3.1: Components of cells

The cell is described as the most basic unit of life. They are small discrete packages that carry out functional and structural roles of organisms and, aside from viruses, all organisms exist as cells or cell assemblages. Major components of bacterial and archaeal cells include the following (Fig. \(3.1\)):

• Cytoplasm: a mixture of substances and structures that are either dissolved or suspended in water and carry out the functions of the cell. Those substances and structures include macromolecules such as proteins and nucleic acids, ribosomes, small organic molecules that are precursors of macromolecules, and inorganic ions. Within the cytoplasm, raw materials from the external environment are enzymatically degraded and new macromolecules are formed.
• Cytoplasmic membrane: a phospholipid bilayer that surrounds the cytoplasm and serves as a permeable barrier. Proteins embedded in the cytoplasmic membrane provide important functions including roles in energy production and the transport of materials in and out of the cell.
• Ribosome: small organelle in the cytoplasm that is responsible for protein production in organisms of all three domains of life. See Chapter 2 for additional details.
• Cell wall: provides cells with structural support and helps determine cell shape (Beveridge, 1988). Cell walls are found in Bacteria and Archaea but also some plants, fungi, and algae of domain Eukaryota.
• Cell capsule: large structure on the outside of some bacterial and archaeal cells that usually consists of polysaccharides. It can help cells resist viruses and toxic chemicals, adhere to surfaces, and remain hydrated.
• Nucleoid: a large double-stranded molecule that aggregates to form a visible mass.
• Flagellum: lash-like appendage of some cells that is primarily used to help the cell move.
• Pilus: (not shown): hair like structure found on the surface of some bacteria and archaea. Numerous pili can be present on each cell and uses vary. They can be used for electron transfer, adherence to surfaces, conjugation, and more.

Eukaryotic cells contain many of the same components as bacterial and archaeal cells (Fig. \(3.1\)). In addition, eukaryotic cells have membrane-enclosed organelles such as the nucleus, nucleolus, mitochondria, and chloroplasts. The nucleus houses DNA, which is packed in a very organized state called chromosomes. The nucleolus exists within the nucleus and produces ribosomes. Mitochondria and chloroplasts are organelles that are used in energy production. Aside from these differences, Eukaryotes also have more DNA than Bacteria and Archaea. The genome is all of the genetic material within an organism. The genome of Eukaryotes is typically much larger, by up to several orders of magnitude, than that of bacteria (Theriot, 2013).

---