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 the other

Question 2

What is the phloem

Answer 2

Tissue that transports biological molecules

Question 3

What is the phloem made up of

Answer 3

Sieve tube elements, long thin structures arranged end to end

Their end walls are perforated to form sieve plates.

Question 4

What are associated with sieve tube elements

Answer 4

Cells called companion cells

Question 5

What happens to sugars produced during photosynthesis

Answer 5

Plants transport them from the site of production (soures) to the places where they will be used or stored for later use - known as Sinks

Question 6

What are sources

Answer 6

The site of production of sugars

Question 7

What are sinks

Answer 7

The site of sugar use, whether thats in respiration or storage

Question 8

Where can sinks be

Answer 8

Anywhere in the plant. Above or below the plant, therefore the translocation of molecules can occur in either direction

Question 9

What type of organic molecules are transported during translocation

Answer 9

Sucrose and Amino acids

Question 10

What type of mineral ions are transported during translocation

Answer 10

Chloride, magnesium, Potassium and phosphate ions

Question 11

What are the 3 stages of the mass-flow theory

Answer 11

1. Transfer of sucrose into sieve elements from tissue
2. Mass flow of sucrose through sieve tube elements
3. Transfer of sucrose from sieve tube elements into storage or sink cells

Question 12

Describe the Transfer of sucrose into sieve elements from photosynthesising tissue

Answer 12

1. Sucrose is manufactured from the products of photosynthesis in cells with chloroplasts
2. The sucrose diffuses down a concentration gradient by facilitated diffusion from the photosynthesising cells into companion cells
3. Hydrogen ions are actively transported from companion cells into spaces within cells walls using ATP
4. These hydrogen ions then diffuse down a concentration gradient through carrier proteins called co-transport proteins into the sieve tube elements
5. Sucrose molecules are transported along with the hydrogen ions in co-transport

Question 13

Describe the mass flow of sucrose through sieve tube elements

Answer 13

1. The sucrose produced by photosynthesising cells is actively transported into the sieve tubes, as described before
2. This causes the sieve tubes to have a lower water potential
3. As the xylem has a much higher water potential, water moves from the xylem into the sieve tubes by osmosis, creating a high hydrostatic pressure within them
4. At the respiring (sink) cells, sucrose is either used up during respiration or converted to starch for storage. These cells therefore have a low sucrose content and so sucrose is actively transported into them from the sieve tubes, lowering their water potential
5. Due to this lowered water potential, water also moves into these respiring cells, from the sieve tubes, by osmosis
6. The hydrostatic pressure of the sieve tubes in this region is therefore lowered
7. As a result of water entering the sieve tube elements at the source, and leaving at the sink, there is a high hydrostatic pressure at the source and a low one at the sink
8. There is therefore a mass flow of sucrose solution down this hydrostatic gradient into the sieve tubes.

Question 14

What is mass flow

Answer 14

The bulk movement of a substance through a given channel or area in a specified time

Question 15

Describe the transfer of sucrose from the sieve tube elements into storage or sink cells

Answer 15

The sucrose is actively transported by companion cells, out of the sieve tubes and into the sink cells.

Question 16

What evidence supports the mass flow theory

Answer 16

1. There is pressure in the sieve tubes, as shown by sap being released when cut
2. The concentration of sucrose is higher in the leaves than in roots
3. Downward flow through the phloem occurs in daylight, but ceases at night
4. Increases in sucrose concentration in the leaves are followed by similar increases in sucrose levels in the phloem later
5. Metabolic poisons/lack of O2 inhibit translocation
6. Companion cells possess many mitochondria and readily produce ATP

Question 17

What evidence questions the mass flow theory

Answer 17

1. The sieve plates function is unclear - as it is likely they hinder mass flow. However there are suggestions they have a structural function
2. Not all solutes move at the same speed, which should happen if it was ‘mass flow’
3. Sucrose is more or less delivered to all regions of the plant at the same speed, rather than quicker areas with low sucrose concentration