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I'm taking a 3-week Bio I summer course, and my textbook says the cell is the smallest/basic unit of life. I asked my professor why organelles aren't a living unit, and her reply was that they aren't self-sufficient. But it seems that cells in multi-cellular organisms need other cells with different specializations in order to survive. You can't just (or can't always) pluck a cell out of a larger organism and leave it to fend for itself outside of it. Same with an organelle. She said only the entire cell fits all the requirements for life, referring me to the textbook, but I couldn't find an actual description of these requirements. So, what is the relevant difference between an organelle and a cell such that only the entire cell is considered alive?

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Proptoplasam is the only living organelle present inside the entire cell –  user6764 May 18 at 9:34

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The definition of life is a controversy in itself, and as it is simply a word that can be understood by everyone however they wish, there is no "correct" definition. It is thus not really possible to give a "correct" answer to your question, but here are a list of things you may want to consider:

  1. Replication of DNA is probably the most agreed common feature to all life on earth. Most organelles do not contain their own DNA but are simply lipid and protein constructs.
  2. Independence is a difficult criterion of life, as parasites require a host for reproduction. However, while the organism may require a host, its cells individually can replicate without a host.
  3. Cells are able to replicate when removed from the body - under the right conditions. (We are getting better and better at culturing all sorts of cell types, for example to avoid having to use animals for research.) Most organelles are not able to replicate under any conditions.
  4. In nature, there are single living cells. In fact, the majority of organisms on earth consist of one cell only. There are no organelles living in nature.

Mitochondria and plastids make a complicated exception to these, as they are originally derived from bacteria themselves. It is indeed debatable whether they could be considered alive, as they possess DNA and it is replicated in order for them to divide, like individual cells do. However, the machinery required is not produced by themselves but rather by their "host" cell, making them subject of a similar argument as viruses.

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Neither Mitochondria nor Plasmids could maintain or replicate themselves outside of a cell because both have passed the genes that encode for key proteins over to the host cell. –  Jack Aidley May 15 '13 at 15:42
    
Which genes exactly are you referring to? As far as I'm aware, mitochondria (I don't know about plastids) replicate their genome themselves and perform a process similar to binary fission themselves as well; but under regulation of the "host" cell. –  Armatus May 15 '13 at 19:09
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In human cells, mitochondrial DNA codes for just 37 genes. All other genes required for mitochondrial function have been transfered to the nuclear DNA. Specifically the POLG and POLG2 genes that are involved in mitochondrial DNA replication are now located on chromosomes 15 and 17, so without being present in the host cell they could not replicate. The replication itself occurs in the mitochondria but the proteins needed are synthesised by the host machinery. –  Jack Aidley May 15 '13 at 20:42
    
So very much alike viruses in regards to their being alive or not then. Thanks :) –  Armatus May 15 '13 at 22:10

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