
Introduction to Cells
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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).

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.


Prokaryotic cells - i.e. bacteria and cyanobacteria.

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 |