Introduction to cell biology

Cells: Fundamental Units of  Life

What are the fundamental properties that characterize living things and distinguish them from nonliving matter? The answer begins with a basic fact that is taken for granted now, but marked a revolution in thinking when first established 175 years ago. All living things (or organisms) are built from cells: small, membrane-enclosed units filled with a concentrated aqueous solution of chemicalsnand endowed with then extraordinary ability to create copies of themselves by growing and then dividing in two. The simplest forms of life are solitary cells.

Let us begin with size. A bacterial cell—say a Lactobacillus in a piece of cheese—is a few micrometers, or μ m, in length. That’s about 25 times smaller than the width of a human hair. A frog egg—which is also a single cell—has a diameter of about 1 millimeter. If we scaled them up to make
the Lactobacillus the size of a person, the frog egg would be half a mile high.

 New cells form by growth and division of existing cells.(A) In 1880, Eduard Strasburger drew a living plant cell (a hair cell from a Tradescantia flower), which he observed dividing into two daughter cells over a period of 2.5 hours. (B) A comparable living plant cell photographed recently through a modern light microscope.

If you cut a very thin slice from a suitable plant or animal tissue and view it using a light microscope, you will see that the tissue is divided into thousands of small cells. These may be either closely packed or separated from one another by an extracellular matrix, a dense material often made
of protein fibers embedded in a polysaccharide gel. Each cell is typically about 5–20 μ m in diameter. If you have taken care of your specimen so that its cells remain alive, you will be able to see particles moving around inside individual cells. To see the internal structure of a cell is difficult, not only because the parts are small, but also because they are transparent and mostly color- less. One way around the problem is to stain cells with dyes that color particular components differently. Alternatively, one can exploit the fact that cell components differ slightly from one another in refractive index, just as glass differs in refractive index from water, causing light rays to be deflected as they pass from the one medium into the other. The small differences in refractive index can be made visible by specialized optical techniques, and the resulting images can be enhanced further by electronic processing.

Cells form tissues in plants
and animals. (A) Cells in the root tip of a
fern. The nuclei are stained red, and each
cell is surrounded by a thin cell wall (light
blue). (B) Cells in the urine-collecting ducts
of the kidney. Each duct appears in this
cross section as a ring of closely packed
cells (with nuclei stained red ). The ring is
surrounded by extracellular matrix, stained
purple.

The cell thus revealed has a distinct anatomy. It has a sharply defined boundary, indicating the presence of an enclosing membrane. A large, round structure, the nucleus, is prominent in the middle of the cell. Around the nucleus and filling the cell’s interior is the cytoplasm, a transparent substance crammed with what seems at first to be a jumble of miscellaneous objects. With a good light microscope, one can begin to distinguish and classify some of the specific components in
the cytoplasm, but structures smaller than about 0.2 μ m—about half the wavelength of visible light —cannot normally be resolved; points closer than this are not distinguishable and appear as a single blur.
In recent years, however, new types of fluorescence microscopes have been developed that use sophisticated methods of illumination and electronic image processing to see fluorescently labeled cell components in much finer detail ( Figure 1–6B ). The most recent super-resolution fluorescence microscopes, for example, can push the limits of resolution down even further, to about 20 nanometers (nm). That is the size of a single ribosome, a large macromolecular complex composed of 80–90 individual proteins and RNA molecules.