what does the phloem structure consist of?
sieve tube element and companion cells
describe the purpose and adaptions of the sieve tube elements
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
describe the purpose and adaptions of the companion cells
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
what does the phloem do?
phloem carries sucrose from where it is made (source: leaves) to where it is needed or will be stored (sink:roots)
what is the evidence for transport of sucrose in the phloem?
- tree ringing
- use of radioactive tracers
- use of aphids
describe tree-ringing as evidence for transport of sucrose in the phloem
- 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
describe use of radioactive tracers as evidence for the transport of sucrose in the phloem
- 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
describe use of aphids as evidence for the transport of sucrose in the phloem
- 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
what is translocation?
the movement of organic molecules e.g sucrose, amino acids from source to sink along with some mineral ions
whats a source?
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
whats a sink?
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
what is mass flow theory?
- sucrose => sieve tube. Done by active transport, through companion cell.
- this lowers the water potential in the sieve tube, so water is drawn in from xylem (osmosis) (down water potential gradient)
- influx of water inc. hydrostatic pressure
- sucrose is transported out of sieve tube at sink, water leaves by osmosis
- lowers hydrostatic pressure at sink (roots)
- there is mass flow of fluid from source to sink down hydrostatic pressure gradient
evidence for mass flow hypothesis
- sap oozes out when phloem is cut due to pressure in phloem
