Introduction to Cells

Introduction to Cells

The cell is the basic unit of an organism and consists of a jelly-like material surrounded by a cell membrane.

It can be seen with a light microscope (LM) but many of the structures within a cell - organelles - can only be seen clearly with an electron microscope (EM). That is partly because an EM has a greater magnifying power (ability to enlarge something).

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However, increasing only magnification has its limits because at some point magnification reveals nothing more - the details only look bigger and vaguer. This is because if 2 objects are a distance of less than half the wavelength of light apart, they cannot be distinguished as separate by a LM. Any object less than half the wavelength of light in size will not be seen at all by a LM.

Using electrons instead of light means that the illumination has a much shorter wavelength than light. This is good because minute detail can be detected. We say that an EM has a bigger resolving power (bigger resolution) than an LM.

There are 2 basic cell types:

  • Prokaryotic: bacteria and cyanobacteria (which used to be called blue-green algae).

  • Eukaryotic: all other cells, such as protoctista, fungi, plant and animal cells.

Eukaryotic cells, i.e. animal and plant cells.

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Prokaryotic cells - i.e. bacteria and cyanobacteria.

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Prokaryotic Eukaryotic
Features Plant Animal
Size(diameter) 0.5 - 5 µm 40 µm 15 µm
Cell wall Yes (contains peptidoglycan) Yes (contains cellulose) No
Genetic Material DNA is naked. A single circular molecule DNA linear, associated with histones (proteins), in a nucleus, surrounded by a nuclear envelope.
Ribosomes 70S ribosomes (smaller) 80S ribosomes (larger)
ER, Golgi apparatus No Yes
Mitochondria No(respiration occurs on an infolding of the cell membrane called the mesosome.) Yes
Chloroplasts No Yes No

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