The document discusses the cell, which is considered the fundamental unit of life. It describes key discoveries in cell biology from Hooke's observation of cells in 1665 to the formulation of the cell theory in 1838. The cell theory states that all living things are made up of cells, cells are the basic functional units of life, and new cells develop from existing cells. The document then discusses various types of cell structures, including the cell membrane, nucleus, organelles, and differences between unicellular and multicellular organisms. It provides details on cell transport mechanisms like diffusion and osmosis.