Translocation is the Movement if Dissolved Substances


The Mass Flow Hypothesis Best Explains Phloem Transport


There is Evidence Both For and Against Mass Flow

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  • Created by: Benedicta
  • Created on: 07-01-11 14:15
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There is Evidence Both For and Against Mass Flow
Supporting evidence
1. If you remove ring of bark (includes phloem, but not xylem) from woody stem a bulge forms
above ring
If you analyse the fluid from the bulge, you'll find it has a higher concentration of sugars than
fluid below ring - evidence there's downward flow of sugars
2. Can investigate pressure in phloem using aphids (they pierce the phloem, then their bodies are
removed leaving mouthparts behind, which allows sap to flow out)
Sap flows out quicker nearer the leaves than further down stem - evidence that there's
pressure gradient
3. If you put a metabolic inhibitor (stops ATP production) into phloem then translocation stops -
evidences that active transport is involved
4. There's an experimental model for mass flow
1. Sugar travels to many different sinks, not just one with highest water potential
2. Sieve plates would create barrier to mass flow.
a lot of pressure would be needed for solutes to get through at reasonable rate
Mass Flow Hypothesis Can be Demonstrated in an Experiment
1. A and B are two containers, each lined with partially permeable membrane
2. Top tube connecting A and B represents phloem, bottom tube represents xylem
3. A represents source end and contains concentrated sugar solution. B represents sink end and
contain weak sugar solution
4. Water enters A by osmosis, increasing pressure - causes sugar solution to flow along top tube
5. Pressure increases in b, forcing water out and back through bottom tube (xylem), which just
transports water
03 January 2011
Translocation is the Movement if Dissolved Substances
1. (e.g. sugars like sucrose, and amino acids) to where they're needed in a plant
Dissolved particles sometimes called assimilates
2. An energy-requiring process that happens in phloem
3. Moves substances from sources to sinks
Source of substance is where its made (so high concentration)
Sink is area where its used up (so at low concentration)
EXAMPLE - source for sucrose is leaves, sinks are other parts of plant, such as food storage
organs and meristems (areas of growth) in roots, stems and leaves
4. Enzymes maintain concentration gradient from source to sink by changing dissolved substances
at sink than inside phloem (e.g. by breaking them down or making them into something else)
Makes sure there's always lower concentration at sink than at source

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EXAMPLE - In potatoes, sucrose converted to start in sink areas, so always lower concentration of
sucrose at sink than in phloem. Makes sure constant supply of new sucrose reaches sink from phloem
The Mass Flow Hypothesis Best Explains Phloem Transport
1. Active transport used to actively load dissolved solutes into sieve tubes of phloem at source
2. This lowers water potential in sieve tubes, so water enters tubes by osmosis
3. Creates high pressure at source end of phloem
1.…read more


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