Inside all cells, from the most simple to the most complicated, there is cytoplasm. Cytoplasm is what all the rest of the stuff inside the cell floats in. Originally, cytoplasm was just the sea water that was inside the lipid membrane bubble when it first formed. Because all cells lived in sea water, it didn’t really matter whether the water flowed in and out of the cell membrane. The sea water was the same inside and outside the cell anyway.
Plant cytoplasm. You can see the chloroplasts floating around in it.
For most cells, that’s what cytoplasm is still today. Even inside plants and animals that have never been near an ocean, cytoplasm (SIGH-toe-plaz-um) is still a lot like sea-water, because that’s what the mitochondria, chloroplasts, DNA molecules, and other things that live inside a cell need to survive. Like the oceans, cytoplasm is mostly made of water and salt. Today we have to drink water and eat salt in order to make our own tiny oceans inside our cells. And our cell membranes have to be careful to keep the right amount of water and salt inside each cell (though cytoplasm isn’t as salty as sea water).
Bibliography and further reading about the parts of a cell: