translocation

Question 1

what does the phloem structure consist of?

Answer 1

sieve tube element and companion cells

Question 2

describe the purpose and adaptions of the sieve tube elements

Answer 2

purpose: to allow movement of organic molecules
adaptions: thin cytoplasm and few organelles
- cells connected to end with perforations in end cell wall
these reduce obstructions for flow of substances

Question 3

describe the purpose and adaptions of the companion cells

Answer 3

purpose: to carry out the metabolic purposes for sieve tube cell + load up sieve tube with substances to be transported
adaptions: dense cytoplasm, so lots of organelles

Question 4

what does the phloem do?

Answer 4

phloem carries sucrose from where it is made (source: leaves) to where it is needed or will be stored (sink:roots)

Question 5

what is the evidence for transport of sucrose in the phloem?

Answer 5

• tree ringing
• use of radioactive tracers
• use of aphids

Question 6

describe tree-ringing as evidence for transport of sucrose in the phloem

Answer 6

• involves removing an outer layer of bark (only removes phloem)
• swelling will occur above the ringing since sucrose cannot pass down so accumulates above the ringing
• tissues below the ringing die since the sucrose cannot arrive at these tissues

Question 7

describe use of radioactive tracers as evidence for the transport of sucrose in the phloem

Answer 7

• use CO2 radiolabelled with ^14C
• 14^C is fixed in photosynthesis into organic molecules which now contain 14^C
• these radioactive organic molecules can be tracked throughout the plant

Question 8

describe use of aphids as evidence for the transport of sucrose in the phloem

Answer 8

• penetrate phloem tissue with stylet
• anaesthetise and sever the stylet
• contents of phloem flow out and can be collected - sucrose and other organic substances detected

Question 9

what is translocation?

Answer 9

the movement of organic molecules e.g sucrose, amino acids from source to sink along with some mineral ions

Question 10

whats a source?

Answer 10

where sucrose is loaded into the phloem (by companion cells), site where sucrose is produced e.g photosynthesis on the leaves or converted from stored products e.g hydrolysis of starch in the roots

Question 11

whats a sink?

Answer 11

where sucrose is unloaded from phloem, site where sucrose is used in metabolic processes (respiration) or converted to starch for storage (condensation reaction) transport of organic molecules needs to be bi-directional both up and down

Question 12

what is mass flow theory?

Answer 12

• sucrose is actively transported by the companion cells using ATP into the phloem sieve tube element at the source
• this decreases the water potential in sieve tube
• therefore water moves into phloem by osmosis from the xylem into phloem
• this increase in volume in sieve tube causes an increase in hydrostatic pressure in the sieve tube element
• sucrose is actively transported out of the phloem at the sink and increases the water potential in phloem
• water moves out of the sieve tube by osmosis and decreases the hydrostatic pressure
• therefore there is a hydrostatic pressure gradient from the source to the sink
• water and dissolved sucrose moves down the sieve tube from higher hydrostatic pressure at the source to lower hydrostatic pressure at the sink by mass flow

Question 13

evidence for mass flow hypothesis

Answer 13

• sap oozes out when phloem is cut due to pressure in phloem