7.8 Transport of organic substances in the phloem

Question 1

What is translocation?

Answer 1

The process by which organic molecules and mineral ions are transported from one part of the plant to another.

Question 2

What is the phloem?

Answer 2

The tissue in flowering plants which transports biological molecules

• made up of end to end long thin structures (sieve tubes)
• walls are perforated to form sieve-tube plates
• products of photosynthesis are moved from sources to sinks by the phloem.
• sieve tube elements are associated with companion cells

Question 3

Why can organic substances not be moved by diffusion?

Answer 3

they move far too quickly, so mass transport is the most widely accepted theory.

Question 4

What is the first phase of mass flow?

Answer 4

The transfer of sucrose into sieve tube elements from photosynthesising tissue.

Question 5

Describe the first phase of mass flow.

Answer 5

1) sucrose is made in cells with chloroplasts from the products of photosynthesis
2) sucrose goes down the concentration gradient by facilitated diffusion from photosynthesising cells into companion cells
3) hydrogen ions are actively transported from companion cells into spaces within cell walls using ATP.
4) hydrogen ions diffuse down the concentration gradient through carrier proteins into sieve tube elements
5) hydrogen ions and sucrose molecules are moved by co-transport into sieve tube elements

Question 6

What is the second stage of mass flow?

Answer 6

flow of sucrose through the sieve tube elements

Question 7

Describe the second stage of mass flow.

Answer 7

1) sucrose from the source is actively transported into the sieve tubes (step 1)
2) this lowers the water potential of the sieve tubes
3) water moves into the phloem from the xylem by osmosis creating hydrostatic pressure
4) At the sink, sucrose is either used in respiration or converted into starch for storage.
5) these cells then have a low sucrose content, so it is easily transported into them by sieve tubes, lowering the water potential
6) hydrostatic pressure is decreased
7) a pressure gradient is created from the source to the sink

Question 8

What is the third phase of mass flow?

Answer 8

Transfer of sucrose from sieve tube elements into storage, or other, sink cells.
- sucrose is actively transported by companion cells, out of sieve tubes and into sink cells.

Question 9

What is the evidence supporting the mass flow hypothesis?

Answer 9

• there is pressure in sieve tubes, shown by sap spilling out when they’re cut.
• concentration of sucrose is higher in the leaves than in the roots
• downward flow in the phloem occurs during the day but not at night
• increases in leaf sucrose levels are shortly followed by an increase in phloem sucrose levels
• metabollic poisons/ lack of oxygen inhibit translocation
• companion cells have lots of mitochondia producing ATP

Question 10

What is the evidence against the mass flow hypothesis?

Answer 10

• sieve plate function is unclear, as they would appear to hinder mass flow
• not all solutes move at the same speed as they should during mass flow
• sucrose is delivered evenly to all areas, rather than faster to those with a low sucrose concentration.