Cell types include prokaryotes, which encompass all bacteria, and eukary-otes, which are cells in all living cells except bacteria and viruses. Bacterial cells are molecules surrounded by a semipermeable membrane and which in turn is covered by a porous cell wall. The protoplasm in the membrane contains nuclear material that is not bounded by any other material separating it from the protoplasm. Organelles are complex and well-defined internal structures surrounded by membranes. Characteristic organelles comprise cell nucleus, mitochondria, and ribosome. Fig. 2.1 shows a diagram of a cell structure of a bacterium.
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