BIOG 1105-1106 | Cornell Introductory Biology, Individualized Instruction

Long-distance transport in the phloem is a pressure-driven system. It results from high pressure in the source-end phloem and lower pressure in the sink-end phloem. How is the pressure in the source generated? Osmotically. In the minor vein phloem of mature leaves, a very high concentration of sugar is established in the sieve elements (S.E.) and companion cells. As always, water follows the sugar. This generates tremendous pressure (> 300 psi), which is resisted by the cell walls.

Sucrose is produced in mesophyll cells. How does this sugar get into the phloem? There are probably two or more mechanisms, but the following is a common one. Sucrose diffuses from one mesophyll cell to another through plasmodesmata. (Remember that diffusion always occurs downhill--from a higher to a lower concentration.) Somewhere in the vicinity of the minor vein phloem, the sucrose exits the cells and enters the cell wall space. Since the cell walls are outside the plasma membrane, they are not part of the true living substance of the tissue. Why does the sucrose leave the cells? Because, if it is outside the cells it can be taken up again, into the companion cells and sieve elements, across the plasma membrane, and since the plasma membrane has carriers capable of active transport, this uptake can take place against a concentration gradient. This builds up the extraordinarily high concentration of sucrose found in the phloem and keeps the concentration in the walls low to encourage diffusion from the mesophyll.

Here's how the pump works. It is secondary in the sense that ATP is not used directly. Instead, ATP is used to pump protons against a concentration gradient from the inside of the cell to the cell wall space. The pH of the cell walls is about 5, compared to 7 inside the cell. The high concentration of protons can now be used as a driving force. On the plasma membrane of the companion cell there are sucrose-proton cotransport proteins. As the protons flow along the downhill concentration gradient through the carriers, they bring sucrose with them. In this way the concentration gradient of protons is used to generate a concentration gradient of sucrose in the opposite direction.

This type of cotransport system is quite common in nature. The carrier proteins are, as you might expect, quite specific. The ones described here will transport sucrose but not glucose or other sugars. Note the similarity between proton-cotransport in plants and sodium-cotransport systems in animals.
At the other end of the phloem, in the sink tissues, sucrose is used for a variety of purposes and this reduces its concentration in the phloem. Since sugar leaves the cells, so does water. The pressure in the sink phloem is therefore lower than it is in the source phloem. It is the difference in pressure between the two ends that drives long-distance flow.