Do proteins in the plasma membrane have special roles?
If we were to weigh all of the components of the plasma membrane we would find that about half the weight of the membrane is protein. However, this is a bit misleading as the much smaller lipid molecules of the plasma membrane tend to outnumber protein molecules by about fifty to one. This means that the proteins tend to be larger and complex, which implies that they have important functions while phospholipids and cholesterol provide more structural support.
Are some membrane proteins involved in the movement of substances in/out?
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Let us go into a little more detail about just how some of the proteins function as doorways in our plasma membranes. Some of these proteins function as channels or pores that will allow the passage of only one specific substance across the membrane. This is like opening the stadium doors for fans before a game. The concentration of fans outside the stadium is much higher than within and the natural flow is for the general movement of people into the stadium, an area of lower concentration. |
Plasma membrane channels allow the passage of ions such as sodium, potassium, chloride, and calcium down their concentration gradient. However, the movement will be in mass amounts resulting in a sudden and significant change in a cell’s environment. As an example, ion channels are especially important in nerve and muscle cells, and drugs often prescribed for people with cardiovascular concerns are calcium-channel blockers.
We should stop for a moment and emphasize a very important concept. In nature, when provided the opportunity, things tend to move from an area of higher concentration to an area of lower concentration. This is referred to as diffusion. The movement of substances across our plasma membranes is an excellent example of diffusion. For example, skeletal muscle cells are told to contract by calcium. Thus for a muscle cell to be relaxed (not contracted) calcium must be pumped out of the intracellular fluid into the extracellular fluid as well as into a special organelle in muscle cells. In fact, the calcium concentration outside the muscle cell will be greater than ten times that inside when a muscle cell is relaxed. Then, when that muscle cell is told to contract, calcium channels on the plasma membrane and the organelle open and calcium diffuses into the intracellular fluid thereby allowing contraction to occur.
Let’s use calcium channel blocker drugs used to treat high blood pressure and angina as an example. Calcium-channel blockers (also called calcium blockers) inhibit the opening of calcium channels (pores) on heart muscle cells and muscle cells lining certain blood vessels. This reduces contraction of the muscle cells and as a result the heart pumps less vigorously and blood vessels relax, both contributing to a lowering of blood pressure and reduced stress on the heart.
Channels or pores are not the only types of proteins found in our plasma membranes. Other proteins can function as carriers that can “transport” substances across the membrane. Here again substances would be moving down their concentration gradient. These carrier proteins tend to transport larger substances such as carbohydrates and amino acids. Perhaps the most famous example of a carrier protein is the glucose transport protein (GluT) which is the primary concern in type 2 diabetes mellitus.
Do some membrane proteins function as pumps?
Not all substances move across the plasma membrane by down their concentration gradient. Since this type of movement seems to go against the natural flow of nature, to make this happen certain membrane proteins must function as pumps. Quite simply, pumps will move substances across a membrane against their concentration gradient or from an area of lower concentration to higher concentration. Pumps need energy which is derived from ATP (see Membrane Pump Figure). In fact, a very respectable portion of the energy that humans expend every day is attributed to pumping substances across cell membranes.
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