- All living things are made of cells.
- All cells arise from preexisting cells.
These two tenets made clear that the cell is the fundamental unit of life.
Cell Size Cells could not be studied until the microscope was developed because they are very small. This fact raises two questions: why are cells so small, and why are living things made up of millions of tiny cells? Cells are small because their surface area and volume must be balanced. In order to stay alive, cells with a larger volume need to carry out more chemical activity than smaller cells do. However, for metabolic activity to take place, the cell must also have enough surface area to allow an adequate supply of nutrients and waste products to move in and out of the cell. Because surface area increases at a slower rate than volume as objects get bigger, the surface area-to-volume ratio in a cell decreases dramatically as the cell gets larger. It turns out that a size of 10 provides the surface area-to-volume ratio necessary for the survival of most cells.
Microscopes
Two major types of microscopes allow scientists to study the miniature world of the cell.
The Light Microscope Light microscopes use light and lenses to magnify their subjects. The most common of these used in the laboratory is the compound microscope, which creates high magnification by combining two relatively low-power lenses. The total power of a compound microscope is the power of the ocular lens, located in the eyepiece, multiplied by the power of the objective lens, located near the slide. For example, an ocular lens of 10x and an objective lens of 11x yield a total magnification of 110x. Typical high school microscopes offer magnifications of up to about 430x. From time to time the SAT II Biology tests your knowledge of the various parts of the compound microscope, usually by showing you an image and asking you to identify the parts. Many parts of the cell are hard to see under microscopes because they are colorless. In order to view them, scientists sometimes employ stains that mark various cell parts differently. One alternative to staining is a technique called phase contrast microscopy, which uses filters to emphasize the contrast between different parts of the cell.
The Electron Microscope At high magnifications, light microscopes produce blurry images. In the 1950s, scientists invented a new type of microscope called the electron microscope, which offers increased image clarity, or resolving power. Electron microscopes are powerful enough to resolve individual fats and proteins. Light microscopes are still widely used, however, because electron microscopes are expensive and can only be used to view matter that is not living.