Hi there, we're going to finish up plant transport by learning about how sugars are transported, which will be through the phloem. Sugars can move all throughout the plant in the phloem. Unlike xylem, which only has transport in one direction from the roots upward, phloem can move sugars and other organic materials in multiple directions all throughout the plant, which is sometimes termed translocation, from one location to another. The phloem will contain a material called sap, which is essentially sugar dissolved in water.
Besides organic sugars, it will also carry other organic molecules like hormones and amino acids, but the major function is to transport sugars. Recall that phloem contains two kinds of cells, sieve cells, which have these sieves in between one cell and the next, and sieve tube elements, which are found only in angiosperms and are slightly more efficient. Unlike the xylem cells, phloem cells are going to be living, and that is necessary for phloem cells because they have to do active transport, which the xylem cells never have to do. However, we still want our phloem cells to be basically hollow, and so they are going to require some support from their companion parenchyma cells, which are connected to them by plasmodesmata, and the companion cells will help with metabolism and waste and structure to allow the sieve cells to still be pretty much empty even though they are living. The model that describes sugar transport in the phloem in plants is termed the pressure flow hypothesis, and in the phloem we are going to be using pressure from water to cause bulk flow of material throughout the phloem.
Sugars will initially enter the phloem from a source. A source is a location where sugars are being generally produced. So it's a source of sugar.
We will put sugars into the phloem by active transport, which is why phloem cells need to be alive. When sugars are pumped into the phloem, that is adding solutes to the solution in the phloem, thereby making it hypertonic. If the solution in the phloem is hypertonic, then water will want to enter, and water will enter the phloem in fact from the xylem. As water enters the xylem, it will cause bulk flow through osmotic pressure.
Because water is increasing in the phloem from the xylem, it creates pressure which is going to push the sap fluid through the phloem like pushing down the plunger on a syringe. That will allow the sugar solution to flow from cell to cell within the phloem by bulk flow without using energy. Eventually that sugar solution will reach a sink, which is a location where sugars are needed.
Sugars will leave the phloem at the sink, so you can think of this as a location where sugars are being drained out of the phloem. Sugars will again leave the phloem by active transport. If you take sugar out of the solution, it will then become hypotonic, so that water will then flow out back to the xylem.
So let's zoom in in a little more detail at what is going on, specifically at the source and the sink. At the source, we are going to do a process called phloem loading, which is loading up the phloem with sugar. Sugars are going to be produced in various locations within the plant. So see if you could be very specific where are sugars produced and during what process.
Once the sugars are created at the source, they will be pumped into the phloem by active transport. So we are going to use energy to get the sugar into the phloem cells, and that active transport is why the phloem cells need to be living. You can only do active transport if you are a living cell. Once those sugars are transported into the phloem, the phloem solution becomes hypertonic because it is getting pumped full of solutes. Since the phloem is hypertonic, water will want to enter the phloem, and it will do so from the xylem.
This is one of the reasons why you see phloem and xylem always right next to each other. The phloem functions by taking water out of the xylem. That water movement will be passive since it is simply diffusing from hypotonic xylem to hypertonic phloem.
It is going to be accomplished through plasmodesmata, so this would be the simplast route by which water enters the phloem from the xylem. As the water enters the phloem, it's going to build up, creating osmotic pressure. That osmotic pressure will then push the phloem solution from cell to cell by bulk flow through the sieve plates.
Once we have loaded up the phloem with sugar, everything else is going to be passive. Water enters passively through plasmodesmata, creating osmotic pressure, which then pushes the solution from cell to cell passively by bulk flow. So it takes energy to put the sugar into the phloem, but the rest of the process doesn't take energy. It's simply through the power of osmosis. That sugar solution will get pressed from one sieve cell to the next through the sieve plates by bulk flow until eventually at the end we can do phloem unloading.
Phloem unloading will occur at sinks. Sinks are areas that need sugar, so what areas of a plant do you think might be sinks? At the sink, we're going to provide sugar to those cells by phloem unloading.
Sugars will again be actively transported out of the phloem and into the sink. This will take some energy to do. Once you extract the sugar from the phloem, that is going to make the phloem hypotonic.
because you are removing solutes. As a result, water will leave the phloem and it will go right back to where it came from, which is the xylem. So water will flow passively through Plasmodasmata out of the phloem and into the xylem, where it can then continue moving throughout the rest of the plant.
So you can see that the phloem functioning is a little bit different from xylem functioning because it does allow material to be moved in any direction and it is a little bit more flexible. and that is based on the active transport of sugars into the phloem through phloem loading at a source and out of the phloem through phloem unloading at a sink. But the phloem is going to be dependent on water from the xylem. So the two portions of the vascular system do work together and that's why you always see them right next to each other throughout the tissues of the plant.
Okay that is all for plant transport. And that finishes up all of our lectures. Yay!