Carbohydrates in Plasma Membranes

Plasma Membranes Continued: Carbohydrates

Carbohydrates are the third major component of plasma membranes. You can always find them on the exterior surface of cells. In fact, they usually bind either to proteins (forming glycoproteins) or to lipids (forming glycolipids) (see image below).

Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins) extend from the outward-facing surface of the membrane. Image Attribution: OpenStax Biology

These carbohydrate chains may consist of 2–60 monosaccharide units and can be either straight or branched. Along with peripheral proteins, carbohydrates form specialized sites on the cell surface that allow cells to recognize each other.

These sites have unique patterns that allow other cells to recognize the cell. This is similar to the way that the facial features unique to each to your friends allow you to recognize him or her. This recognition function is very important to cells. This is because it allows the immune system to differentiate between body cells (called “self”) and foreign cells or tissues (called “non-self”). You can find similar types of glycoproteins and glycolipids on the surfaces of viruses. These may change frequently, preventing immune cells from recognizing and attacking them.

These carbohydrates on the exterior surface of the cell—the carbohydrate components of both glycoproteins and glycolipids—are collectively referred to as the glycocalyx (meaning “sugar coating”). The glycocalyx is highly hydrophilic and attracts large amounts of water to the surface of the cell. This aids in the interaction of the cell with its watery environment and in the cell’s ability to obtain substances dissolved in the water.

As discussed above, the glycocalyx is also important for cell identification, self/non-self determination, and embryonic development. In addition, it is used in cell-cell attachments to form tissues.

Evolution Connection: How Viruses Infect Specific Organs

Glycoprotein and glycolipid patterns on the surfaces of cells give many viruses an opportunity for infection. HIV and hepatitis viruses infect only specific organs or cells in the human body. HIV is able to penetrate the plasma membranes of a subtype of lymphocytes called T-helper cells, as well as some monocytes and central nervous system cells. The hepatitis virus attacks liver cells.

These viruses are able to invade these cells. This is because the cells have binding sites on their surfaces that are specific to and compatible with certain viruses (see image below). Other recognition sites on the virus’s surface interact with the human immune system, prompting the body to produce antibodies. Your body makes antibodies in response to the antigens or proteins associated with invasive pathogens. Your body could also make them in response to foreign cells, such as might occur with an organ transplant. These same sites serve as places for antibodies to attach and either destroy or inhibit the activity of the virus.

HIV binds to the CD4 receptor, a glycoprotein on the surfaces of T cells. Image Attribution: modification of work by NIH, NIAID

Unfortunately, these recognition sites on HIV change at a rapid rate because of mutations. This makes the production of an effective vaccine against the virus very difficult, as the virus evolves and adapts. A person infected with HIV will quickly develop different populations, or variants, of the virus that are distinguished by differences in these recognition sites.

This rapid change of surface markers decreases the effectiveness of the person’s immune system in attacking the virus. This is because the antibodies will not recognize the new variations of the surface patterns. In the case of HIV, the problem gets worse as a result of the fact that the virus specifically infects and destroys cells involved in the immune response, further incapacitating the host.