9.2 - transport in the phloem of plants

Question 1

what is the phloem?

Answer 1

long continuous tube of live cells called sieve tube cells.
separated by perforated sieve plates.
each sieve tube cell is closely associated with a companion cell which carries out metabolic functions.

Question 2

what is translocation?

Answer 2

-the movement of solutes can move in both directions.
- it usually goes from a source such as a leaf or stems with excess solutes, to deficient areas, sinks, roots and developing roots.

Question 3

transportation of sucrose

Answer 3

sucrose enters the phloem via phloem loading - either the apoplast or symplast route.

Question 4

what is the apoplast route?

Answer 4

moves THROUGH the cell wall.
1. H+ pumped out of companion cell into surrounding tissue - uses ATP bc its active transport.
2. H+ defuses through a co-transporter with energy transporting sucrose into the phloem.

Question 5

what is the symplast route?

Answer 5

simple diffusion of sucrose through small gaps in the plasma membrane, known as plasmodesmata.
sucrose is converted to an oligosaccharide to maintain concentration gradient.

Question 6

translocation - sucrose

Answer 6

movement of sucrose due to changes in pressure.
1. water is drawn into the phloem from xylem via osmosis.
2. builds hydrostatic pressure in the phloem.
3. sap flows to a lower pressure area down the hydrostatic gradient.
4. sucrose is drawn into sink decreasing osmotic pressure.
5. water moves back into the xylem decreasing hydrostatic pressure.

Question 7

phloem adaptations

Answer 7

folded membrane to increase apoplast route.
rigid cell walls allow pressure gradients to form.
wide plasmodesmata to accommodate oligosaccharides.
many mitochondria for active transport of sucrose.

Question 8

how are aphids used to model phloem transport?

Answer 8

1. inject radioactive carbon into leaves.
2. place aphids along the stem.
3. cut off aphid stylets
4. identify rate of phloem transport
   rate decreases as distance increases.

Question 9

what does the phloem do?

Answer 9

• links parts of plant that need sugars and amino acids to other plants that have extra
• it moves the sugars and amino acids from the source to sinks. (source and sink can swap) - so the sieve tubes transport both up and down the plant
• is an active process

Question 10

what are sources?

Answer 10

photosynthetic tissues:
- mature green leaves
- green stems
storage organs that are unloading:
- storage tissues in germinating seeds
- tap roots or tubers at start of growth season

Question 11

what are sinks?

Answer 11

roots that are growing or absorbing mineral ions using energy through cell respiration.
- developing fruit
- developing seeds
- developing tap roots or tubers
- growing leaves

Question 12

what is hydrostatic pressure?

Answer 12

the pressure in a liquid including water

Question 13

how does hydrostatic pressure occur?

Answer 13

• sucrose and carbs draw water into companion cells by osmisis
• the pressure builds because water cant be compressed and the walls are ridged

Question 14

what is phloem loading?

Answer 14

bringing sugars into the phloem.

Question 15

what does mass flow do at the source?

Answer 15

• after photosynthesis, there is a high concentration of solutes at source (sucrose)
  -sucrose moves into companion cells then into phloem.
• high concentration of solutes (sap solution) in phloem is hypertonic so water uptake from xylem through osmosis.
• buildup of water causes hydrostatic pressure so forces sap solution to move to lower pressure causing mass flow when water and solutes move away from source and to sink.

Question 16

what does mass flow do at the sink?

Answer 16

• sucrose is withdrawn from phloem used as energy or stored as starch.
• sap solution decreases and solutes become hypotonic.
• reduction in osmotic pressure causes water that carried solute, to be drawn out of phloem.
• water goes back to transpiration stream in xylem
• so hydrostatic pressure is lower at sick than source and sap moves from source to sink.

Question 17

why does the phloem have limited cytoplasm and no nucleus?

Answer 17

so that sieve cells can maintain sucrose and organic molecule concentration by active transport.

Question 18

what is the function of the cell wall?

Answer 18

• it is ridged to resist high pressure inside sieve tube and allow for flow of phloem.

Question 19

what is the function of the sieve plates?

Answer 19

• parts of cell wall that separate cells.
• strengthens sieve tube and pores allow sap to pass through in both directions.

Question 20

what is the function of the cell membrane?

Answer 20

• holds sap inside sieve tube
• used to load and unload sucrose

Question 21

what is the function of the lumen of sieve tube?

Answer 21

• space with no organelles to allow for easy flow of sap along phloem.

Question 22

what is the function of the plasmodesmata?

Answer 22

connects cytoplasm of companion cell with adjacent sieve tube cells.

Question 23

what is the function of the companion cell?

Answer 23

• perform metabolic functions of sieve tube.
• have many mitochondria to make ATP for active transport of sucrose.
• has active trans proteins and enzymes to make oligosaccharides.

Question 24

what is the function of the plasma membrane of companion cells?

Answer 24

• infolding to increase phloem loading capacity using apoplastic route.