topic 9.2- transport in the phloem of plants

Question 1

describe the structure of the phloem (4)

Answer 1

• porous sieve tube plate
• companion cells (folded); undertake metabolic and genetic functions for the sieve cell; many mitochondria
• sieve tube member cell; specialised cell (no organelles)
• plasmodesmata; gaps in cell wall for apoplast pathway

Question 2

why are companion cells folded?

Answer 2

to give a large SA for the transport of sucrose

Question 3

describe movement in the phloem

Answer 3

both ways, contrasts with the xylem

Question 4

state the function of the phloem

Answer 4

to move around products of photosynthesis from sources to sinks

Question 5

give 4 examples of sources

Answer 5

photosynthetic tissues;
- mature green leaves
- green stems
storage organs unloading their stores:
- storage tissues in germinating seeds
- tap roots of tubers at the start of the growth season

Question 6

give 2 examples of sinks

Answer 6

roots growing/absorbing mineral ions
parts of the plant with developing food stores (developing fruits, seeds, leaves, tap roots/tubers)§

Question 7

why is sucrose transported rather than glucose

Answer 7

• it is a non-reducing sugar so will have no immediate reactions with other molecules; glucose is quite reactive so would be taken up by cells on route
• osmotic effect
• sucrose is more of an efficient energy story (disaccharide more ee than monosaccharide)

Question 8

describe translocation

Answer 8

1. Sucrose can be loaded into the phloem at sources such as leaves via the symplastic or apoplastic pathway.
2. In the apoplastic pathway, sucrose is co-transported into companion cells along with hydrogen ions. The concentration of sucrose increases in the companion cells so it diffuses into sieve tubes from the companion cells, through plasmodesmata.
3. In the symplastic pathway, sucrose diffuses from cytoplasm to cytoplasm of neighbouring cells.
4. The high solute concentration causes water to move, by osmosis, into the sieve tubes from xylem vessels.
5. The flow of water into the sieve tubes increases the hydrostatic pressure which causes water to flow to areas of lower pressure, such as sinks. Sinks are areas of the plant which don’t synthesise organic molecules, such as the roots.
6. At sinks, sucrose is unloaded from the sieve tubes, which decreases the solute concentration.
7. This leads to water moving back into the xylem vessels by osmosis and as a result, the hydrostatic pressure in the sieve tubes decreases.

Question 9

describe how sucrose is moved into/out of different cells

Answer 9

sucrose is moved from cells to companion cells and sieve tube elements by diffusion along a concentration gradient, but is moved into companion cells and sieve tube elements by an active process

Question 10

how does phloem loading occur

Answer 10

via the apoplastic pathway

Question 11

state the evidence that backs up the ideas of translocation

Answer 11

• companion cells have membrane folding for a large SA for transport of sucrose
• also have many mitochondria to release the ATP needed for active transport
• if these mitochondria are poisoned, translocation in the phloem stops
• the pH of the companion cells is higher than the surrounding cells, supporting the idea of a hydrogen pump
• the flow of sugars in the phloem is about 10000 times faster than it would be by diffusion alone, suggesting an active process is driving the mass flow

Question 12

Why are sieve elements are unable to sustain independent metabolic activity without the support of a companion cell?

Answer 12

This is because the sieve element cells have no nuclei and fewer organelles (to maximise flow rate)

Question 13

State how the structure of phloem sieve elements and companion cells is adapted to their relationship

Answer 13

Plasmodesmata exist between sieve elements and companion cells in relatively large numbers
These connect the cytoplasm of the two cells and mediate the symplastic exchange of metabolites

Question 14

look at xylem and phloem images of stem and root