Science Questions

How can active transport in plants possibly work? Michael in Cambridge

In a plant, you have a leaf which is receiving sunlight and taking in carbon dioxide from the atmosphere to make sugars and some proteins from photosynthesis. The sugars then get redistributed round the plant. How does that actually happen? There are two transport systems in a plant. One of them just works in one direction. Essentially there's a series of long tubes between the roots and the leaves called xylem. These are dead cells which have been waterproofed with a special material called lignin and they have cellulose around them. They are about 0.01 millimetres across, and at that kind of thickness, the column of water that flows into them has a tensile strength stronger than that of steel. As the water evaporates from the leaf at the top end of the plant, it pulls the water up from the root. That's fairly intuitive. But how do the sugars go from the leaf down to the root? Well scientists have wondered about this for quite a long time, and thought that it might be just 'source and sink'. In other words, if you make a lot of something in one place, it tends to move away from where there's a lot of it to where there's not much of it. That's one possibility of how things are moved around. However, there's a second transport system called phloem and these are individual living cells that have individual plates between the cells that act like sieves. They can actually control the direction in which things move in those cells. Although it's pretty sketchy exactly how they know how to control whether something goes up or down, there's very good evidence that it does work like that because plants do distribute things in the right direction according to which way they need it to go. If you kill those cells, the process stops. This shows that it uses energy. That's really the extent of our knowledge at the moment.

If you look at the cells in your body, there's an electrical gradient between the inside and outside of the cell. In other words, the inside of a cell is slightly more negative than the outside. The way cells manage to do that is that they have the cellular equivalent of a revolving door. It picks up three versions of a sodium on the inside of the cell and boots it out, and picks up two potassiums on the outside of the cell and brings it in. Overall, the cell is exchanging three plusses for two plusses, which creates an electrical difference. You can use that electrical difference to do some work for you, such as grabbing something that you want. This all requires energy, and is active transport. Cells are doing it all around your body to transport glucose into your cells. That's how cells like the liver, even though they're stuffed with glucose most of the time, an get even more packed in. I think plants are probably up to the same trick.

February 2006