7.2: Cells—The Lives Within Our Life

The truth is that each of us is but a society of cells which live and work together to survive and reproduce. Human cells vary enormously in size, but generally they’re too small for all but microscopic view. Most range in size from half a micron to 20 microns—that is, from about a fifty-thousandth of an inch in diameter to something less than one-thousandth of an inch. Nerve cells are our largest, for they serve much like living wires. Many nerve cells are small, but they can be as long as several feet. Some kinds of human cells are shown in Figure 7-1.

To describe the cells as separate lives is no mere poetic license. It’s a simple fact. With just a few exceptions, each cell, within its own outer membrane, carries its complete hereditary plan and can reproduce itself.

It has its own systems for taking in nutrients. On the average, it produces hundreds of complex chemicals—some cells make thousands—to break those nutrients down, rearrange its atoms, and replace damaged or aging cell parts. It has its own system of waste disposal and its ways to communicate with other cells. It has its specific work to do in supporting and controlling its fellow cells. It even breaks itself down into harmless chemical bits when death comes.

It’s almost too much to believe—that these cells can conduct such lives when, in many cases, a million of them can fit onto the head of a pin. It’s hard to comprehend the concept that we have no life aside from these tiny lives, that all of life on earth, from insect to oak tree, is nothing more than colonies of cells.

But it’s true. Take away the cells, and we are only puddles of salty, chemical-laden water, with a bit of grease and protein—which the cells have made.

Even for biologists, the concept is a little unnerving. Yet it explains why some tiny microbes (bacteria are single cells) can strike down man with disease. The invading microbial cells are really at war only with other cells—generally their equals in size and strength. And much of modern medicine is based on the understanding that the war is at this microscopic level. In fact, the vast bulk of modern pharmacology—from penicillin to the exotic drugs which combat leukemia, from sulfa to the birth control pill—is aimed at guarding, healing, and controlling, not the whole organ or whole body, but the cell.

Figure 7-1: Some Kinds of Human Cells. (a) Goblet cell. (b) Cartilage cell, (c) Bone cell. (d) Two kinds of blood cells. e) One kind of nerve cell. (f) Cells from the lining of the trachea, or windpipe. (g) Epithelial cells from the lining of the urinary bladder. (h) Cells from the outer layer of the skin. (j) a sperm, male reproductive cell. (k) an ovum, female reproductive cell. (l) a smooth muscle cell from the stomach.

Cells differ very widely in some respects. The blood cells which initiate clotting, for example, are extremely small. Other cells, such white blood cells, are especially large and can actually swallow up and digest invading bacteria.

Figure 7-2: Some parts of a “typical” cell. Mitochondria: where oxygen is used to release energy from food. Nucleus: home of DNA, the genetic material. Ribosomes: sites of protein synthesis. Cytoplasm: fluid that fills the cell.

Some cells, such as the muscle cells, can relax and contract. Some cells tend to stack themselves like stones in a wall, as do skin cells. Some have deep indentations, as do the goblet cells in glands. Some rapidly produce chemicals which send electrical impulses along a chain, as nerve cells do.

But all human cells have certain attributes in common. A few key features of a typical cell are shown in Figure 7-2; those which don’t concern us here, are omitted.

First, we might notice that the cell has an outer skin, known as its membrane. Inside the membrane lies the cytoplasm (“cell plasma”), the fluid of the cell. The cytoplasm has a great many small bodies—such as mitochondria and ribosomes— within it. Around these wash nutrients, wastes, and chemicals, circulating freely.

Mitochondria are the power centers of the cell. It’s in these tiny bodies that the body’s fuel is finally burned to free the energy we need to live. Ribosomes are the protein factories of the cell.

Finally, we reach the core of the cell, the nucleus. Isolated by its own special membrane, this is the computer, data bank, and administrator of the cell. For it holds the ultimate substance of life—DNA.

DNA is the genetic material passed from generation to generation. To DNA is attributed the differentiation of all life’s forms and processes. It determines the difference between a nose and a leaf, between a grape and an asparagus, between a gnat and an elephant. How does DNA provide such a variety of life? How does it control life processes within our own bodies?