Transport In Plants

Question 1

How is a water potential gradient established across the mesophyll cells of a leaf?

Answer 1

Mesophyll cells lose water by evaporation due to heat from the sun, these cells have a lower water potential so water enters via osmosis from neighbouring cells, this is turn occurs with other neighbouring cells. So water moves along this gradient.

Question 2

What causes cohesion tension in the water of the xylem?

Answer 2

Water molecules are cohesive so form a column of water in the xylem being pulled up by transpiration, this is the transpiration pull.
This puts the xylem under tension, there is negative pressure in the xylem results in cohesion-tension.

Question 3

What evidence is there for the cohesion-tension theory?

Answer 3

• Diameter of tree trunks changes due to changes in the rate of transpiration.
• If a xylem is broken and air enters, the tree is no longer able to draw up water so the continuous column is broken.
• When a xylem is broken, air does not leak out, air is drawn in.

Question 4

What is the source?

Answer 4

Site of sugar production.

E.g. The leaf.

Question 5

What is the sink?

Answer 5

Site of sugar use or storage.

E.g. Roots

Question 6

What does the phloem transport?

Answer 6

The products of photosynthesis or organic substances to other parts of the plant.
E.g. Sucrose and glucose

Question 7

What direction does translocation occur?

Answer 7

Either.

Question 8

What is phloem loading?

Answer 8

Sugar made from photosynthesis going from the lead to the phloem. There’s a high concentration gradient from the source.

Question 9

Why does water enter the phloem?

Answer 9

Lower water potential in the phloem.

Question 10

Why does water return to the xylem from the phloem when travelling down?

Answer 10

Low water pressure at the sink.

Question 11

How do the sugars go into the sink?

Answer 11

Via active transport, travels through companion sink cells and is stored in the vacuole as starch.

Question 12

Why does water flow occur?

Answer 12

Transpiration.

Question 13

What’s the favoured explanation for translocation?

Answer 13

Mass flow theory

Question 14

How is sucrose transferred into sieve elements from the photosynthesising tissue?

Answer 14

Sucrose diffuses down a concentration gradient by facilitated diffusion from the source into companion cells. Hydrogen ions diffuse down a concentration gradient through carrier proteins into the sieve tube elements. The molecules and ions are transported by co-transport by active transport.

Question 15

What is the mass flow theory of Sucrose though sieve tube elements?

Answer 15

Sucrose is actively transported from source to phloem sieve tubes, so they have a lower water potential causing water from the xylem to enter by osmosis, creating a high hydrostatic pressure within them.
At the sink, sucrose is used up by respiration of stored as starch.
These cells have a low sucrose concentration so move into them lowering the water potential of the sieve tubes causing water to go back into the xylem and lowering the hydrostatic pressure.
So there’s a mass flow of sucrose solution down this hydrostatic gradient in the sieve tubes.

Question 16

What evidence supports mass flow theory?

Answer 16

• There’s pressure within sieve tubes, which is seen when cut.
• The conc. of sucrose is higher in the source than the sink.
• Flow of the phloem occurs in daylight but ceases at night.
• Increases in sucrose levels in the leaf are followed by similar increases in sucrose levels in the phloem later.
• Lack of oxygen inhibit translocation of sucrose in the phloem.
• Companion cells possess many mitochondria and readily produce ATP.

Question 17

What is the evidence against mass flow theory?

Answer 17

• Function of sieve plates is unclear, as they seem to hinder mass flow. (May help prevent bursting under pressure).
• Not all solutes move at the same speed, but should if by mass flow.
• Sucrose is delivered more or less at the same rate to all regions, not by concentration gradient, which mass flow would suggest.

Question 18

What causes movement of water out of the xylem?

Answer 18

The humidity of the atmosphere is usually lesson than the air spaces next to the stomata so there is a water potential gradient. Steam diffuses out of the air space by a passive process.

Question 19

How is transpiration affected by increasing temperature?

Answer 19

Transpiration increases with temperature due to increased rate of evaporation.

Question 20

What effect does increasing wind speed have on transpiration?

Answer 20

The wind has to be storing enough to blow away the barrier layer of water before it is able to increase the rate of transpiration.

Question 21

What effect does increased light intensity have on transpiration?

Answer 21

Light stimulates photosynthesis and the opening of stomata to take in CO2 but simultaneously let out water, so transpiration increases however as there is a limited number of stomata, the rate levels off.