A cell is the basic unit of living thing. It is the smallest independently functioning unit in the structure of an organism, usually consisting of one or more nuclei surrounded by cytoplasm and enclosed by a membrane.
Cells also contain organelles such as mitochondria, lysosomes, and ribosomes. Multicellular organisms are made up of cells with different shapes and sizes.
Animal cells are capable of changing their shape, but plant cell is fixed due to the presence of cellulose. In this article, the different types of cells and their characteristics shall be examined.
The diameter of most cells ranges between 0.5 and 20µ (µ or micron = 0.001 mm). Table 1 shows the range of size variation of different kinds of cells. Cells vary widely in shape and depend upon the environmental conditions, surroundings, function, and need of the organism.
Range of Size Variation of Different Kinds of Cells
| Cell Type | Size |
|---|---|
| Egg of ostrich | 170 x 135 mm |
| Egg of hen | 60 x 45 mm |
| Human egg | 0.1 mm = 100 μ |
| Amoeba | 100 μ |
| Sea urchin egg | 70 μ |
| Red blood corpuscle | 7 μ |
| Typhoid bacillus | 2.4 x 0.5 μ |
| Escherichia coli | 1.5 x 0.7 μ |
| Diplococcus pneumoniae | 200 x 100 mμ |
| Influenza virus | 100 mμ |
| Tobacco mosaic virus | 300 x 15 mμ |
| T3 bacteriophage | 45 mμ |
Cells of multicellular organisms have different shapes. The red blood cells of human beings are biconcave, whereas muscle cells are long and with pointed ends. Animal cells are capable of changing their shape, but the shape of plant cells is fixed due to the presence of a cellulose wall.
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Variation in Cell Shape (Sinha and Sinha, 1982)
- Tetracoccus
- Streptococcus
- Micrococcus
- Bacillus
- Spirillum
- Diatom
- Amoeba
- Dinoflagellate
- Acetabularia
- Desmid
- Human red blood cell
- Melanocyte
- Smooth muscle cell
- Striated muscle cell
- Tracheidal cell
- Parenchymatous cell
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Summary of Cell Structure in Living Organisms
Cell is the basic unit of living thing. The cell is the smallest independently functioning unit in the structure of an organism; usually consisting of one or more nuclei surrounded by cytoplasm and enclosed by a membrane. Cells also contain organelles such as mitochondria, lysosomes, and ribosomes.
Cells are the smallest structures capable of basic life processes, such as taking in nutrients, expelling waste, and reproducing. All living things are composed of cells. Some microscopic organisms, such as bacteria and protozoa, are unicellular, meaning they consist of a single cell.
Plants, animals, and fungi are multicellular; that is, they are composed of a great many cells working in concert. But whether it makes up an entire bacterium or is just one of trillions in a human being, the cell is a marvel of design and efficiency.
Cells carry out thousands of biochemical reactions each minute and reproduce new cells that perpetuate life. Cells of multicellular organism have different shapes and sizes. Animal cells are capable of changing shape, but the shape of plant cell is fixed due to the presence of cellulose wall.