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Unformatted text preview: for life. In addition to containing large amounts of water, all the body cells are constantly bathed in a dilute saltwater solution (something like seawater) called interstitial fluid, which is derived from the blood. All exchanges between cells and blood are made through this fluid. Cells vary tremendously in length ranging from 2 micrometers (1/12,000th of an inch) in the smallest cells to over a meter (3 feet) or more in the nerve cells that cause you to wiggle your toes. Furthermore, a cells structure often reflects its function; this will become clear later in this chap- ter. Cells can have amazingly different shapes. Some are disk-shaped (red blood cells), some have threadlike extensions (nerve cells), others are like toothpicks pointed at each end (smooth muscle cells), and still others are cubelike (some types of epithelial cells). Cells also vary dramatically in the functions, or roles, they play in the body. For example, white blood cells wander freely through the body tissues and protect the body by destroying bacteria and other foreign substances. Some cells make hor- mones or chemicals that regulate other body cells. Still others take part in gas exchanges in the lungs or cleanse the blood (kidney tubule cells). Anatomy of a Generalized Cell Although no one cell type is exactly like all others, cells do have the same basic parts, and there are certain functions common to all cells. Here we will talk about the generalized cell, which demon- strates these many typical features. In general, all cells have three main regions or parts a nucleus (nu 9 kle-us), cytoplasm (si 9 to-plazm ), and a plasma membrane (Figure 3.1). The nucleus is usually located near the center of the cell. It is sur- rounded by the semifluid cytoplasm, which in turn is enclosed by the plasma membrane, which forms the outer cell boundary. (Figure 3.4 on p. 67 shows the more detailed structure of the generalized cell as revealed by the electron microscope.) The Nucleus Anything that works, works best when it is con- trolled. For cells, headquarters, or the control center, is the gene-containing nucleus ( nucle 5 kernal). The genetic material, or deoxyribonucleic acid (DNA), is much like a blueprint that contains all the instructions needed for building the whole Chapter 3: Cells and Tissues 63 Chromatin Nuclear envelope Nucleus Nucleolus Nuclear pores Rough ER (a) (b) Nucleus Cytoplasm Plasma membrane FIGURE 3.1 Anatomy of the generalized animal cell nucleus. (a) Orientation diagram: The three main regions of the generalized cell. (b) Structure of the nucleus. body; so, as one might expect, human DNA differs from that of a frog. More specifically, DNA has the instructions for building proteins . DNA is also ab- solutely necessary for cell reproduction. A cell that has lost or ejected its nucleus (for whatever rea- son) is programmed only to die....
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