Phloem

Question 1

Structure + Function

Answer 1

• transports organic substances like sugars both up and down the plant
• transports organic solutes around the plant
• phloem formed from cells arranged in tubes
• sieve tube elements are living cells that form the tube for transporting solutes
• no nucleus and few organe;;es
• companion cells for each
• carry out living functions
• providing energy needed for active transport of solutes

Question 2

translocation

Answer 2

• movement of solutes to where they’re needed in a plant
• energy requiring process that happens in phloem
• moves solutes from sources to sink
• source = where assimilates are produced = high conc
• sink = where assimilates are used up = lower conc
• enzymes maintain a concentration gradient from source to sink by changing the solutes at sink making sure they always at lower concentration

Question 3

Source

Answer 3

• active transport used to actively load the solutes from companion cells into sieve tubes of phloem at the source
• lowers water potential inside sieve tubes
• water enters tubes by osmosis from the xylem and companion cells
• creates a high pressure inside the sieve tubes of source end of phloem

Question 4

sink

Answer 4

solutes removed from phloem to be used up

• increase water potential inside sieve tubes so water also leaves the tubes by osmosis
• lowers the pressure inside the sieve tubes

Question 5

flow

Answer 5

• pressure gradient from, source end to sink end
• this gradient pushes solutes along sieve tubes towards the sink when it reaches the sink solutes can be used or stored
• higher conc of sucrose at source higher rate of translocation

Question 6

supporting evidence

Answer 6

• ring of bark removed from woody stem a bulge forms above the ring. fluid from the bulge has a higher conc of sugars then the fluid from below the ring so sugars can’t move past the area where bark has been removed so this shows evidence that it can be a downward flow of sugars
• pressure in phloem can be investigates using aphids by piercing phloem then their bodies are removed leaving the mouthparts behind which allows sap to flow out the sap flows out quicker nearer leaves them further down the stem which is evidence for pressure gradient
• radioactive tracer such as radioactive carbon can be used to track movement of organic substances in plants
• metabolic inhibitor put into phloem translocation stops so its evidence active transport is involved

Question 7

objections

Answer 7

• sugar travels to many different sinks not just one with the highest water potential
• sieve plates would create a barrier to mass flow a lot of pressure would be needed for solutes to get through a reasonable rate