all living things have cells. every cell has a cell membrane on the outside, and a jelly-like material that fills the inside, called cytoplasm. the membrane is important because it separates the chemicals inside and outside. some molecules can pass through the membrane, but others cannot. living cells have genes, made of dna. genes say to the cell what to do, like a language. one dna molecule, with many genes, is called a chromosome. cells can copy themselves to make two new cells.