|MadSci Network: General Biology|
Hi Roopa, Thanks for your question. The difference between the two situations you described it that the lichen represents a form of symbiosis called mutualism where both organisms benefit from the interaction. With respect to the algae you studied, this association represents an *obligate* mutualistic relationship, since the algae cannot reproduce without the interaction. Mitochondria actually form part of individual cells; they're not a species that could on its own form a symbiosis with another species. The cell has been designated 'the fundamental unit of life,' so a part of the cellular machinery wouldn't generally be considered 'alive'. Hope this helps, Chris Reigstad
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