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7.5 Transport of assimilates - Coggle Diagram
7.5 Transport of assimilates
Phloem structure
Sieve tube elements
Living cells, unlike xylem vessels
Perforated end plates reduces resistance to flow
No nucleus, little cytoplasm to reduce resistance to flow
Line up to form a sieve tube
Companion cells
Much like normal plant cell
Increased number of mitochondria and ribosomes
Very close to sieve tube elements
Many plasmodesmata connecting companion cells and sieve tube elements
Phloem sap
Water
Sucrose
Some mineral ions
Movement of phloem sap
Active process
Phloem loading
Sucrose produced at sources such as leaves via photosynthesis
Sucrose is dissolved in water and moves towards phloem tissue
May move via symplast pathway, through cytoplasm and plasmodesmata
May move via apoplast pathway, through cell walls
Companion cell pumps H+ ions into cell walls
Uses proton pump
Active transport
Creates concentration gradient
H+ ions diffuse back into companion cell via symporter transport proteins, and bring sucrose with them
Sucrose moves against concentration gradient into companion cells, as the symporters need both H+ ions and sucrose to work
Sucrose can then diffuse into sieve tube via plasmodesmata connecting companion cell with sieve tube element