The basic unit of life is the cell. All organisms have at least one cell, and the cell is the smallest unit that expresses all of the characteristics of life. Most cells are very small, and the first cell was not seen until the invention of microscopy in the 1600s. Generally, the simplest bacterial cell type has a rigid outer cell wall protecting the rest of the cell and a selectively permeable membrane enclosing its interior. This cell type also has a jellylike cytoplasm containing its genetic material and other important structures or substances. However, the cytoplasm in this cell type does not contain specialized, membrane-bound organelles. Such unicellular organisms, called prokaryotes, were the first life-forms to appear on Earth. Eukaryotes, with more complex cells containing a nucleus to enclose genetic material and additional specialized, membrane-bound organelles to perform other functions, appeared later. Some eukaryotes are unicellular, while others are multicellular. Scientists classify cells according to their characteristics and genetic relationships. All life is sorted into three domains: Bacteria, Archaea, and Eukarya. Domains Bacteria and Archaea are the prokaryotes. All eukaryotes belong to the domain Eukarya, which includes the kingdoms of plants, animals, fungi, and protists.
At A Glance
Living things are made of one or more cells; living things grow, reproduce, and respond to their environments.
- In 1839 scientists Matthias Schleiden and Theodor Schwann proposed the unified cell theory, stating that all living things are made of at least one cell, that cells are the basic units of life, and that new cells arise only from existing cells.
- The cell functions as the basic unit of life because each cell is able to grow, use energy, and reproduce, which are the characteristics that define life.
- All cells exhibit key features, including a cell membrane, cytoplasm, genetic material, and ribosomes.
- Cells come in a range of sizes, but most are very small.
- All cells descended from an early ancestral cell.
- According to the endosymbiotic theory, some ancient prokaryotes took up residence inside other cells, giving rise to the first eukaryotic cells millions of years ago.
- Cells of prokaryotes and eukaryotes have fundamental structural differences.
- Scientists classify organisms partially based on cell type, with prokaryotes sorted into two domains, Bacteria and Archaea, and eukaryotes placed into their own domain.
Prokaryotes, which include true bacteria and archaea, live in many different kinds of habitats.
- Some major eukaryotic groups include plants, which have cell walls and carry out photosynthesis; animals, which are multicellular and obtain nutrition by eating other organisms; and fungi, which may be single-celled or multicellular and digest food outside of their bodies.
All cells are surrounded by a membrane and contain genetic material but otherwise are structurally diverse.