There are two distinct types of cells : prokaryotic cells (. bacterial cells) and eukaryotic cells (. plant or animal cell). The main difference between the two is a well-defined nucleus surrounded by a membranous nuclear envelope present only in eukaryotic cells. Despite this difference they share a number of common features: the genetic information is stored in genes , proteins serve as their main structural material, ribosomes are used to synthesize proteins , adenosine triphosphate is the main source of metabolic energy to sustain various cellular processes, and a cell membrane that controls the flow of substances into and out of the cell.