
Temperature 
Air movement 
Soil water availability

Question 21

How does light intensity affect transpiration?

Answer 21

Increased sunlight means increased photosynthesis
This increases the number of open stomata
More water vapour diffuses out
Increased light intensity = increased rate of transpiration

Question 22

How does humidity affect transpiration?

Answer 22

High humidity - reduce WP gradient between surrounding air and the leaf
Reduces rate of water loss
High humidity = low rate of transpiration

Question 23

How does temperature affect transpiration?

Answer 23

Increase in temperature increases kinetic energy of water molecules - increased evaporation from the spongy mesophyll into air spaces of the leaf
Increase in temperature increases concentration of water vapour that the external air can hold before it becomes saturated
Both factors increase diffusion gradient therefore increasing rate of transpiration

Question 24

How does air movement affect transpiration?

Answer 24

Each leaf has a layer of air around it trapped by the shape of the leaf
Water vapour that diffuses out the leaf accumulates here so the water vapour potential around the stomata increases - decrease diffusion gradient
Increased air movement = increased rate of transpiration

Question 25

How does soil water availability affect transpiration?

Answer 25

If soil is very dry, plant will be under water stress and the rate of transpiration will be reduced