The solutes not only need to get into or out of cells, but they need to be moved from one area of a plant to another. We have already seen how water moves through the plant in the xylem: by transpiration! For soil mineral solutes, this is the pathway to get into the plant and up the xylem too. These solutes are dissolved in the water that we have seen passing through that pathway.
We have also seen how those mineral solutes might get into and out of cells by diffusion, facilitated diffusion, and active transport. We have observed in laboratory how minerals might become concentrated in leaf mesophyll by the evaporative cooling process. These minerals are critical for a range of enzyme and other metabolic functions. But what about other solutes? Would up-and-out be the only pathway?
No, indeed there must be ways for the solutes from photosynthesis and other biochemical pathways to depart from the leaf and go both down to the root and up to flowers, fruits, and apical meristems to provide fuel for respiration carried out in these non-photosynthetic areas. So leaves are the likely source of small organic molecules, and the rest of the plant organs are sinks for those molecules. The flow of these solutes then must be able to be both upward and downward in the plant. The solutes could be any of the subunits of the macromolecules; examples include sugars, amino acids, nucleotides, and fatty acids.
The flow of these organic molecules is called translocation and this process occurs mostly through the phloem. While phloem lies alongside the xylem in veins in leaves, vascular bundles in stems, and the vascular cylinder of roots, it is a completely different tissue conducting in different directions and by different mechanisms!
The diagram below shows "whole" and "longitudinal sections" of phloem cells. The phloem cells, unlike xylem conducting cells, are alive at maturity. The conducting cells are called sieve tube elements and have lateral and end walls with holes through which the cytoplasm is continuous from cell to cell (remember symplast?). You can see the smaller diameter plasmodesmata on the side walls and very large diameter ones in the end walls. The idea of this cell as perforated like a sieve (colander!) gives you some idea of how it was named.
The translocation pathway is of course mostly up-down, explaining the large pores in the sieve plates. It also makes sense that these cells have to develop from meristematic cells by cytoplasmic degradation. The nucleus, mitochondria, plastids, vacuole, and other large organelles need be lost to avoid blocking the pores as flow occurs from cell to cell! These cells have to have simplified cytoplasmic organelles! This simplification means, however, that to stay alive, the sieve cell must have a companion cell nearby to provide it with the required elements of life from the nucleus and other major organelles through the plasmodesmata in the side walls. So indeed a view of the cross section of phloem would show sieve tube elements with very simple cytoplasm, and adjacent companion cells with dense and complex cytoplasms!
We can get an idea about the relative diameters of the lateral and terminal pores by looking at a longitudinal section through the phloem...check out the cytoplasms too!
Now of course, to get these conducting elements working, we need to make the solutes by photosynthesis and other biochemistry and load them into the phloem. We will see how photosynthesis produces these solutes later, but for now we will focus instead on how we load them from other cells into the phloem of the leaf. In order to collect lots of small organic molecules from leaf mesophyll cells, the apoplastic fluids must find their way into the phloem at high rates.
Transfer cells, adjacent to the phloem have highly corrugated cell membranes to increase their surface area for absorption of solutes using the transport proteins we have been discussing recently! Here you can see that a transfer cell has lots of surface area because of the infoldings, and also has lots of mitochondria to drive active transport to pump small organic molecules from the apoplast into the transfer cells and to load them into the adjacent phloem sieve tube elements!
Let us remind ourselves of where the materials are being produced for loading into the phloem. Below is the cross section of a leaf, where photosynthesis occurs in the chloroplasts of the palisade mesophyll cells. Of course we must get carbon dioxide and water to these mesophyll cells and light will be penetrating the upper epidermis for the energy source.
In the close-up of a single vein below, you can see that carbon dioxide gets into the internal leaf atmosphere via the stomata, and is taken into the mesophyll cells. Water comes into the leaf by transpiration through the xylem and the dilute mineral solution bathes all of the mesophyll cells. As photosynthesis produces trioses, they are assembled into hexose, and then into sucrose in the cytosol. Of course we have learned that dissolved materials can move within the leaf by via symplast or by apoplast in plants. Below you can see how sugars from photosynthesis in mesophyll cells may travel by symplastic pathways through plasmodesmata directly into phloem cells. Anywhere along this pathway (not shown but should have been) apoplastic solutes can enter any of these cells. The diagram only shows apoplastic flow at the "branching" point of this enlargement. Just realize that every cell here could be doing that!
Some of those same membrane transport proteins are at work in our phloem cells. Here is an example of how a proton pump and a sucrose-proton symport work in a bundle-sheath or companion cell is actively pushing protons out into the apoplast. These protons are returned by the sucrose symporter to shuttle sucrose from the apoplast into the symplast against the concentration gradient.
Having the organic molecules loaded into the phloem, where do they go? Below is an example of an experiment to show this. The plant shown on the left was treated on leaf 14 with radioactive carbon dioxide. The red shading on the leaves indicates how much radioactivity is found in the other leaves of the plant a week later. As you can see, the isotope moves upwards into leaves that are on the same side of the stem. There is more radioactivity in leaves directly above the treated leaf than in those at other angles. Little to no radioactivity appears in leaves on the opposite side of the stem.
The plant above on the right, represents a second trial in which leaf 10 was treated with radioactive carbon dioxide, but all the mature leaves (8, 11, 6, 9) on the opposite side of the stem were removed. A week later the plant was autoradiographed to show where the isotope was translocated as small organic molecules. As you can clearly observe, the small leaves on both sides are receiving the "feed" from the treated leaf. Those young leaves had lost their natural sources but received organics from a different leaf in response to the excisions. These results indicate that phloem translocation is dynamic: responding to "need."
What is not shown in the experiment above is that photosynthate must obviously pass down the plant too...going to roots for their respiratory fuel and to support the full range of biochemistry that takes place there in the darkness of the soil environment. Thus the flow in phloem is bidirectional...the solutes must travel up to young leaves and apical buds, flowers, and fruits...but also downwards to the root system.
Now what exactly are the substances translocated in phloem? A partial list is given here with concentrations in an example plant.
Well "sugars" is a pretty broad category...just what are we talking about? It turns out that most simple six-carbon sugars have reducing groups (aldehyde or ketone) and these generally are not translocated in phloem. Just below are some example reducing sugars showing the reducing group.
Instead the reducing sugars are "conjugated" or polymerized into short-chain oligosaccharides. Sucrose is arguably the most commonly-translocated sugar. Raffinose, stachyose, and verbascose are related oligosaccharides transported in phloem. Because the reducing group is lost in the polymerization, these sugars are called non-reducing sugars. They are presumably less-reactive and so are arguably better candidates for transport.
The diagram above also shows mannitol which, like sorbitol in our osmosis project, is a sugar alcohol. Notice how the reducing aldehyde (=O) of mannose is reduced into an additional hydroxyl (-OH) group. Perhaps the name sugar alcohol now makes more sense? With the reactive group inactivated from their starting sugars, mannitol and sorbitol are far less permeant and this is a plus for transport monosaccharides...at least during the transport process itself. Of course you noticed that our potato tuber tissue sticks never came into equilibrium with their sorbitol solutions. Do you now have an explanation for that?
The phloem also transports amino acids, but we also know this group is up to 20 or more compounds, so which ones are most commonly transported? Well from leaf to root we generally have glutamic acid, glutamine, aspartic acid, and asparagine.
Of course we think of amino acids as good sources of nitrogen. But nitrogen is transported in xylem too...from soil to leaf. In plants lacking symbioses with nitrogen-fixing bacteria, the nitrogen is transported as ammonium, nitrate, or nitrite ions in soil water. Those species that do associate with nitrogen-fixing microorganisms often transport nitrogen up the plant in the transpiration stream as organic nitrogen molecules such as allantoic acid, allantoin, and citrulline.
The basic idea of how flow in the phloem works was described by Ernst Münch in 1930. The concept is a pressure-driven bulk flow model. To understand this you have to realize that materials flowing from sieve tube element to sieve tube element do not go through any membranes...so the phloem is basically one cell, continuous from leaf to root and leaf to apical bud. A basic diagram here helps us to understand how this works:
As you can see, the cells of the leaf produce sugars that are loaded by bundle sheath, transfer, and/or companion cells into the phloem. The resulting reduced Ψs here lowers the water potential and water moves from the xylem into the phloem cell too. The combination of sugar and water moving into the phloem cell increases the pressure Ψp in the cell, pushing phloem cytoplasm by bulk flow away from the leaf and toward the sink organs (root, etc.).
A pressure gradient is established along the phloem column from leaf to sink. The pressure is reduced along its length as sugar is taken out by adjacent cells and water follows osmotically. In the ultimate sink tissue, there is still sugar present to be unloaded and for water to follow. Thus, from source to sink, the pressure gradient is what provides the power for bulk flow. It is subject more to Poiseuille's than to Fick's factors.
Because the source is in the middle of the plant, with sinks at each end of the plant, the idea of phloem as a huge symplast with unloading at each end, and loading in the middle source area, explains the required bidirectional flow of solutes translocated in phloem.