Why are bacteria and viruses so much smaller than animal cells? - I don't have more information about the question, sorry if this is too vague.

  • $\begingroup$ I actually think that this is an interesting question. The problem is that the format for this site would make this a difficult question to answer properly. Viruses use the cells they infect to make more copies of themselves, so right there, they would need to be much smaller than the cell itself, otherwise it couldn't reproduce itself. There are also energy efficiency issues, replication times, simpler genomes, fewer organelles, and on and on to explain bacterial smaller size. $\endgroup$
    – AMR
    Jan 24 '16 at 18:41

Cells/viruses vary in size mainly according to their function, components & requirements imposed by cell division/virus replication.

Bacterial cells are so small because they need a large surface area to cell volume to take in nutrients. Bacteria accumulate nutrients from the environment by diffusion alone, and so adopt certain sizes and shapes so they can import nutrients most efficiently. other evolutionary advantages are also existent in response to the demands of the environment and predators

Viruses function is pretty minimalistic. they pretty much consists of nucleic acid surrounded by a protective coat of protein called a capsid. this is the primitive virus, all it nedds is to infect cells and pass the information stored inside them.

The size of animal cells however depends partially on the type of cell and its function. Red blood cells are pretty small in comparison to other animal cells, they don't need to divide (so do not own a nucleus) and need a large surface area in order to diffuse Oxygen efficiently. Egg cells for example serve to store nutrients and don't need to carry out active metabolism, and so they are much bigger.

I took some of the examples from here, and for more technical information about the upper limits of cell size and plant cells click here.

  • $\begingroup$ All cells take nutrients in from their environment, including those of multicellular organisms, even mammals, so your second paragraph is irrelevant. Viruses can be packages with enzymes such as reverse transcriptase and integrase, etc. so your third paragraph is incomplete. Your fourth paragraph is somewhat true, but neurons would be a better example. Also you make no mention of the hypothetical size limit of all cells due to surface area to volume to diffusion rate limits. And your answer lacks references. I know the OP accepted the answer, but it lacks quality and should consider removing, $\endgroup$
    – AMR
    Jan 24 '16 at 18:50
  • $\begingroup$ I agree with you @AMR that this topic is broad. I added reference to some more detailed article for the specifics. I think the answer fits the question. A more specific answer will meet a more specific question. $\endgroup$
    – A. Steiner
    Jan 24 '16 at 20:11
  • $\begingroup$ @AMR 's arguments are valid but you can answer them if you explain a parameter regarding the shape: surface area/volume ratio. While increasing surface area will allow a greater chance of nutrient uptake and synthesis, increase of volume will result in higher energy consuption. The cell has to strike an optimum. Therefore bigger eukaryotic cells are dependent on organelles for energy. $\endgroup$
    Jan 25 '16 at 6:07

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