Pressure flow hypothesis: if phloem flow is bidirectional, how can one pressure gradient work? Also, transpiration vs translocation?
Lakshmi TripathiDropper Sample
26 Sept 2026 · 1 min read
I don't post often, but this one has been bothering me for two days.
NCERT says the pressure flow hypothesis explains translocation in the phloem. Sugar (sucrose) is loaded actively into the sieve tubes at the source (like a leaf). Water follows by osmosis, so turgor pressure rises. This high pressure pushes the sap towards the sink, where the sugar is unloaded and water goes out, and the pressure drops. Fine, that makes sense to me.
But then NCERT also says translocation can go in both directions, up and down, depending on source and sink. If there's a single pressure gradient in a tube, how can it be in both directions? Is it two different tubes, or does the direction change over time?
Second doubt, which looks silly but isn't: transpiration and translocation sound alike and the MCQ options use them as traps. Can somebody give me a clean contrast (what moves, which tissue, direction, energy)?
Third: which step in the pressure flow needs ATP? I had "all of it" in my notes, which I'm quite sure is wrong.
I prefer to read slowly, so long answers are welcome. Thanks all.
7 answers & replies
Sign in to join the conversation.
[+][–]Ishita GoyalDropperSample· 24d ago
Bidirectional but not in the same tube at the same time. In one sieve tube, flow is in a single direction, from where the sugar is loaded (source) to where it's consumed or stored (sink). Different sieve tubes in the same plant can carry sap in different directions at the same time. Also the same organ can change from source to sink: a tuber is a sink while it is storing starch during growth, but becomes a source when it sprouts and sends sugar to the new shoot.
[+][–]Lakshmi TripathiDropperSample· 23d agoAuthor
That tuber example answers it. Source and sink are jobs a plant part can have at different times.
[+][–]Farhan ShaikhDropperSample· 24d ago
Quick contrast:
Transpiration Translocation What loss of water vapour movement of organic/inorganic solutes (mainly sucrose) Tissue water goes up the xylem and leaves through stomata phloem Direction upward only (water in xylem) any direction (source → sink) Energy passive (driven by the evaporation) loading needs ATP, the flow is passive Remember xylem = one-way up, phloem = source to sink.
[+][–]Kavya NaikDropperSample· 24d ago
ATP is needed for the loading of sugar into the sieve tube at the source (active transport of sucrose, with H+ gradient coupling, which is more detail than you need). Once the loading is done, the water enters osmotically, and the flow itself is by pressure difference, which doesn't need metabolic energy directly. Unloading at the sink can be passive or active depending on the tissue. If you want a single sentence: "loading is active, flow is passive".
[+][–]Mehak AroraDropperSample· 24d ago
Is it true that xylem can also be bidirectional? My friend said so.
[+][–]Rahul DubeyDropperSample· 24d ago
Mehak, no. In xylem the movement is unidirectional, bottom to top, from roots to the tops. Phloem is the one that can be bidirectional (source to sink wherever they are). Also phloem sap is rich in sucrose and the xylem has water and minerals. This one is a very standard 1-mark question.
[+][–]Siddharth RaoDropperSample· 23d ago
A model to picture it: two connected balloons filled with sugar solution at different pressure (the source balloon is pumped hard, the sink balloon is leaky). Sap flows from the pumped one to the leaky one. If you add another pair of balloons somewhere else with the opposite arrangement, flow goes the other way in that pair. That's how one plant can have flow in both directions.
