ABOUT CELLS

 what you mean by cells?

Hold your finger up and look at it closely. What do you see? Skin. It looks solid and smooth, creased with lines and flexible to the touch. But if you were able to remove a bit and examine it under a microscope, it would look very different—a sheet of tiny, irregularly shaped bodies crammed together like tiles on a floor.  takes you on a journey into your fingertip. The crammed bodies you see in panels3 and 4 are skin cells, laid out like a tiled floor. As your journey inward continues, you travel inside one of the cells and see organelles, structures in the cell that perform specific functions. Proceeding even further inward, you encounter the molecules of which the structures are made, and finally the atoms shown in panels8 and 9. While some organisms are composed of a single cell, your body is composed of many cells. A single human being has as many cells as the stars in a galaxy, nearly one hundred trillion. All cells, however, are small. In this chapter we look more closely at cells and learn something of their internal structure and how they communicate with their environment.

The Cell Theory

Because cells are so small, no one observed them until micro-scopes were invented in the mid-seventeenth century. Robert Hooke first described cells in 1665, when he used a micro-scope he had built to examine a thin slice of nonliving plant tissue called cork. Hooke observed a honeycomb of tiny, empty (because the cells were dead) compartments. He called the compartments in the cork cellulae (Latin, small rooms), and the term has come down to us as cells. For another century and a half, however, biologists failed to recognize the importance of cells. In 1838, botanist Matthias Schleiden made a careful study of plant tissues and developed the first statement of the cell theory. He stated that all plants "are aggregates of fully individualized, independent, separate beings, namely the cells themselves." In 1839, Theodor Schwann reported that all animal tissues also consist of individual cells. The idea that all organisms are composed of cells is called the cell theory. In its modern form, the cell theory includes three principles

 1. All organisms are composed of one or more cells, within which the processes of life occur.

 2. Cells are the smallest living things. Nothing smaller than a cell is considered alive.

 3. Cells arise only by division of a previously existing cell. Although life likely evolved spontaneously in the environment of the early earth, biologists have concluded that no additional cells are originating spontaneously at present. Rather, life on earth represents a continuous line of descent from those early cells.

Most Cells Are Very Small

 Cells are not all the same size. Individual marine alga cells, for example, can be up to 5 centimeters long—as long as your little finger. In contrast, the cells of your body am typically from 5 to 20 micrometers (an) in diameter, too small to see with the naked eye. It would take anywhere from 100 to 400 human cells to span the diameter of the head of a pin. The cells of bacteria are even smaller than yours, only a few micrometers thick.

Why Aren't Cells Larger?

 Why are most cells so tiny? Most cells are small because larger cells do not function as efficiently. In the center of every cell is a command center that must issue orders to all parts of the cell, directing the synthesis of certain enzymes, the entry of ions and molecules from the exterior, and the assembly of new cell parts. These orders must pass from the core to all parts of the cell, and it takes them a very long time to reach the periphery of a large cell. For this reason, an organism made up of relatively small cells is at an advantage over one composed of larger cells.

 Another reason cells are not larger is the advantage of having a greater surface-to•volume ratio. As cell size in-creases, volume grows much more rapidly than surface area. For a round cell, surface area increases as the square of the diameter, whereas volume increases as the cube. To visualize this, consider the two single cells. The large cell to the right is 10 times bigger than the small cell, but while its surface area is 100 times greater , its volume is 1,000  times the volume of the smaller cell. A cell's surface Provides the interior's only opportunity to interact with the environment with substances passing into and out of the cell across its surface, and large cells have far less surface for each unit of volume than do small ones. Some larger cells, however, function quite efficiently in Pan because they have structural features that increase surface . Cells in the nervous system, for example, called neurons,