Could someone explain what are single celled organisms and how they differ from multicellular organisms?
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I feel like you’re going to have to be more specific, because you already understand what’s different between single-sell and multicell organisms…the number of cells.
Any individual cell in any organism is fully alive…all it needs from the outside world is energy, water, and raw materials.
A single-cell organism can extract that directly from the environment. The whole organism is one cell. Bacteria, algae, yeast all fall in this category.
A bunch of single-cell organisms can group up to make a bigger group but, if it’s really simple, the cells might not be cooperating in any way, they’re just growing in a blob. If you cut them apart, they’re all just fine.
If you get enough cells in a blob they may start to specialize…some cells capture energy, some move water, some extract raw materials. They can mutually survive better by cooperating. Now you have a mutli-cell organism…you can’t just cut it apart and have it live, the cells have evolved to need each other in their specialized roles. Multi-cell organisms covers a *huge* range from just a few cells to us and things bigger than us.
The basic life function is reproduction. Realistically that requires some way to gather and process the materials and energy needed for reproduction and a way to dispose of waste material.
Single celled organisms can do that. They can allow molecules into the cell through various mechanisms; some let it bubble through them cell membrane, some have primitive “mouths”, some envelope the substance. They can make a copy of their DNA and split in half (some of them, such as paramecia can even exchange DNA with other single-celled organisms first to implement a primitive form of sexual reproductions). They can also expel molecules they don’t need.
Cells can come together in 2 basic ways; they can be mostly the same or they can differentiate.
Cells can get an advantage by clustering together. That can, for example, change the environment to make it more advantageous for that organism (yeast does this).
They can get an even bigger advantage by specializing. Some cells can be really hard and be a support structure. Some cells can get good at transporting materials. Some can turn into light, sound or chemical detectors. The end result is a significantly higher ability to ensure that it can survive to reproduce.
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