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Translocation is the movement of sugars, mainly sucrose, through the phloem from sugar sources to sugar sinks. A source is an organ that makes or releases sugar, such as a mature leaf; a sink is an organ that uses or stores it, such as a root, a growing shoot tip or a developing fruit. The pressure-flow hypothesis, proposed by Ernst Münch, explains the movement: loading sugar at the source draws in water and raises pressure, unloading at the sink lets water leave and lowers pressure, and sap flows in bulk from high to low pressure.
Build a two-bag model: a sugar-filled dialysis bag and a water-filled one joined by a tube, both set in water. Water enters the sugar bag, pressure builds, and liquid flows through the tube. Then map the model onto a real plant and ask which organs are sources and which are sinks in different seasons.
A leaf makes sugar, but a root in the dark cannot, and neither can a growing fruit. The plant must move sugar from where it is made to where it is needed. That job belongs to the phloem.
Phloem is made of living cells. The conducting cells are sieve-tube elements, joined end to end through perforated walls called sieve plates. A mature sieve-tube element has no nucleus and few organelles, so sap can flow through it easily. Each one is paired with a companion cell, which keeps its nucleus and supports the sieve-tube element through plasmodesmata. The main sugar carried is sucrose, a disaccharide.
Biologists describe the route in terms of sources and sinks. A source supplies sugar to the phloem. A sink takes sugar out to use or store it. Mature leaves are the usual sources. Roots, shoot tips, flowers, fruits and seeds are common sinks. The same organ can change roles. A storage root fills with starch in summer and acts as a sink. In spring it breaks the starch down and acts as a source for new growth.
The pressure-flow hypothesis, proposed by the German botanist Ernst Münch, explains how sap moves. At the source, sucrose is loaded into the sieve tubes. In many plants this loading is active: proton pumps build a proton gradient, and a cotransporter uses that gradient to move sucrose into the companion cells and sieve tubes. The high sugar concentration lowers the water potential inside the tube, so water enters from the nearby xylem by osmosis. This raises the pressure at the source end. At the sink, sucrose is removed and used or stored. Water then leaves the tube, and pressure drops. The difference in pressure pushes sap in bulk from source to sink, much as water flows through a hose from the tap to the open end.
Several observations support the model. Aphids feed by inserting their mouthparts into a single sieve tube. If an aphid's body is cut away, sap keeps oozing out of the mouthparts, which shows the sap is under pressure. Sap can move at speeds of up to about 1 meter per hour, too fast for diffusion alone. Girdling a tree, which removes a ring of bark and its phloem, stops sugar from reaching the roots.
Xylem and phloem thus complement each other. Xylem moves water up by tension in dead cells. Phloem moves sugar wherever sinks are, pushed by pressure in living cells.
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