Q.Explain pressure flow hypothesis of translocation of sugars in plants.

The pressure flow (mass flow) hypothesis, proposed by Ernst Munch, explains the translocation of sugars (mainly sucrose) through the phloem's sieve tubes as follows:

• At the source (e.g., a photosynthesising leaf), sugars manufactured by photosynthesis are actively loaded into the sieve tube elements of the phloem, using energy (ATP) — typically via a proton-sucrose co-transport mechanism.
• This active loading raises the solute concentration inside the sieve tubes at the source, lowering their water potential.
• Water then moves into the sieve tubes at the source by osmosis from the adjacent xylem, increasing the hydrostatic (turgor) pressure inside the sieve tubes at the source end.
• At the sink (e.g., a root, fruit, or growing tissue), sugars are continuously unloaded (removed) from the sieve tubes, either for immediate use or storage, which raises the water potential inside the sieve tubes there (since solute concentration falls), and water then tends to leave the sieve tubes at the sink, lowering the pressure there. …

The pressure flow hypothesis, also called the mass flow hypothesis, was proposed by the German plant physiologist Ernst Munch to explain how sugars (chiefly sucrose) synthesised in one part of the plant are transported (translocated) through the phloem to other parts of the plant that require them.

Step-by-step mechanism:

1. Loading of sugar at the source: In a photosynthetically active tissue such as a mature leaf (the "source"), sugars produced during photosynthesis are actively transported ("loaded") into the sieve tube elements of the phloem. This loading is an active process, requiring ATP — typically achieved via a proton (H⁺)-sucrose co-transport mechanism, in which protons pumped out of companion cells by an ATP-driven proton pump re-enter along with sucrose through a co-transporter protein.
2. Lowering of water potential at the source: As sugar concentration builds up inside the sieve tube elements at the source, the solute potential there becomes more negative, which lowers the overall water potential of the sieve tube contents at that end.
3. Osmotic entry of water at the source: Because the water potential inside the sieve tubes at the source is now lower than that of the adjacent xylem (which is typically carrying relatively dilute sap), water moves into the sieve tubes by osmosis from the xylem, increasing the volume and, since the sieve tube is a relatively confined, walled structure, this raises the hydrostatic (turgor/pressure) potential within the sieve tubes at the source end.
4. Unloading of sugar at the sink: At the destination tissue — the "sink" — such as roots, developing fruits/seeds, or actively growing regions, sugars are continuously removed ("unloaded") from the sieve tubes, either to be used immediately in respiration/growth or converted to storage forms (such as starch).
5. Rise in water potential at the sink: As sugar is removed at the sink, the solute concentration (and hence the negative solute potential) inside the sieve tubes there decreases, so the water potential inside the sieve tubes at the sink rises relative to the source end.
6. Osmotic exit of water at the sink: Water then tends to move out of the sieve tubes at the sink end (into surrounding cells or back into the xylem for recirculation), which lowers the hydrostatic pressure in the sieve tubes at that end. …

Walk through the mechanism in strict causal sequence: active sugar loading at the source → lowered water potential → osmotic water entry → raised pressure at source; sugar unloading at the sink → raised water potential → osmotic water exit → lowered pr …

• Describing the entire process as 'active' — only the loading (and sometimes unloading) step at the membrane requires ATP; the actual bulk flow along the sieve tubes is passive, driven by the pressure gradient.
• Omitting the role of water entering/leaving via the adjacent xylem at the source/sink ends, which is essential to building and releasing the pressure. …