3. Cells were observed first in the late seventeenth century, when Robert Hooke viewed cork with a crude lens. Antonie van Leeuwenhoek viewed many cells under the light microscope he invented. Stains revealed subcellular details.
4. The cell theory states that all life is composed of cells, that cells are the functional units of life, and that all cells come from preexisting cells.
3.2 Variations on the Cellular Theme
5. Bacteria are unicellular and lack nuclei and have DNA, ribosomes (structures that help manufacture proteins), enzymes for obtaining energy, and various other biochemicals. A cell membrane and usually a rigid cell wall enclose the cell contents.
6. Archaea are unicellular and lack nuclei. They share some characteristics with bacteria and eukaryotes but also have unique structures and biochemistry.
3.3 Organelles Divide Labor
7. Eukaryotic cells sequester certain biochemical activities in organelles. A eukaryotic cell houses DNA in a membrane-bounded nucleus; synthesizes, stores, transports, and releases molecules along a network of organelles (endoplasmic reticulum, Golgi apparatus, vesicles); degrades wastes and digests nutrients in lysosomes; processes toxins and oxygen in peroxisomes; extracts energy from digested nutrients in mitochondria; and in plants and some protists, extracts solar energy in chloroplasts. A cell membrane surrounds eukaryotic cells. Cell walls protect and support cells of many organisms.
8. The endosymbiont theory proposes that chloroplasts and mitochondria evolved from once free-living bacteria engulfed by larger archaea. Evidence for the endosymbiont theory is that mitochondria and chloroplasts resemble small aerobic bacteria in size, shape, and membrane structure and in the ways their DNA, RNA, and ribosomes interact to manufacture proteins. Gene and genome sequence data also support the endosymbiont theory.