Transport in plants

Question 1

What does the xylem do?

Answer 1

Transports water and mineral ions in a solution. Substances move up the plant from roots to leaves

Question 2

Describe the xylem vessels

Answer 2

Long, tube like structures formed from dead cells with no end walls

Question 3

Explain the cohesion - tension theory

Answer 3

Helps water to move up plants against the force of gravity:
1. water evaporates from leaves at the top of the xylem
2. this creates tension pulling more water into the leaf
3. water molecules are cohesive which means that when some are pulled others follow
4. water enters through roots

Question 4

What is transpiration?

Answer 4

Evaporation from a plants surface:
- water evaporates from moist cell walls and accumulates in spaces between cells in leaf
- stomata open which moves leaf down the concentration graident

Question 5

What 4 factors increase the rate of transpiration?

Answer 5

High light intensity
High temperatures
Low humidity
High winds

Question 6

What does the phloem do?

Answer 6

Transports organic substances up and down the plant

Question 7

What do companion cells do?

Answer 7

Carry out living functions for sieve cells

Question 8

What do sieve tubes do?

Answer 8

Form a tube for transporting substances

Question 9

What is translocation?

Answer 9

Movement of solutes to where they are need in a plant (requires energy)

Question 10

What does the mass flow hypothesis help to explain?

Answer 10

How solutes are transported from source to sink by translocation

Question 11

Explain the mass flow hypothesis

Answer 11

1. Active transport actively loads solutes from companion cells into sieve tubes at the source.
2. A decrease in water potential means water enters tubes by osmosis. This increases the pressure in sieve tubes at the source end.
3. At the sink end, solutes are removed. This increases the water potential in the sieve tubes so water leaves by osmosis. This decreases pressure.
4. This results in a pressure gradient from the source to sink
5. Gradient pushes solutes along sieve tubes towards the sink. When they reach the sink the solutes are used or stored.

Question 12

Explain the arguments for mass flow hypothesis

Answer 12

• radioactive tracer can track movements of organic substances
• metabolic indicators
• removing a ring of bark
• aphids

Question 13

Explain the arguments against mass flow hypothesis

Answer 13

• sugar travels to many different sinks
• sieve plates would create a barrier to mass flow