Explain pressure-driven mass flow in phloem
Loading at sources and unloading at sinks create a hydrostatic-pressure gradient that drives phloem sap from source to sink.
AS Level · Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; identifying tissues from real slides, drawing biological sections, supervised practical competence and human subject-teacher review remain necessary.
Before you begin
- Distinguish plant cell walls from cell membranes and recognise that vascular tissues form pathways through roots, stems and leaves.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the main driving difference for phloem mass flow from source to sink?
What you will learn
- Explain how source and sink activities create a hydrostatic-pressure difference
- Describe mass flow in phloem sieve tubes down that pressure gradient
Loading a source raises pressure
Assimilates are loaded into sieve tubes at a source. Their concentration in the sieve-tube solution increases and its water potential becomes lower. Water enters from adjacent xylem by osmosis, increasing hydrostatic pressure at the source end.
At a sink, assimilates are removed from the sieve tube for use, growth or storage. This raises the water potential of the remaining sieve-tube solution relative to the source region; water can leave towards xylem, so hydrostatic pressure is lower near the sink.
Pressure difference drives bulk flow
The source region has higher hydrostatic pressure than the sink region. The pressure gradient drives phloem sap, with dissolved assimilates, by mass flow through sieve tubes from source to sink.
Mass flow is bulk movement of solution under a pressure difference. It is not the same as individual sucrose molecules diffusing all the way from source to sink. The model explains direction and driving gradient; it does not specify a measured transport speed.
Pause and explain
Where is hydrostatic pressure typically higher in the source–sink model?
Worked example
Explain how loading at a leaf source and unloading at a root sink can produce mass flow in phloem.
Show the worked solution
- Loading at the source lowers sieve-tube water potential; water enters by osmosis and pressure rises.
- Unloading at the sink allows water to leave; pressure is lower there, so sap flows down the gradient.
Answer At the leaf source, assimilate loading lowers the water potential of sieve-tube sap, so water enters from xylem by osmosis and raises hydrostatic pressure. At the root sink, assimilates are unloaded and water can leave, giving lower hydrostatic pressure. The resulting pressure gradient drives phloem sap by mass flow from source to sink.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Outline the pressure changes that occur at a source and a sink during phloem mass flow.
Give me a hint
Track assimilates, water and hydrostatic pressure at both ends.
Compare my reasoning
- At the source, loading lowers water potential and water enters by osmosis, raising pressure.
- At the sink, unloading allows water to leave and pressure is lower.
At a source, assimilates are loaded into sieve tubes, lowering water potential; water enters from xylem by osmosis and raises hydrostatic pressure. At a sink, assimilates are removed and water can leave, so hydrostatic pressure is lower.
Look for these in your work
- Source loading and water entry
- Sink unloading and water exit
- Pressure directions correct
Why is pressure-driven mass flow not the same as sucrose diffusing from a leaf all the way to a root?
Give me a hint
Compare bulk solution movement with individual molecule diffusion.
Compare my reasoning
- Mass flow moves the phloem solution under a hydrostatic-pressure gradient.
- Diffusion describes random net movement of particles down a concentration gradient.
Mass flow is bulk movement of phloem sap driven by a hydrostatic-pressure gradient from source to sink. Diffusion is net movement of individual particles down a concentration gradient; it does not describe the whole solution moving through the sieve tube under pressure.
Look for these in your work
- Mass flow defined
- Diffusion distinguished
- Pressure gradient included
At a source the measured phloem pressure is 520 kPa and at its sink it is 390 kPa. State the direction of mass flow and calculate the pressure difference.
Give me a hint
Flow is down the hydrostatic-pressure gradient.
Compare my reasoning
- Compare source and sink pressures.
- Subtract 390 kPa from 520 kPa.
Mass flow is from the source towards the sink, down the hydrostatic-pressure gradient. The pressure difference is 520 − 390 = 130 kPa.
Look for these in your work
- Direction correct
- Subtraction shown
- Unit retained
Common mistakes
- Reversing the source and sink pressure difference
- Calling mass flow the diffusion of each sucrose molecule along the entire tube
- Claiming that water enters at both ends without explaining pressure change
What can you explain now?
What is the main driving difference for phloem mass flow from source to sink?
Compare with the explanation
A hydrostatic-pressure gradient
Sap flows from higher to lower hydrostatic pressure.
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Make it stick
How do loading, osmosis and unloading create a hydrostatic-pressure gradient that drives phloem mass flow?
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