- 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
- role of seive plates unclear
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