Are Some Cell Membrane Proteins Receptors?

Last, but certainly not least, not all proteins in the plasma membrane function in transport operations. Some proteins function as receptors for special communicating substances in our body such as hormones and neurotransmitters. Typically, receptors will interact with only one specific molecule and ignore all other substances. In a way, then, these proteins can also be viewed as being involved in transport processes; however what’s being transported isn’t ions or molecules but information.

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.

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

What Is the Composition of the Plasma Membrane?

Each cell is enveloped by a very thin membrane measuring only about 10 nanometers (nm) thick. A nanometer is one-billionth of a meter— pretty thin indeed. The makeup of the plasma membrane is a very clever

combination of lipids and proteins with just a touch of carbohydrate and other molecules. Interestingly, plasma membranes use the basic principle of water solubility to allow for its barrier properties and it is the lipid that provides this character. Molecules that are somewhat similar to triglycerides (fat) called phospholipids are arranged to provide a water-insoluble capsule surrounding cells. What that means is that water-soluble substances such as sodium, potassium, and chloride, carbohydrates, proteins, and amino acids are not able to move freely through the membrane whereas some lipid substances and gases move more freely. The plasma membrane will also contain the lipid substance cholesterol. Cholesterol appears to increase the stability of the plasma membranes.
 

Since the plasma membrane functions as a barrier between the outside and inside of the cell, there must be a means (or doorways) whereby many water-soluble substances can either enter or exit a cell. One of the roles of proteins in the plasma membrane is to function as doors, thereby allowing substances such as sodium, potassium, chloride, glucose, and amino acids to enter or exit a cell. This is shown in Figures 2.1 and 2.2.

Photosynthetic membranes

The reaction centres of purple and green sulphur bacteria are localised in membranes, often called chromatophore membranes, which lie close to or include the outer cell membrane. In purple photosynthetic bacteria. the LH proteins are also intrinsic to the chromatophore membrane. However, in green bacteria the very large LH chlorosome, packed with many thousands of molecules of bacteriochlorophyll, is stacked into rodlike structures attached to the cytoplasmic side of the photosynthetic membrane, which does not invaginate as it does in purple bacteria (Fig.l.8)

In oxygenic photosynthetic organisms, the photosynthetic apparatus involved in light reactions is embedded in the specialised thylakoid membrane (see Fig.l.3). In cyanobacteria, the thylakoid membranes tend to form concentric rings within the cytoplasm and are characterised by the presence of the large LH phycobilisomes attached to their surfaces, which induces a considerable spacing between them. In the green oxyphotobacteria (prochlorophytes), the same concentric rings are present but the membranes lie more closely together because of the absence of bulky phycobilisomes. The presence of phycobilisomes in the chloroplast of red algae leads to a thylakoid membrane organisation reminiscent of cyanobacteria. In striking contrast, the thylakoid membranes of higher plant chloroplasts, and to a lesser extent those of green algae, are arranged in stacked (grana) and unstacked regions (see Fig.l.3). The granal thylakoids are highly enriched in PSII, while PSI is found in the unstacked regions. However, this extreme lateral separation does not seem to occur in the thylakoid membranes of cyanobacteria and many forms of algae and therefore cannot be an absolute requirement for oxygenic photosynthesis to occur.