The document summarizes key concepts in cell structure and function: 1) Early microscopists like Hooke, Leeuwenhoek, and Brown made discoveries that led to the development of the cell theory stating that all living things are composed of cells, cells are the basic units of structure and function, and new cells are produced from existing cells. 2) Cells are either prokaryotic, lacking organelles and a nucleus, or eukaryotic, having a nucleus and membrane-bound organelles. Eukaryotic cells have structures like the plasma membrane, mitochondria, ER, Golgi bodies, lysosomes, and chloroplasts that allow specialization of function. 3)