Acts as semi-permeable barrier to the outside of the cell. Allows flow of certain molecules in and out as needed.
Brain of the cell
Only plants have cell membranes so the are not important for human cells.
Convert glucose to usable energy for the cell
Only prokaryotes have a nucleus
DNA is kept in the nucleus
Eukaryotic cells have a nucleus
It is better to have many cells than to have large cells
Surface area to volume ratios
Cells get bigger as organisms get bigger
Cells need to move between very small areas