Describe translocation in phloem (pressure-flow)

WHAT is translocation?

WHY does anything move? (the driving idea)

Xylem uses transpiration pull (negative pressure). Phloem is different: it is a living, positive-pressure system. The engine has two coupled tricks:

Active loading of sugar at the source (needs ATP).
Osmosis doing the rest for free.

HOW it works — step by step (derive the mechanism)

Let $\Psi$ = water potential, $\Psi_s$ = solute (osmotic) potential, $\Psi_p$ = pressure potential.

At the SOURCE:

Sucrose is loaded into companion cells / sieve tubes by active transport (often a proton–sucrose cotransport driven by an $H^+$ gradient made by ATP). Why active? Sugar must go against its concentration gradient into an already sugary tube.
Loading makes $\Psi_s$ very negative → total $\Psi$ drops.
Water enters from nearby xylem by osmosis → volume rises → turgor pressure $\Psi_p$ rises. Why? The sieve tube is a closed-ish pipe; adding water with nowhere to go builds pressure.

At the SINK: 4. Sucrose is unloaded (used in respiration, or converted to starch which is osmotically inactive). 5. $\Psi_s$ rises (less solute) → water leaves by osmosis back to xylem → turgor pressure $\Psi_p$ falls.

In between: 6. Now $\Psi_p(\text{source}) > \Psi_p(\text{sink})$ . This pressure gradient drives bulk (mass) flow of the whole sap solution from source to sink.

Figure — Describe translocation in phloem (pressure-flow)

Evidence that supports pressure-flow

Common mistakes (steel-manned)

Recall

Recall Feynman: explain to a 12-year-old

Imagine two water balloons joined by a straw. You keep squirting sugar-water into the first balloon (the leaf). Sugar is thirsty, so it sucks water in — the balloon swells and gets tight and high-pressure. The second balloon (the root) is having its sugar taken out, so it lets water leave and goes floppy and low-pressure. A tight balloon always pushes into a floppy one, so the sugary water whooshes down the straw from leaf to root. The plant just keeps loading sugar at one end and taking it out the other, and the flow never stops. Turn it around (root feeds the shoot in spring) and the water whooshes the other way!

Flashcards

What is translocation?

The movement of organic solutes (mainly sucrose) through the phloem from a source to a sink.

Define a source in translocation.

A region where sugar is made or released (e.g. photosynthesising leaf, mobilising storage organ).

Define a sink.

A region where sugar is used or stored (roots, fruits, growing tissues, tubers).

Why is sugar loading at the source described as active transport?

Sucrose moves against its concentration gradient into the phloem, requiring ATP (often via H⁺–sucrose cotransport).

What happens to water potential in the sieve tube when sucrose is loaded?

It falls (becomes more negative) because solute lowers water potential.

Why does water enter the phloem at the source?

Osmosis: the lowered water potential draws water in from the neighbouring xylem.

What directly drives phloem flow?

A hydrostatic (turgor) pressure gradient, high at the source and low at the sink → bulk/mass flow.

Write the water potential equation.

Ψ = Ψs + Ψp (solute potential + pressure potential).

Why can't sugar just diffuse from source to sink?

Diffusion over metres is far too slow; bulk flow moves the whole solution together, quickly.

How does the aphid stylet experiment support pressure-flow?

Sap oozes out of a cut sieve tube under its own pressure, proving positive hydrostatic pressure exists in phloem.

Are phloem sieve tube elements living or dead?

Living (but enucleate), supported by living companion cells that supply ATP.

Why is phloem transport bidirectional across a plant while xylem is one-way?

Flow follows the pressure gradient toward any sink, so direction depends on source/sink location, not gravity.

Give an example of a tissue that switches from sink to source.

A potato tuber: sink in summer (stores starch), source in spring (feeds new shoot).

Connections

Water potential (Ψ = Ψs + Ψp)
Osmosis and turgor pressure
Xylem and transpiration — contrast: dead cells, negative pressure, one-way
Active transport and proton pumps — sucrose loading mechanism
Photosynthesis — makes the source sugar
Companion cells and sieve tube structure
Sources and sinks in plants

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, phloem me sugar transport ko translocation kehte hain, aur iska mechanism hai pressure-flow hypothesis. Idea simple hai: source (jaise patti, jahan photosynthesis se sugar banti hai) par sucrose ko phloem sieve tube me active transport se load kiya jaata hai — matlab ATP kharch hota hai, kyunki sugar apne gradient ke against ja rahi hai. Jab sugar andar aati hai, tube ka water potential neeche gir jaata hai (zyada negative), aur osmosis se paani xylem se andar ghus aata hai. Paani andar aaya, jagah nahi, toh pressure (turgor) badh jaata hai.

Ab sink par (jaise root, fruit, ya growing shoot) sugar bahar nikal li jaati hai — use ho jaati hai ya starch me convert. Solute kam, toh water potential badh gaya, paani wapas bahar chala jaata hai, aur wahan pressure kam ho jaata hai. Toh ek taraf high pressure, doosri taraf low pressure — aur high se low ki taraf poora ka poora sugary liquid bulk flow me beh nikalta hai. Ye diffusion nahi hai, ye pure solution ka ek saath bahna hai, isliye fast aur long-distance hota hai.

Important baat: phloem me flow do-taraf ho sakta hai — hamesha neeche nahi. Jahan sink hoga, wahan flow jaayega. Spring me potato tuber source ban jaata hai (starch todkar sugar bhejta hai new shoot ko), toh flow ulta ho jaata hai. Yaad rakho ki xylem dead cells se banta hai aur negative pressure (transpiration pull) use karta hai, jabki phloem living cells (sieve tube + companion cell) aur positive pressure use karta hai. Aphid stylet experiment isko prove karta hai — sap khud hi pressure se bahar nikalti hai. Bas ye chain yaad rakho: Load karo → potential Lower → paani Lure → pressure Lift → sink par Let-go — aur flow chalta rehta hai.

Test yourself — Plant Biology

Connections