BIOLOGY Volume 1 - A Guide to General Biology - 2004

5. CELLS

5.2. Cells under the Light Microscope

By the end of the 19th century, most of the structures visible with a Light Microscope (i.e., a microscope that uses visible light to illuminate the specimen) had already been discovered. At that time, a Cell was viewed as a small lump of living protoplasm, invariably enclosed by a Plasma Membrane and occasionally — as in plants — by a non-living Cell wall. The most prominent cellular Structure was the nucleus, containing a readily stainable material called Chromatin (a term derived from Greek meaning "colored material"). Chromatin represents the uncoiled form of Chromosomes. Prior to Cell Division, chromosomes appear as long, thin threads. They contain DNA, the genetic material. DNA regulates cellular activity and possesses The ability to replicate, thereby ensuring The formation of new Cells.

Fig. 5.1, A and 5.2 show generalized animal and plant cells as viewed under a light microscope. (A "generalized" cell displays all the typical structures found in any given cell.) The only structures shown here that had not yet been discovered by the late 19th century are Lysosomes. Fig. 5.1, B, as well as Fig. 6.2 and 6.16, present photomicrographs of various animal and plant cells.

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Fig. 5.1. A. Generalized animal cell as seen under the light microscope. B. Epithelial cells lining the inside of the human cheek, showing Characteristic Features of an animal cell. Each cell contains a nucleus surrounded by Cytoplasm, which houses numerous distinct Organelles, such as Mitochondria (x400).

Fig. 5.2. A generalized plant cell as seen under a light microscope. The asterisk indicates structures characteristic of PLANT CELLS AND absent in animal cells.

The living Contents of the cell, filling the space between its nucleus and plasma membrane, are called the cytoplasm. The cytoplasm contains numerous diverse organelles. An organelle is a distinct structural cellular component that performs a specific function (see Figs. 5.12 and 5.13, as well as Section 5.10). The only structure found in animal cells and absent in plant cells is the centriole. Overall, plant cells are very similar to animal cells, but they contain a greater variety of structures. Unlike animal cells, plant cells feature:

1) a relatively rigid cell wall covering the outside of The Plasma Membrane; fine strands known as plasmodesmata pass through pores in The Cell wall, connecting the cytoplasm of adjacent cells into a single continuous system;

2) METABOLISM/14.html">Chloroplasts, where Photosynthesis takes place;

3) a large central vacuole; animal cells contain only small vacuoles used, for example, in phagocytosis.

How to use a light microscope is explained in Section 5.11.