State the three tenets of cell theory and compare the structure of prokaryotic and eukaryotic cells.
Cell theory, one of biology's oldest unifying ideas, rests on three tenets built up over roughly two centuries of microscope work: all living things are composed of one or more cells; the cell is the basic unit of structure and function in an organism; and all cells arise from preexisting cells. Robert Hooke first used the word "cell" in 1665 to describe the small, box-like compartments he saw in cork under an early microscope (he was actually looking at dead plant cell walls); Antonie van Leeuwenhoek later observed living, moving single-celled organisms he called "animalcules"; and by the mid-1800s, Schleiden, Schwann, and Virchow had combined and extended these observations into the formal theory.
Every cell falls into one of two broad categories. Prokaryotic cells (bacteria and archaea) are small and structurally simple: their DNA floats free in a region called the nucleoid rather than being enclosed in a nucleus, they lack membrane-bound organelles, and most have a protective cell wall outside the cell membrane. Eukaryotic cells (found in protists, fungi, plants, and animals) are larger and far more compartmentalized: their DNA is enclosed within a true, membrane-bound nucleus, and their cytoplasm is packed with specialized membrane-bound organelles, each with a distinct job -- the subject of next week.
Mastery looks like: They can state all three tenets of cell theory from memory and correctly classify a described cell as prokaryotic or eukaryotic with a reason.
Common stumbles: Confusing which cell type has a true nucleus; forgetting that eukaryotic cells are typically much larger, not just more complex, than prokaryotic ones.