mass transport in plants

Question 1

What is the Xylem?

Answer 1

The tissue that transports water in the stem and leaves of plants

Question 2

Describe Xylem structure [3]

Answer 2

• Tough, waterproof lignin walls (with small holes through which water can escape)
• Dead cells
• No end wall between cells

Question 3

Describe the cohesion-tension theory of water transport in the xylem [8]

Answer 3

• How water moves up the xylem against gravity via the transpiration stream
• Water evaporates from the leaves via the (open) stomata due to transpiration
• Reducing water potential in the cell and increasing water potential gradient
• Water drawn out of xylem to the cells
• Creating tension
• Cohesive forces between water molecules pull water up as a column
• Water lost from xylem enters the roots via osmosis
• Water is moving up, against gravity

Question 4

Give evidence of the Cohesion-tension theory [3]

Answer 4

• Cut flowers do not drip water
• Broken xylem no longer takes up water
• Trunk diameter (expands in day to take up more water)

Question 5

What is the Phloem?

Answer 5

the tissue that transports organic substances in plants

Question 6

Describe Phloem Structure [3]

Answer 6

• Made of living tissue
• Sieve tubes (containing sieve plates)
• Companion cells

Question 7

Give evidence for mass flow [2]

Answer 7

• Aphids eat from phloem as it is a good source of sugars
• If strip of bark removed from centre of tree, the bark above or below it would expand due to a build up of solutes

Question 8

What is translocation?

Answer 8

Movement of solute from source to sink in the plant

Question 9

Describe the mass flow hypothesis for the mechanism of translocation in plants [13]

Answer 9

At the source:
- High concentration of solute
- Active transport loads solutes from companion cells to sieve tubes of the phloem
- Lowering the water potential inside the sieve tubes
- Water enters sieve tubes by osmosis from xylem and companion cells
- Increasing pressure inside sieve tubes at the source end
At the sink:
- Low concentration of solute
- Solutes removed to be used up
- Increasing the water potential inside the sieve tubes
- Water leaves tubes via osmosis
- Lowering pressure inside sieve tubes
Mass flow:
- Pressure gradient from source to sink
- Pushes solutes from source to sink
- Solutes used or stored at the sink

Question 10

Give 4 environmental factors which affect the transpiration rate

Answer 10

• Light
• Temperature
• Humidity
• Wind

Question 11

Explain how light affects the transpiration rate [5]

Answer 11

• The higher the light intensity, the faster the transpiration rate
  HIGH LIGHT INTENSITY
• Stomata open in light to let in CO2 for photosynthesis
• Allowing more water to evaporate faster
  LOW LIGHT INTENSITY
• Stomata close when it’s dark as there is no need to let in CO2 for photosynthesis
• So there is a low transpiration rate

Question 12

Explain how temperature affects the transpiration rate [4]

Answer 12

• The higher the temperature, the faster the transpiration rate
• Water molecules gain kinetic energy as temperature increases
• Move faster
• Water evaporates faster

Question 13

Explain how humidity affects the transpiration rate [4]

Answer 13

• The lower the humidity, the faster the transpiration rate
• Because as humidity increases, more water is in the air
• Decreasing the water potential gradient from leaf to air
• Water evaporates slower

Question 14

Explain how wind affects the transpiration rate [5]

Answer 14

• The windier, the faster the transpiration rate
• Wind blows away water molecules from around the stomata
• Decreasing the water potential of the air around the stomata
• Increasing the water potential gradient from leaf to air
• Water evaporates faster