ch03.1 - Cellular Basis of Life

Although the four major elements build most of the

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Although the four major elements build most of the cell’s structure (which is largely protein), the trace elements are very important for certain cell func- tions. For example, calcium is needed for blood clotting (among other things), and iron is neces- sary to make hemoglobin, which carries oxygen in the blood. Iodine is required to make the thyroid hormone that controls metabolism. In their ionic form, many of the metals (such as calcium, sodium, and potassium) can carry an electrical charge; when they do they are called electrolytes (e-lek tro-l¯ ıts). Sodium and potassium ions are es- sential if nerve impulses are to be transmitted and muscles are to contract. (A more detailed account of body chemistry appears in Chapter 2.) Strange as it may seem, especially when we feel our firm muscles, living cells are about 60 percent water, which is one of the reasons water is essential 62
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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 cell’s 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).
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  • Spring '10
  • S
  • DNA, cells, Cell nucleus

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