Cellular Customs and Border Patrol
Transport of material into and out of a cell can be passive, where no energy is used, or active, where the process must be helped through the expenditure of energy. In passive transport, molecules move from areas of high concentration to areas of low concentration.
Diffusion and Osmosis
Simple diffusion is a process by which molecules in areas of high concentration spread out into areas of low concentration. Spritz some air freshener in the trash can and pretty soon the whole kitchen will have a lovely lilac scent. This is an example of diffusion.
You can think of the process of diffusion as a lot like riding a bicycle downhill. The only energy required was what you used to get up to the top of the hill. Afterward, it’s simply letting gravity coast you downhill. For cellular transport, the uphill push is the creation of a high concentration of molecules. This buildup of molecules might happen when, for example, food is digested into the nutrients your body needs to function. The nutrient molecules pile up. This process requires energy. But the distribution of the piled-up molecules—having them topple downhill (so to speak)—doesn’t (necessarily) require additional energy.
Thus, the top of the hill is the area of high concentration and the bottom of the hill is low concentration. (If you have a stack of molecules at the bottom of the hill, the molecules at the top of the hill wouldn’t have any place to go, so diffusion could not occur.)
Oxygen and carbon dioxide freely diffuse through the membrane during respiration.
Anatomy of a Word
respiration
Respiration is the process of bringing oxygen into cells and getting rid of carbon dioxide (a waste product) from cells.
As the cell does its work, it uses up its oxygen, converting it to carbon dioxide. The carbon dioxide piles up until the concentration is high enough that it can topple downhill (that is, through the cell membrane and out into the wild where it can be carried away). As the carbon dioxide builds up inside the cell, oxygen builds up outside the cell. Once the carbon dioxide moves out of the way, the high concentration of oxygen outside the cell can tumble inside the cell, evening out the distribution of oxygen molecules. And the cycle continues to infinity and beyond, or at least for a very long time.
Water is also capable of freely diffusing through the plasma membrane; however, the diffusion of water is termed osmosis.
Water molecules tend to dilute materials to an equal extent. If water on one side of a membrane has more solute added, such as sugar or salt, water will flow from the side of less solute (lesser concentration, or hypotonic) and into the side rich in solutes (hypertonic) in order to attempt to equalize the concentration of water and stuff on both sides of the membrane.
Carrier-Mediated Transport
Molecules that can’t diffuse through the plasma membrane (because they are too large or because they carry a charge) are transported through the membrane via protein channels. The molecule is still moving from an area of high concentration to an area of low concentration.
Glucose and charged ions such as sodium are among the molecules and ions that must use a protein channel for diffusion into or out of a cell. No energy is used since this is still diffusion. The only difference is the specialized tunnel through which these molecules can diffuse.
Active transport is a type of transport that is distinct from passive diffusion in that molecules are actively moved in and out of the cell, often with the help of transport proteins. The molecule in need of moving adheres to a transport protein, which brings it where it needs to go and releases it. This type of facilitated diffusion is still a matter of moving molecules from an area of high concentration to an area of low concentration.
However, sometimes molecules need to be transported from an area of low concentration to an area of high concentration (against their concentration gradient). This is the opposite of diffusion, and it requires transmembrane proteins to carry the molecules plus a lot more energy to make the trip. (This is more like riding your bike up hill than it is coasting downhill.)
Active transport protein channels bind the transport molecules and use energy from ATP to change their shape in such a way as to move the molecules across the membrane and against the gradient. The final step of the active transport process is to reset the channels so that the next active transport cycle may begin.
Sometimes a cell uses membrane vesicles to transport molecules into and out of a cell. In this case, instead of using a transmembrane protein as a sled, the molecule is encased by a pocket in the cell membrane (creating a vesicle) and moved into or out of the cell.
· Endocytosis is the process of using vesicles to transport molecules into a cell.
· Exocytosis is the process of using vesicles to transport molecules out of a cell.