Despite their distinctive anatomical characteristics and specialized functions, the cells of your body have many similarities. There is no cell that truly represents or contains all of the various components found in the many types of human body cells. As a result, students are often introduced to the anatomy of cells by studying a so-called typical or composite cell—one that exhibits the most important characteristics of many different human cell types. Such a generalized cell is illustrated in Figure 3-1. Keep in mind that no such “typical” cell actually exists in the body; it is a composite structure created for study purposes. Refer to Figure 3-1 and Table 3-3 often as you learn about the principal cell structures described in the paragraphs that follow.

Ideas about cell structure have changed considerably over the years. Early biologists saw cells as simple, fluid-filled bubbles. Today’s biologists know that cells are far more complex than this. Each cell is surrounded by a plasma membrane that separates the cell from its surrounding environment. The inside of the cell is composed largely of a gel-like substance called cytoplasm (literally, “cell substance”). The cytoplasm is made of various organelles and molecules suspended in a watery fluid called cytosol, or sometimes intracellular fluid. As Figure 3-2 shows, the cytoplasm is crowded with large and small molecules—and various organelles. This dense crowding of molecules and organelles actually helps improve the efficiency of chemical reactions in the cell.

The nucleus, which is not usually considered to be part of the cytoplasm, is generally at the center of the cell. Each different cell part is structurally suited to perform a specific function within the cell—much as each of your organs is suited to a specific function within your body. In short, the main cell structures are (1) the plasma membrane; (2) cytoplasm, including the organelles; and (3) the nucleus (see Figure 3-1).

Figure 3-3 shows a simplified view of the evolving model of cell membrane structure. This concept of cell membranes is called the fluid mosaic model. Like the tiles in an art mosaic, the different molecules that make up a cell membrane are arranged in a sheet. Unlike art mosaics, however, this mosaic of molecules is fluid; that is, the molecules are able to slowly float around the membrane like icebergs in the ocean. The fluid mosaic model shows us that the molecules of a cell membrane are bound tightly enough to form a continuous sheet but loosely enough that the molecules can slip past one another.

What are the forces that hold a cell membrane together? The short answer to that question is chemical attractions. The primary structure of a cell membrane is a double layer of phospholipid molecules. Recall from Chapter 2 that phospholipid molecules have “heads” that are water soluble and double “tails” that are lipid soluble (see Figure 2-21 on p. 49). Because their heads are hydrophilic (water loving) and their tails are hydrophobic (water fearing), phospholipid molecules naturally arrange themselves into double layers, or bilayers, in water. This allows all the hydrophilic heads to face toward water and all the hydrophobic tails to face away from water (see Figure 2-22 on p. 50).

Because the internal environment of the body is simply a water-based solution, phospholipid bilayers appear wherever phospholipid molecules are scattered among the water molecules. Cholesterol is a steroid lipid that mixes with phospholipid molecules to form a blend of lipids that stays just fluid enough to function properly at body temperature. Without cholesterol, cell membranes would break far too easily.

Each human cell manufactures various kinds of phospholipid and cholesterol molecules, which then arrange in a bilayer to form a natural “fencing” material of varying thickness that can be used throughout the cell. This “fence” allows many lipid-soluble molecules to pass through easily—just like a picket fence allows air and water to pass through easily. However, because most of the phospholipid bilayer is hydrophobic, cell membranes do not allow water or water-soluble molecules to pass through easily. This characteristic of cell membranes is ideal because most of the substances in the internal environment are water soluble. What good is a membrane boundary if it allows just about everything to pass through it?

Just as there are different fencing materials for different kinds of fences, cells can make any of a variety of different phospholipids for different areas of a cell membrane. For example, some areas of a membrane are stiff and less fluid; others are somewhat flimsy. Many cell membranes are packed more densely with proteins than seen in Figure 3-3; other membranes have less protein.

Some membrane lipids combine with carbohydrates to form glycolipids, and some unite with protein to form lipoproteins easily. Recall from Chapter 2 that proteins are made up of many amino acids, some of which are polar, some nonpolar (see Figure 2-26, p. 53). By having different kinds of amino acids in specific locations, protein molecules may become anchored within the bilayer of phospholipid heads and tails or attached to one side or the other of the membrane.

The different molecular interactions within the membrane allow the formation of lipid rafts, which are stiff groupings of membrane molecules (often very rich in cholesterol) that travel together like a log raft on the surface of a lake (Figure 3-4). Rafts help organize the various components of a membrane. Rafts play an important role in the pinching of a parent cell into two daughter cells during cell division. Rafts may also sometimes allow the cell to form depressions that pouch inward and then pinch off as a means of carrying substances into the cell (Box 3-1). Human immunodeficiency virus (HIV), for example, enters cells by first connecting to a raft protein in the plasma membrane and then subsequently being pulled into the cell.

Box 3-1

Caveolae

The list of organelles inside human cells that have been identified with new techniques of cell imaging (see Tools of Microscopic Anatomy online at A&P Connect) and biochemical analysis has continued to grow. Among the more recently discovered organelles are the “little caves,” or caveolae (singular, caveola). Caveolae are tiny indentations of the plasma membrane that indeed resemble tiny caves (see the figure). Caveolae appear to form from rafts of lipid and protein molecules in the plasma membrane that pinch in and move inside the cell. Caveolae can capture extracellular material and shuttle it inside the cell or even all the way across the cell (see the figure). Although there is much yet to understand about the many functions of caveolae, one possible problem that they may cause has already been outlined. Some caveolae in the cells that line blood vessels may have CD36 cholesterol receptors that attract low-density lipoproteins (LDLs, which carry the so-called bad cholesterol). As the figure shows, once the LDLs attach to the receptor, the caveola closes and migrates to the other side of the cell. There, the LDL molecules are released to build up behind the lining of the blood vessel. As the LDLs accumulate, the blood vessel channel narrows and obstructs the flow of blood—a major cause of stroke and heart disease.

Researchers believe that other diseases, such as certain forms of diabetes, cancer, and muscular dystrophy, may also result from inappropriate actions taken by caveolae.

Embedded within the phospholipid bilayer are a variety of integral membrane proteins (IMPs). As their name implies, they are integrated into structure of the membrane itself. Proteins that have some functional regions or domains that are hydrophilic and other domains that are hydrophobic can be integrated into a phospholipid bilayer and remain stable. IMPs have many different structural forms that allow them to serve various functions (see Table 3-3).

A cell can control what moves through any section of membrane by means of IMPs that act as transporters (see Figure 3-3). Many of these transporters have domains forming openings that, like gates in a fence, allow water-soluble molecules to pass through the membrane. Specific kinds of transporterss allow only certain kinds of molecules to pass through—and the cell can determine whether these “gates” are open or closed at any particular time. We consider this function of integral membrane proteins again in Chapter 4 when we study transport mechanisms in the cell.

Some IMPs have carbohydrates attached to their outer surface—forming glycoprotein

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