Cells are the fundamental units of life, the smallest units that can independently acquire energy, metabolize, develop, and reproduce. A lipid-containing plasma membrane surrounds all cells and separates the cell's interior from its surroundings. Within the cell is a region packed with DNA, the hereditary material. The space between the plasma membrane and the DNA is filled with cytoplasm, which contains structures that carry out life processes.
Prokaryotic cells lack a nucleus and other membrane-bound organelles.
Just as diversity occurs in a community, diversity also occurs within an organism. Many different kinds of chemicals occur, yet similarities between chemicals allow us to assign them to similar categories, e.g., monosaccharides, nucleotides. These different types of chemicals must occur within almost all cells for them to function normally. Differences between cells may be an expression of differences in amounts of the same classes of chemicals, or of differences in which compounds are present or absent. How cells perform these duties allow us to look at some structures within cells that are common to most cells, as well as identify some structures that are associated with only specific, very specialized cells.