T Flashcards
(10 cards)
Translocation
The movement of dissolved products from photosynthesis from a source to a sink
(Active process)
Products : Organic Substances
Sucrose
Amino acids
Hormones
Source cells
Organs that perform photosynthesis ( Pallisade mesophyll)
Storage organs that contain starch that can be readily hydrolysed
Roots which absorb minerals
Sink cells
Cells in parts of a plant actively growing
Roots (High mitotic index)
Exam tip : (Specific)
Vague references to sugar being transported to “the sink” can lose marks in exams. Always aim to describe the movement of assimilates from or to specific groups of cells, e.g.:
photosynthetic cells in a leaf are a source
dividing cells in a growing root tip would be a sink
Phloem structure (Can be compared to xylem)
Tube of elongated LIVING cells - sieve tube
No nucleus / organelle only thin layer of cytoplasm to reduce flow resistance
Companion cells help as they contain lots of mitochondria to produce ATP for active transport
Sieve plates allow passage of dissolved substances between cells
Mass flow hypothesis ( Transportation of sucrose )
1) Companion cells use ATP to actively transport to pump H+ ions out source cell and then H+ ions are co-transported back in down their concentration gradient with sucrose molecules.
2) Sucrose molecules diffuse through companion cell and diffuse into sieve tubes through carrier proteins both by facilitated diffusion
3) Sucrose in the phloem decreases water potential so water from xylem or neighbouring tissue moves into phloem near source cell by omosis
4) This increase hydrostatic pressure and generates a hydrostatic pressure gradient between source cell and sink cells so sucrose moves towards sink cell down pressure gradient
5) Sucrose moves into sink cell by facilitated diffusion down concentration gradient while this is happening water potential increases again and moves back into xylem by osmosis
6) DONE :D
Sieve plates
End walls between cells have sieve plates - Allow solutes through
Evidence for mass flow theory
Removal of phloem prevents passage of sugar
Puncture problem - Sap comes out due to positive pressure
Water in by osmosis at source and out by osmosis at sink
Metabolic inhibitors stop translocation - ATP needed for mass flow
Evidence against mass flow theory
Sieve plates act as a slight barrier - No reason to have evolved them
Amino acids travel more slowly than sucrose at the phloem when they should travel at the same rate
Substances flow in opposite directions but should only be flowing in one direction (Source to sink)
