Translocation and the Mass Flow Hypothesis
Sugars are loaded into the phloem at a source, water follows by osmosis, and the pressure built there pushes sap towards a sink. The hypothesis is judged against ringing experiments and tracer evidence.
What a learner can do afterwards
- Separates the loading step, the pressure step and the unloading step.
- Explains why phloem transport needs energy while xylem transport does not.
- Weighs one piece of evidence for the hypothesis against one difficulty with it.
1 · Read
Translocation moves sugary sap from sources to sinks through the phloem. Sources are usually leaves making sugar, while sinks are roots, fruits or buds that consume or store it. Keep three steps separate in your head: loading at the source, pressure driven flow through the tubes, and unloading at the sink.
The pressure gradient is built with water. Sucrose is actively loaded into the sieve tubes at the source, water follows by osmosis, and pressure rises there. At the sink, sucrose is unloaded for use or storage, water leaves too, and pressure falls. Sap therefore flows downhill in pressure from source to sink, and different tubes can run different ways as sources and sinks shift with the season.
That loading step is why phloem transport needs energy while xylem transport does not. Pushing sucrose in against its gradient burns ATP, supplied by companion cells packed with mitochondria. The middle flow step is passive, and xylem water simply rides the transpiration pull with no loading cost at all.
Judge the mass flow hypothesis like an examiner: one strength against one weakness. Aphids tapped into phloem ooze sap under pressure, ringing a stem stops flow below the ring, and tracers fed to a leaf turn up in the roots. But sieve plates between tube elements look like they should choke the flow, and different substances can travel opposite ways at once. Weigh one piece of evidence against one difficulty and you answer evaluation questions the way examiners want.
Loading builds pressure at the source, sap flows to the sink, and evidence and difficulties are weighed together.
More to read
30.5 Transport of Water and Solutes in Plants
Translocation moves sugary sap from sources to sinks through the phloem, and the mass flow hypothesis explains how. First, sucrose is actively loaded into the sieve tubes at the source leaf. Water follows by osmosis, which raises the pressure there. At the sink, sucrose is unloaded for use or storage, water leaves too, and pressure falls. Sap therefore flows downhill in pressure from source to sink.
Keep the three steps separate in your head: loading, pressure driven movement, unloading. The middle step is passive, but the loading step at the start burns ATP. That single energy cost is the deepest difference between phloem and xylem transport.
30.5 Transport of Water and Solutes in Plants
The hypothesis survives because the evidence lines up on its side. Aphids piercing a stem to drink phloem sap show the contents are under pressure, exactly as predicted. Cut a ring of bark to remove the phloem and sugars pile up above the cut while tissues below starve, proving the downward route. Radioactive tracers fed to a leaf turn up in the roots, moving at speeds that fit bulk flow.
Honest science also notes the difficulties. Sieve plates between tube elements look like they should choke the flow, yet sap moves fast. Different substances can travel in opposite directions within one stem, which a single pressure gradient cannot easily explain. Weigh one strength against one weakness like this and you answer evaluation questions the way examiners want.
30.2 Stems
Stems provide the plumbing the hypothesis needs. Rings of vascular bundles carry xylem on the inside and phloem on the outside, running unbroken from leaves to roots. The phloem's sieve tube elements, stacked end to end with companion cells alongside, form living pipelines built for bulk flow.
Seeing the tissue makes the mechanism concrete. Companion cells, packed with mitochondria, supply the ATP that loads sucrose into the tubes. Sources are usually photosynthesising leaves, while sinks are roots, fruits, or growing buds that consume or store sugar.
2 · Watch
به انگلیسیTake it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.