Tutorial: Basic Cell Biology I: Answers

What are the differences between a bacterium and a virus?

I guess there are lots of differences that one could suggest, but for me the defining one is the fact that bacteria can replicate itself without the help of a host cell. A virus cannot replicate independently.

There are several other differences, all of which are directly or indirectly related to the above:

  • Viruses do not carry their own enzymes for making proteins, nor do they contain ribosomes which are contained in bacteria for protein production.
  • Viruses do not have a cell membrane or a cell wall.
  • Viruses tend to be much smaller than bacteria and are often only visible with the use of an electron microscope.
  • Viruses are not treatable with antibiotics. Some viruses however can be treated with effective anti-viral compounds.
  • Viruses cannot be cultured on agar plates like most bacteria. They require living cells in the form of cell cultures.



What are the main constituents of a bacterial cell?

Bacteria are efficient replicating machines and survival experts thus all their consituents serve a function to these purposes.

Best to “break down” a bacterium into what is inside the cell, what is on the surface of the cell and what is attched to the surface.

Inside a bacterium:

  • Nucleoid area containing DNA. Note this is not a nucleus
  • Ribosomes: Area where proteins are synthesized.
  • Cytoplasm: Contains nutrients, proteins and enzymes, mostly associated with allowing the bacterium to replicate.

On the surface of a bacterium: (from inside to out)

  • Cell membrane: Mediates transport of substances in and out of teh bacterium. Conatains enzymes
  • Cell wall: Thinner in Gram negative bacteria than in Gram Positive. Confers rigidity to the cell. Also acts as a permeability barrier.
  • Capsule (not in all bacteria) : Protects against phagocytosis. Can prevent dessication of the cell.

Attached to the surface of a bacterium

  • Sex Pili (not in all bacteria): Allows DNA transfer between bacteria by conjugation.
  • Flagellae (not in all bacteria): Gives motility to the bacterium, allowing it to find an area where it can survive best.
  • Fimbriae: also called common pili. Facilitates attachment to cells eg epithelial cells.

 Try and draw a schematic bacterium to see if you know the basic structure.


Of course there are some bacteria which have more specialisd components and functions. If you are looking for a distinction in an exam you will need to know about these. Consider things like plasmids, cytoskeleton, vesicles, slimes layers. The list goes on…

Further reading:

Todar’s bacteriology: Cell structure.

Wikipedia: Bacterial cell structure.


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