Introduction to Cells Cell Theory 1. 2. 3. All organisms are composed of one or more cells. The cell is the basic unit of structure and organization of organisms. All cells come from pre-existing cells. Organelles Small specialized structures that have a specific function. Contained in all cells. Some are surrounded by membranes. Prokaryotic Cells Those cells that do not contain membranebound organelles. Most unicellular organisms, such as bacteria, are prokaryotes. Eukaryotic Cells Those cells that do contain membrane-bound organelles. Most multicellular organisms are eukaryotes. (However, so are some single-celled organisms such as amoeba.) Nucleus Central membrane-bound organelle which controls all cellular functions in eukaryotes. Benefits of Having MembraneBound Organelles Separation of cell functions into distinct compartments. Ability to carry out various chemical reactions at the same time.