Cell Biology Page

Cellular Transport

Water, molecules, and ions move in and out of cells all of the time. If the cells are surrounded by such a tough membrane, how can these particles move. The cell membrane is not impermeable, but merely semi-permeable. So some things, like small ions can move freely through the cell membrane. This passive process is called passive transport, and it follows a simple physical phenomenon that everyone is familiar with.

Think about a barbecue that you've been invited to. As you walk to your the barbecue site, the rich aroma of the cooking meats and vegetables drift towards you. As you walk closer, you find yourself following the smells of ribs, steaks, hamburgers, bell peppers, and corn. The smells sets you mouth watering as they get stronger. How is it that you can tell exactly what is on the grill? Diffusion.

The process of diffusion is very simple, and operates on each separate molecule drifting in the air, or in water based upon it concentration. Diffusion is the process of molecules moving from areas of high concentration to areas of lower concentrations. There are definitely more molecules of aroma near your friends' barbecue, because that is where the food is being cooked. As more molecules drift in the air, they push against each other, pushing each other away from their source, the barbecue pit. Of course currents will direct the movement of the odors, but even without a breeze, the molecules will move to areas where there are fewer odors of ribs.

This is the same process that move ions through cell membranes. But in this case, we call it osmosis. Osmosis is simply the process of diffusion, except that the particles are moving through a semi-permeable membrane. For fish, molecules of O2, which are small enough to move through the cell membranes, move from the water, and into the blood stream of the fish through their gills. Gills use the process of diffusion by moving the blood carrying O2 away from the gills, replacing it with blood that has very little O2. In this way, the blood in the gills will always have a lower O2 concentration than the water that the fish is in, so O2 will always move into the gills.

A simple process, but what if the molecule is too big to squeeze through the lipid bilayer? Cell membranes have hundreds of proteins called channel proteins floating in the lipid bilayer. These channel proteins act like doors through the cell membrane, allowing larger molecules like glucose passage into or out of the cell. This process is called facilitated transport, or facilitated diffusion, because the channel proteins facilitate, help, molecules move through the cell membrane.

But sometimes the cell needs a lot of some particular molecule or ion, much more than could be supplied by diffusion. When ions and molecules are moving up the concentration gradient, from and area of low concentration to an area of high concentration, then energy is needed. This process is called active transport, and it needs ATP. The channel protein acts like a pump, grabbing the molecules on one side of the membrane, and moving it to the other side. ATP powers the movement.