1.2.3 - Transport in Plants (Transpiration and Xerophytes) Part 17 & 18

Question 1

Describe how a low availability of water can affect transpiration rate.

Answer 1

• A low availabilty of water means the water that has been lost cannot be replaced.
• The plant tries to reduce transpiration rates by closing some of its stomata or by shedding leaves.

Question 2

List as many factors as possible that affect the rate of transpiration.

Answer 2

• Number / size of leaves
• Number / size / position of stomata
• Presence of a waxy cuticle
• Light
• Temperature
• Humidity
• Wind
• Water availability

Question 3

Describe how a potometer can be used to estimate transpiration rates.

Answer 3

• As water is lost from the leaves, it is replaced by the water in the capillary tube.
• The meniscus at the end of the tube moves upwards as water is replaced.
• This movement can be measured with a scale.
• The more it moves, the water is replacing the water lost. The more water lost means a higher transpiration rate.
• Ensure there are no air bubbles inside the apparatus.

Question 3

List some features that allow xerophytes to adapt to dry conditions.

Answer 3

• They may have smaller / needle-like leaves.
• They have a densely-packed mesophyll.
• A thicker waxy cuticle.
• They may close their stomata.
• They may have hair on the surface of their leaves.
• They may have pits containing stomata.
• Leaves are rolled.
• A high salt concentration is maintained in the plant.

Question 4

Define the term ‘transpiration’.

Answer 4

Transpiration - The loss of water by evaporation from the upper parts of the plant such as the leaves.

Question 5

Describe how marram grss is adapted to living on sand dunes.

Answer 5

• Leaf is rolled up to trap air with high water vapour potential.
• Thick waxy cuticle to reduce water evaporation from the surface.
• Hairs on the lower surface to trap more air.
• Stomata in pits to trap air saturated with moisture.

All these features reduce water loss by transpiration, allowing for the marram grass to survive in dry conditions.

Question 6

How does light affect the rate of transpiration?

Answer 6

• In light, the stomata opens.
• This provides an easy route for water vapour to diffuse through.
• The higher the light intensity, the more stomata will open, the higher the rate of transpiration.

Question 6

Explain how maintaining a high salt concentration in a xerophyte helps it survive in dry conditions.

Answer 6

• A high salt concentration means a low water potential gradient between the cells and spaces in the leaves.
• This means the amount of water evaporated from the cell surfaces is reduced, also reducing the water potential between the leaf and the air.

Question 7

Explain how the rolling of leaves reduce the water loss by transpiration.

Answer 7

• When the leaves of xerophyte are rolled, this reduces the amount of air and wind exposed to the lower epidermis.
• This means less water vapour can diffuse out into the air.

Question 9

How does temperature affect the rate of transpiration?

Answer 9

• A higher temperature increases the rate of evaporation.
• This causes a high water vapour potential in the leaf, making the difference in water vapour potential steeper between the leaf and the air.
• This makes water vapour diffuse out quicker.
• The water vapour molecules will also have more kinectic energy to move faster.

Question 10

Explain how water leaves from xylem into the air surrounding the leaves.

Answer 10

• Water enters the xylem in the leaves and passes to the mesophyll cells by osmosis.
• The water evaporates from the surface of the mesophyll cells.
• The water vapour collects in the air spaces between the spongy mesophyll, increasing the water vapour potential.
• Once the potential is higher than the surrounding air, the water vapour diffuses out of the leaves through the open stomata.

Question 12

Explain why transpiration is a consequence of gaseous exchange.

Answer 12

• Plants exchange gases with the atmosphere via their stomata.
• Their stomata opens during the day for photosynthesis.
• Here, the stomata provides an easy route for the water vapour from the spongy mesophyll to diffuse out.

Question 13

Describe how the presence of a cuticle affects the rate of transpiration.

Answer 13

A waxy cuticle which is impermeable to water prevents evaporation from the surface. This decreases the transpiration rate.

Question 14

Explain how hairs on the surface of the leaves of a xerophyte helps it to survive in dry conditions.

Answer 14

• The hairs traps a layer of air around the leaves.
• In this layer of air, moisture becomes saturated. This reduces the water potential between the leaves and the air surrounding the leaves, preventing diffusion.
• This means less water is lost by transpiration.