Could someone explain what are single celled organisms and how they differ from multicellular organisms?
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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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