The cell is the basic structural, functional, and biological unit of all known living organisms. Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. Organisms can be classified as unicellular (consisting of a single cell; including bacteria) or multicellular (including plants and animals).
The cell was discovered by Robert Hooke in 1665, who named the biological unit for its resemblance to cells inhabited by Christian monks in a monastery. Cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, that all cells come from preexisting cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.
- Prokaryotes are single-celled organisms, while eukaryotes can be either single-celled or multicellular.
- The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of membrane-bound organelles (compartments) in which specific metabolic activities take place.
- All cells possess DNA, the hereditary material of genes, and RNA, containing the information necessary to build various proteins such as enzymes, the cell’s primary machinery.
- Embedded within this membrane is a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell.
- Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
- The ER has two forms: the rough ER, which has ribosomes on its surface that secrete proteins into the ER, and the smooth ER, which lacks ribosomes.
- In complex multicellular organisms, cells specialize into different cell types that are adapted to particular functions.