Substances move by active transport if: 1) they are small molecules, but they need to move against their concentration gradient; or 2) they are really large, possibly even whole cells. So there are two types of active transport:
Now I'll explain each one.
Active transport using pumps
Again, this is how small molecules cross the membrane. But for these molecules to move, they need energy. Why? Because they cannot move by diffusion! That is because active transport moves molecules against the concentration gradient. What does that mean? Active transport pushes molecules from where they are in low concentration to where they are in high concentration; that means that it builds up a high concentration of the molecule.
In order for active transport to work, there have to be special channels in the membrane that use the energy (ATP) to push the molecules through the membrane. These special channels are called pumps. Your book calls them "carriers," but I don't like that term. It can get confused with other things. And every single protein that acts as a pump has the word "pump" in its name. For example, your book shows you the sodium-potassium pump in Figure 4-17. A web site that shows you the sodium-potassium pump is http://www.students.stir.ac.uk:80/biology/cellmemb/pump.htm . When you go there, do not feel the need to understand everything there, but it should just be an additional way to see this pump.
Active transport using vesicles
Big things can't get across the membrane. The membrane would have to rupture to get them in. But if a single-celled organism were swimming around and food passed by, it would need to be able to grab onto that food by bringing it inside itself somehow. How can it do that? By using a vesicle to cross the membrane.
You learned in the web page on membrane structure that the plasma membrane is an oily film. This film can pinch off a piece when needed. The piece that comes off is a rounded circle of membrane called a vesicle. This is also like when you play with bubbles (as I have done a lot with my nephews and my cousin's sons). When two bubbles touch, they sometimes fuse into one larger bubble. A vesicle can also fuse with a cell, causing the cell to get larger. Both of these are shown in the diagram here. In each row, the first drawing is the "before" image and the last drawing is the "after" image. The one in the middle, between the arrows, is the "during" image. The arrows are meant to indicate time passing.
Active transport using vesicles can be used to either take in material (like the second row under "pinching off") or spit out material (like the bottom row under "fusing together.") Each direction of movement has a name.
© 2006 STCC Foundation Press, content by Dawn A. Tamarkin, Ph.D.
Last changed: January 21, 2007