9.1: Cell Structure

A cell (from cella, Latin for small room or cubicle) is the basic unit of life. With few exceptions, our many kinds of cells have common features (see Figure 9.1): A cell membrane surrounds the cell, which is filled with a fluid called cytoplasm (cell plasma). Within the cell are the cell nucleus, ribosomes for protein production, mitochondria (where oxygen-requiring reactions take place), and a store of energy-providing nutrients— glycogen granules (carbohydrate) and fat droplets (triglyceride). The cell membrane is discussed below. Other parts of the cell are discussed where relevant in this chapter and the next.

Figure 9.1: Some Part of Cells

Cell Membrane

Cell membranes are made of a double layer of lecithin (Figure 5.5) interspersed with cholesterol (Figure 9.1) and studded with proteins (called membrane proteins or surface proteins). Membrane proteins differ according to a cell’s function. Two were discussed earlier: lactase (enzyme that digests lactose) and LDL-receptors that allow cells to take in cholesterol from the blood.

With just these two examples, we see how important these membrane proteins are. Lactose intolerance occurs when there isn’t enough lactase in the membrane of cells lining the small intestine (Chap. 6), and heart disease can occur at a young age when there aren’t enough LDL-receptors (Chap. 8).

Insulin receptors are another example. They are found in the membrane of almost all our cells. Insulin (a hormone made in the pancreas) works by attaching to insulin receptors, which then triggers a number of events within the cell, including the production of more glucose transporters—membrane proteins that allow glucose to enter a cell (Figure 6.2). (This is how insulin “tells” a cell what to do.)

Membrane proteins also play a role in AIDS (and COVID-19, discussed in Chap. 10). HIV (like all viruses) can do damage only by entering cells. HIV enters (infects) by attaching to a membrane protein called CD4—much like the way LDL-cholesterol enters by attaching to LDL-receptors. CD4 is mostly found in the membrane of certain white blood cells, making them the prime target for HIV infection.

In the search for ways to control the infection, CD4 was made by biotech for use as a possible drug. The hope was that HIV would attach to this “loose CD4” drug in the blood, instead of the actual CD4 in the membrane of the white blood cells. In other words, the drug was intended as a decoy to prevent HIV from infecting these cells (it worked in the test tube, but was too hard to continuously keep enough of the CD4 drug in a person’s blood).

Cells use membrane proteins to interact with the “outside world.” Think of cells as houses. Someone wanders around with a key. Only if the key fits, does the person get in.