mass transport in plants

Question 1

What is the function of the xylem

Answer 1

The xylem tissue transports water and mineral ions in solution up the plant from the root to the leaf

Question 2

Describe the cohesion-tension theory in transpiration

Answer 2

• Water evaporates from the leaves via the open stomata due to transpiration
• Reducing the water potential in the cell and increasing the water potential gradient
• Water is drawn out of the xylem creating tension
• Cohesive forces between water molecules pull water up as a column
• Water lost enters roots via osmosis
• Water is moving up, against gravity
• Water is adhesive so it sticks to the edges of the column

Question 3

Describe the adaptations of the xylem

Answer 3

1. Elongated cells arranged end to end to form a continuous column
2. Hollow due to lignification so there is no cytoplasm/nucleus to slow water flow
3. Thick cell walls with lignin make it rigid so it’s less likely to collapse under low pressure
4. Waterproof preventing water loss
5. Pits allow lateral water movement
6. Narrow lumen increases height water can rise due to cohesion tension and capillary action

Question 4

List the factors affecting transpiration

Answer 4

1. Light
2. Temperature
3. Wind
4. Humidity

Question 5

Describe how light affects the rate of transpiration

Answer 5

The lighter it is the faster the transpiration rate because the stomata open when it gets light to let in CO2 for photosynthesis, when its dark the stomata is closed so there’s little transpiration

Question 6

Describe how temperature affects the rate of transpiration

Answer 6

The higher the temperature the faster the transpiration rate, warmer water molecules have more energy so they evaporate from cells inside the leaf faster increasing the concentration gradient between the inside and outside of the lead, making water diffuse out of the leaf faster

Question 7

Describe how humidity affects the rate of transpiration

Answer 7

The lower the humidity, the faster the transpiration rate, if the air around the plant is dry the concentration gradient between the leaf and the air is increased, which increases the transpiration rate

Question 8

Describe how wind affects the rate of transpiration

Answer 8

The windier it is the faster the transpiration rate, lots of air movement blows away water molecules from around the stomata which increases the concentration gradient, which increases the rate of transpiration

Question 9

What is the function of the phloem?

Answer 9

Phloem tissue transports solutes sugars (like sucrose) around plants

Question 10

What are sieve tube elements?

Answer 10

They are living cells that form the tube for transporting solutes, they have no nucleus and a few organelles

Question 11

What are companion cells

Answer 11

There is a companion cell for each sieve tube element, they carry out living functions for sieve cells such as providing energy needed for the active transport of solutes

Question 12

What is translocation

Answer 12

The movement of solutes like sucrose and amino acids where they are needed in a plant

Question 13

Describe the mass flow hypothesis of translocation

Answer 13

1. Sucrose made by photosynthesis moves out of the source cell through facilitated diffusion via carrier proteins
2. Companion cells have a low concentration of sucrose so they will use active transport to move it into the phloem, this requires energy and the hydrolysis of ATP
3. Water moves by osmosis from the xylem to the phloem via pits
4. There is a high hydrostatic pressure at the top of the phloem and low hydrostatic pressure at the bottom of the phloem so there is a high hydrostatic pressure gradient
5. This moves the water and sucrose to the bottom of the phloem
6. There is a lower concentration of sucrose in the phloem, sucrose moves from the phloem to the companion cell via active transport

Question 14

What are the adaptations of the phloem

Answer 14

Sieve tube elements have no nucleus and no organelles so they have companion cells which carry out the living functions for the sieve cells

Question 15

Describe the use of tracers as evidence for the mass flow hypothesis

Answer 15

A radioactive tracer such as carbon can be used to track the movement of organic substances in a plant

Question 16

Describe the use of ringing as evidence for the mass flow hypothesis

Answer 16

• If a ring of bark is removed from a wood stem, a bulge forms above the ring
• The fluid from the bulge has a higher concentration of sugars than the fluid from below the ring
• This is evidence that there’s a download flow of sugars

Question 17

Describe the use of aphids as evidence for the mass flow hypothesis

Answer 17

• Aphids pierce the phloem, their bodies are removed leaving the mouthparts behind which allows the sap to flow out
• The sap flows out quicker nearer the leaves than further down the stem which is evidence for the pressure gradient

Question 18

Describe the use of metabolic inhibitors as evidence for the mass flow hypothesis

Answer 18

If a metabolic inhibitor that stops ATP production is put into the phloem translocation stops - this is evidence active transport is involved

Question 19

What are the objections against the mass flow hypothesis

Answer 19

1. Sugar travels to many different sinks, not just to the one with the highest water potential, as the model suggests
2. The sieve plates would create a barrier to mass flow, a lot of pressure would be needed for the solutes to get through at a reasonable rate