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strand 3 (part 4) - Coggle Diagram
strand 3 (part 4)
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translocation
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Tracer experiment - Radioactive isotopes such as
14C used to label organic compounds. Subsequent autoradiography shows labelled compounds are in the phloem.
The bark of a tree contains the active phloem if you remove a ring of bark the downward movement of organic compounds stops and it accumulates above the ring.
Sap-sucking insects
– mouthpart is found to be in the phloem, removing sap from the plant. Organic compounds are then found in the mouthpart.
structure of phloem
Sieve tube elements - Stacked one on top of another. No nucleus, thin layer of cytoplasm around large central ‘vacuole’, few organelles, P-protein in cytoplasm. Large pores in end walls (sieve plates) with cytoplasm passing from element to element. Smaller pores in side walls with strands of cytoplasm (plasmodesmata) running through to adjacent.
Companion cells - Smaller cells with nucleus, cytoplasm and organelles. Have relatively large number of mitochondria. Each controls the activity of the adjacent sieve tube element. Many plasmodesmata in lateral walls.
process
loading from source
In the leaves, solutes (sucrose) pass from photosynthesising cells into the companion cells by facilitated diffusion. H+ actively transported from companion cells into spaces within cell walls. H+ diffuse with sucrose into sieve tube elements through co-transporters.
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unloading to a sink
As sucrose is pushed down a pressure gradient, it becomes surrounded by cells with a lower sucrose concentration. Sucrose moves from the phloem into these cells, lowering their water potential. Water then moves from the phloem into the xylem and is re-circulated. Cells in the sink use the sucrose or store it.