We have cells that reproduce by cell division. How do our cells differ from cells that are considered independent organisms?
That is a very, very, very broad question. Here are some diffences between prokaryotes and eukaryotes:
But it goes far beyond that. You can ask about the differences between a white blood cell and an amoeba, a neuron and a diatom, a simple squamous epithelial cell and a euglena, a skeletal muscle cell and a yeast…
I’m not really looking for a technical biological answer as to the literal differences. I’m looking for a high-level answer as to why a bacterium, for example, is an organism but a somatic cell (I’ve been Googling) is not.
Because a bacteria contains and uses all of the genes to (hopefully, from its point of view) survive in its environment. Cells in multicellular organisms specalize into organs and tissues, with every type having a different set of genes active than every other type. Which means that they no longer do everything they need to survive. Some tasks are trusted to pancreas cells, some to liver cells, etc. Remove a single cell from a multicellular organism and it will be missing something critical to surviving. You could, in theory, have cells that switch from specialists back to generalists if seperated from the whole organism, and some of the most simple multicellular organisms can do that, but there doesn’t seem to have been any evolutionary pressure for more complex organisms to retain that option. And there are probably strong pressures against it, because a cell would have to have at least a few of those “emergency backup systems” running in idle at all times “just in case”–being seperated from your normal food and oxygen supply is no time to start building the organelles needed for independent living. That would be like trying to put on your parachute after jumping out of the plane. (Johnny Utah excluded.)
A single-cell organism’s ultimate imperative is, basically, to multiply - to consume as much as possible, grow, and split into new cells. The innovation of multi-cell organisms is how certain cells managed to suppress this drive so that they could coexist harmoniously with similar cells, and eventually evolved specialized purposes.
Of course, ever so often this suppression mechanism goes haywire, and certain cells revert to their old single-cell states and begin multiplying like crazy. We call this “cancer”.
Interesting video I just watched today on the topic:
The interesting part for me is that your cells have a whole bunch of “prove you’re not cancer” surface proteins, and cells that go around killing other cells that display them wrong (or not at all).
I’m not sure that’s really an answer to the OP’s question, though, because there are plenty of single-celled eukaryotes, too, and yet those eukaryotes behave very differently from the cells in our body.
Of course, for single-celled organisms, the default isn’t to assume that an unknown cell is benign. The default is that everything is cancerous, or the equivalent of it.
Which is why my post continued with this:
But it goes far beyond that. You can ask about the differences between a white blood cell and an amoeba, a neuron and a diatom, a simple squamous epithelial cell and a euglena, a skeletal muscle cell and a yeast…
A multicellular organism consists of germline cells (sperm/eggs) and somatic cells (everything else). Neither type of cell is individually an organism. But germline cells do share a key property with single-celled organisms: they have a direct evolutionary future in progeny. Whereas the lineages of all somatic cells die with the organism.
Paraphrased from a prior thread on cancer:
The remarkable thing is that multicellular life exists at all. For billions of years, all of life was single-celled organisms, and most of it still is. This is a more natural evolutionary state, since every cell has direct descendants. Cells compete for survival, with every cell lineage having a potential evolutionary future. Successful single-celled organisms usually follow a straightforward strategy of maximum proliferation, something like scarf up nutrients from the environment, metabolize those nutrients as efficiently as possible, make as many copies of myself as possible as quickly as possible.
By contrast, from an evolutionary perspective, multicellular life seems an odd strategy. In complex multicellular organisms, billions or trillions of somatic cells have no direct descendants at all, their lineages die out when the organism dies. The vast majority of cells in the organism sacrifice their individual procreative potential for the sake of a tiny number of germline cells. This only makes sense because all cells in a multicellular organism share the same DNA. Thus, the self-sacrificing somatic cells do have an indirect evolutionary future, because the successful passage of germline cells into the next generation means that identical copies of their own DNA will proliferate.
Thus, in order for multicellular life to be feasible, one genome (much larger than the genome of most single-celled organisms) must incorporate complex mechanisms that control differentiation into all the different somatic cell types, and then regulate the growth of the different cell types in specific limited quantities in specific places for the benefit of the organism as a whole. Make a liver so big, then stop proliferating. Make a certain thickness of skin, then stop proliferating. Make as many leukocytes as necessary to protect the organism, then stop. All of this is dedicated to increase the chance of the copies of this genome that are carried in the organism’s sperm or eggs making it into the next generation.
The problem is that the body of a multicellular organism is itself an evolutionary environment where cells are constantly replicating and acquiring new DNA mutations, by routine copying errors exacerbated by carcinogens and the bombardment of radiation. There are many redundant checks and balances to regulate the growth of cells as appropriate for the good of the organism as a whole, but sooner or later some cells will acquire enough DNA mutations in key places (usually at least 6 mutations) to override all of the checks and balances. Eventually, the organism partially reverts to the ancestral condition of competition among lineages of selfish individual cells. The most “successful” cell lineages in that environment are those that follow the crude ancestral evolutionary strategy: scarf up nutrients from the environment, metabolize those nutrients as efficiently as possible, make as many copies of myself as possible as quickly as possible. This is cancer.