Sucrose enters the phloem through an active process
ATP used by companion cells to actively transport H+ions out of cytoplasm
Sets up diffusion gradient for H+ions to diffuse back into companion cells
Diffusion occurs through co-transporter proteins: allow hions to bring back sucrose moleclues
Conc of sucrose increases in companion cell, diffuse through plasmodesmata into seive tube elements
At source, the water potentional in seive tube element reduces due to high conc of sucrose
Water molecules move into seive tube from surrounding cells by osmosis
Hydrostatic pressure increases
At sink, cells surrounding seive tube use sucrose for metabolic processes/starch
Reduces conc in cells
Sucrose moves out by diffusion/active transport to surrounding cells
This increases water potentional in seive tube, water moves out by osmosis to surrounding cells
Decreases hydrostatic pressure
Mass flow: The flow of water, from source to sink moving down a hydrostatic pressure gradient, which carries assimulates along phloem
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Evidence for translocation
Leafs: Produce sugars by photosynthesis, loaded in phloem once converted to starch, act as source, in summer. In early spring, need energy provided by stores in plant, act as sink
Roots: Stored carbohydrtaes released into phloem, in spring, source, During summer, store sugars as starch, sink
How do we know its Phloem?
if supplied with radioactively labelled CO2, labelled c appears in phloem
Ringing tree to remove bark (phloem in it), sugars collect above ring
Aphid feed on plant stem, mouth parts take food from phloem
How do we know its metabolic energy/ ATP?
Companion cells = lots of mitochondria
matabolic poison stops production of ATP= Translocation stop
How do we know its this mechanism?
pH of companion cells (hions)= higher than surrounding, conc of sucrose higher at source
Evidence against mechanism?
Not all solutes in phloem sap move at same rate
Sucrose moves to all parts of plant at same rate, rather than faster to areas of low conc
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