Cells are the foundational level of organization of life. An organism may be unicellular, as with prokaryotes and most eukaryotic protists, or multicellular where the functions of an organism are divided into different collections of specialized cells. In multicellular eukaryotes, cells are the building blocks of complex structures and can have various forms and functions.
Cells are the building blocks of all living organisms, whether it is a single cell that forms the entire organism (e.g., a bacterium) or trillions of them (e.g., humans). No matter what organism a cell is a part of, they share specific characteristics.
A living cell has a plasma membrane, a bilayer of lipids, which separates the watery solution inside the cell, also called cytoplasm, from the outside of the cell.
Furthermore, a living cell can replicate itself, which requires that it possess genetic information encoded in DNA. DNA can be localized to a particular area of the cell, as in the nucleoid of a prokaryotic cell, or it can be contained inside another membrane, such as the nucleus of eukaryotes. Eukaryote means "true nucleus." The word prokaryote, hence, implies that the cell is from a group which arose before membrane-bound nuclei appeared in the history of life.
Prokaryotic cells lack internal membranes. In contrast, eukaryotes have internal membranes that enclose compartments called organelles. Each organelle fulfills a specific cellular function, such as the membrane-bound nucleus and mitochondria.
The presence or absence of certain organelles is used to classify organisms. While plant cells possess chloroplasts in which they convert light energy to sugar, animal cells contain lysosomes, a membrane-enclosed compartment in which enzymes break up larger molecules.
