Much of our knowledge of cellular organization has been made possi¬ble by the development of better and more powerful compound microscopes. In the detailed analysis of subcellular structure, three attributes of certain microscopes like compound microscopes are of particular importance: magnification, resolution, and contrast. Magnification of the microscope is a means of increasing the apparent size of the object being viewed. Resolution of the microscope is the capacity to separate adjacent forms or objects-to show them as distinct. Contrast of the microscope is important in distinguishing one part of a cell from another.

Although ordinary light microscopes can be endowed with very high microscope magnification, their resolving power is limited. It is about 500 times better than that of the unaided human eye, but this is still not enough for viewing some, of the smaller subcellular structures. Contrast is often obtained in microscopy by fixing and staining the material being studied. Since different parts of the material often differ in their af¬finities for various dyes, it is possible to stain these parts different colors and make them stand out from each other when being viewed under a microscope.

The electron microscope (EM) has opened up exciting new vistas in the study of cells. This microscope, as its name implies, uses a beam of electrons instead of light as its source of illumination. The specimen to be examined must be sliced into extremely thin sections to allow a beam of electrons to pass through and fall on a photographic plate, where an image of the specimen is produced. Because the electrons pass through the specimen, the EM is often referred to as the trans¬mission electron microscope or transmission EM. Electron microscopes are capable of resolving objects about 10,000 times smaller than those distinguishable by the unaided human eye.

Though the transmission EM has very high magnification and reso¬lution, it does not give direct information about the three-dimensional shape of the specimen. Another kind of electron microscope, the scan¬ning electron microscope, gives a surface view of the specimen being studied and a sense of depth lacking in the transmission photograph. In the scanning EM the electron beam moves back and forth across the specimen in a manner similar to the electron beam in a television tube. The beam does not pass through the specimen but instead causes electrons to be emitted from the surface of the specimen. The scattered electrons are collected to produce a picture with a three-dimensional effect. Another not inconsiderable advantage is that the specimen need not be sectioned. Though the resolution of a scanning EM is not as great as that of a transmission EM, the three-dimensional image it provides is of great importance for many kinds of studies.

CELL SIZE

Most cells are very small and can be distinguished only with certain micro¬scopes like a compound microscope. Some, however, such as bird egg cells, can be seen with the naked eye. Others, like nerve cells, may be very small in some of their dimensions, but extremely long; a single human nerve cell may be over a meter in length. To say that cells are generally small is not saying much, however, because even among microscopic cells there is a wide range in size. The diameter of a human red blood cell is about 35 times greater than that of some very tiny microorganisms, while that of a human egg cell is about 14 times greater than that of a red blood cell. The diameter of an ostrich egg cell, in turn, is about 1,500 times greater than that of a human egg cell. Most cells, however, have a diameter ranging between 0.5 and 40 micrometers.

One probable reason why cell size has remained restricted through¬out the course of evolution is that the ratio of surface area to volume strongly affects the functioning of cells. Cells, as studied under a compound microscope, obtain necessary materials such as oxygen and nutrients from the area surrounding them. These materials must enter across the surface of the cell, and waste products must leave by the same route. As cell size increases, the volume increases much more rapidly than the surface area (volume increases as the cube of the cell radius, surface area as the square of the cell radius). Thus increasing size entails the problem of adequate exchange surface for support of the greatly increased volume. This whole question of the ratio of surface area to volume is not limited to single cells; we shall encounter it again and again in reference to whole organisms.

Another factor limiting cell size is the ability of the control center (the nucleus) to regulate the rest of the cell. As a cell increases in size, more and more of its parts must be located far from the control center and proper interaction becomes progressively more difficult.