Cell Transport

Transport across the cell membrane is not ensured simply by applying a sufficient free energy gradient. The membrane must be permeable to some extent to allow passage through it. Some solutes (steroid hormone, fat soluble vitamins, oxygen and carbon dioxide) are lipid (fat) soluble and dissolve in the lipid bilayer portions of the membrane then diffuse to the other side. Many other important solutes (ions, glucose and amino acids) are more polar and as a consequence are soluble in water but not lipids. These substances move through special pathways provided by proteins that span the membrane.

Small solutes like Na+ (ions) pass easily through channels, but larger ones like glucose enter the cell by facilitated diffusion - they bind to a protein carrier that "rocks" back and forth or moves in some other way, exposing the binding site first to one side then to the other side of the membrane. The solute hops on or off the site, depending on the concentration. If there is a higher concentration outside the cell, then binding site will have a greater chance of picking up a solute on the outside and more solutes will move in than out. This will continue until the concentrations on both sides are equal. At this point, movement in one direction is just balanced by movement in the other so net movement ceases. It is a purely passive transport because any glucose movement is always down its concentration gradient. Similar facilitated diffusion systems exist for many other substances.

Proteins also provide pathways for solute movements against concentration gradients. Primary active transport is (probably) similar to facilitated diffusion. The transported molecule binds to a site on a protein that can "rock" or otherwise expose the binding site first to one side and then to the other side of the membrane. In contrast, however, to the passive facilitated diffusion, suppose the binding site properties change and depend on which side of the membrane it faces. If the solute can bind on only one side of the membrane, say on the surface facing the inside of the cell, then transport is in only one direction, from inside to out, but never the reverse. If the concentration is less inside than out, the protein will transport against a gradient - it will be an active transport system. Energy for the transport will have to be supplied in order to change the binding site properties each time it cycles back and forth. This energy is generally derived from splitting ATP.

Solutes can also move against a gradient by co- and countertransport. Both utilize the passive transport of one solute to transport a different solute. The example of co-transport is similar to facilitated transport but now protein carrier has binding sites for two different solutes, Na+ and glucose. The carrier will not "rock" if only one of the sites is occupied. Both sites must be either empty or occupied, both Na+ and glucose must be bound. Outside the cell, Na+ is much more concentrated than glucose, but inside the cell, the concentration of Na+ is very low because it is continually being pumped out by an active transport process operating elsewhere in the membrane. Both Na+ and glucose will move into the cell, but few molecules will come back out because of the low concentration of intracellular Na+. This makes it difficult for glucose to find a Na+ partner to ride the co-transport system in the reverse direction. Glucose can be pulled into the cell even against its concentration gradient.

The energy to transport glucose against the concentration gradient comes from energy dissipated by Na+ as it moves down its concentration gradient. The concentration gradient for Na+ is maintained by a primary active transport pump, which is driven by energy released by the splitting of ATP, so that ATP is indirectly involved in this co-transport. Similar co-transport systems exist for other solutes. Counter-transport is similar to co-transport but the two solutes move in opposite directions. There are binding sites for two different solutes (Na+ and Ca++, for example). The carrier will not "rock" if only one of the two sites is occupied (by Na+ and Ca++). The Na+ concentration is much higher than Ca++ and tends to dominate and keeps the countertransporter moving in the direction that allows Na+ to flow down its gradient (into the cell). Ca++ flows out of the cell, even though the Ca++ concentration is higher outside the cell than in. Once again, the energy dissipated by Na+ moving down its gradient is coupled to the uphill transport of another solute.