Are Some Membrane Proteins Involved in the Movement of Substances In and Out?

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.
 

Proteins in the plasma membrane act as receptors, transporters, channels, pumps, and enzymes.

Plasma membrane channels allow the passage of ions such as sodium, potassium, chloride, and calcium down their concentration gradient. The movement can be in massive 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, which will be discussed more in just a bit and also in Chapter 13.
 

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 (Ca2+). 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, which are used to treat high blood pressure and angina, as an example. Calcium-channel blockers (also called calcium blockers or CCBs) 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 diabetes mellitus. We will spend much more time on glucose transporters later on.
 

Not all substances move across the plasma membrane by moving 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. In fact, a very respectable portion of the energy that humans expend every day is attributed to pumping substances across cell membranes. We will go into much more detail about this later on in this chapter and other chapters.