How are unicellular organisms and multicellular organisms similar? How are they different?

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Similarities:

  • One important distinction here is whether we're talking about prokaryotes, i.e. organisms without a complex cell structure, and/or eukaryotes, organisms that have nuclei and other structures. This would significantly affect the comparison.
  • However, generally speaking, we can say that unicellular and multicellular organisms are alike in that they exhibit all the functions of life, such as a metabolism and reproduction, they contain DNA and RNA, they can exhibit a wide range of lifestyles, and they are essential to almost every ecosystem that we currently know of.

Differences:

  • Unicellular organisms can reproduce faster and in greater numbers. Some of them are also capable of actually sharing DNA between living individuals, which is impossible for more complex life without technological intervention.
  • Multicellular organisms are almost always larger.
  • Unicellular organisms are more commonly found in extreme environments; the archaea, the third domain of life, counts among its members a number of "extremophiles" - single-celled organisms that thrive in environments of extreme heat, acidity, salinity, etc.
  • Multicellular organisms typically experience severe stress or death if a certain number of cells die or are separated from the group; unicellular organisms have no comparable problem.
  • Multicellular organisms require more food and living space and typically occupy a higher link in the food web.
  • Multicellular organisms may be better equipped to survive a harmful DNA mutation due to redundancy in other cells, but they are also at risk of cancer and other related diseases that have no comparison in the unicellular organism.
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