3.1–3.3: What is a cell?

Human cell, packed with organelles.

Where do we begin if we want to understand how organisms work? Given their complexity, this task can be daunting. Whether we are studying a creature as small as a flea or as large as an elephant or giant sequoia, all organisms are made of smaller units that are more easily studied and understood (FIGURE 3-1). The most basic unit of any organism is the cell, the smallest unit of life that can function independently and perform all the necessary functions of life. Understanding cell structure and function is the basis for our understanding of how complex organisms are organized.

Figure 3.1: What do these diverse organisms have in common? Cells.

The term “cell” was first used in the mid-1600s by Robert Hooke, an English scientist also known for his contributions to philosophy, physics, and architecture. When he was made Curator of Experiments for the Royal Society of London, Hooke suddenly had access to many of the first microscopes available, and he began to examine everything he could get his hands on. Because Hooke thought the close-up views of a very thin piece of cork resembled a mass of small, empty rooms, he named these compartments cellulae, Latin for “small rooms.”

After sufficient improvements were made to early microscopes in the 19th century, the central role of the cell in biology could be understood. By the 1830s, scientists realized that all plants and animals were made entirely from cells. Subsequent studies revealed that every cell seemed to arise from the division of another cell. You, for example, are made up of at least 60 trillion cells, all of which came from just one cell: the single fertilized egg produced when an egg cell from your mother was fertilized by a sperm cell from your father.

The facts that (1) all living organisms are made up of one or more cells and (2) all cells arise from other, preexisting living cells are the foundations of cell theory, one of the unifying theories in biology, and one that is universally accepted by all biologists (FIGURE 3-2). As we see in Chapter 10, the origin of life on earth was a one-time deviation from cell theory: the first cells on earth probably originated from free-floating molecules in the oceans early in the earth’s history (about 3.5 billion years ago). Since that time, however, all cells and thus all life have been produced as a continuous line of cells, originating from these initial cells.

Figure 3.2: The cell is the basic unit of life.

Today, we know that the cell is a three-dimensional structure, like a fluid-filled balloon, in which many of the essential chemical reactions of life take place (such as the breakdown of carbohydrates for energy and the translation of the genetic code for protein production). Generally, these reactions involve transporting raw materials and fuel into the cell and exporting finished materials and waste products out of the cell. In addition, most, but not all, cell types contain DNA (deoxyribonucleic acid), a molecule that contains the information that directs the formation of various cellular products within the cell, the chemical reactions in the cell, and the cell’s ability to reproduce itself. We explore all of these features of cell functioning in this and the next three chapters.

To see a cell, you don’t have to work in a lab or use a microscope. Just open your refrigerator. Chances are you’ve got a dozen or so visible cells in there: eggs. Although most cells are too small to see with the naked eye, there are a few exceptions, including hens’ eggs from the supermarket. As long as they are unfertilized, which most store-bought hens’ eggs are, each egg tends to contain just one cell. The ostrich egg, weighing about three pounds, contains the largest of all animal cells. (We should note, however, that by the time the ostrich lays a fertilized egg, the embryo inside has already gone through multiple divisions.)

In addition to being among the largest cells around, eggs are also the most valuable. Almas caviar, eggs from the beluga sturgeon, sells for nearly $700 per ounce. This value is exceeded only by that of human eggs, which currently fetch as much as $25,000 for a dozen or so eggs on the open market (FIGURE 3-3). (Human sperm cells command only about a penny per 20,000 cells!)

Figure 3.3: Not all cells are tiny. And some cells are extremely valuable!

Most cells are much smaller than hens’ eggs and ostrich eggs. Consider that, at this very moment, there are probably more than seven billion bacteria in your mouth—even if you just brushed your teeth! This is more than the number of people on earth. It is possible to squeeze so many bacteria in there because most cells (not just bacteria) are really, really tiny—so tiny that 2,000 red blood cells, lined up end to end, would just extend across a dime.

In this chapter, we investigate the two different kinds of cells that make up all of the organisms on earth, the processes by which cells control how materials move into and out of the cell, and how cells communicate with each other. We also explore some of the important structures found in many cells and the specialized roles these structures play in a variety of cellular functions. Along the way, we learn about some of the health consequences when cells malfunction.