Prokaryotic Cell Structure

The prokaryotic cell is the structural plan shared by bacteria (and archaea): a cell whose genetic material lies free in the cytoplasm without a membrane-bounded nucleus, bounded by a plasma membrane and, in most bacteria, a cell wall, and equipped with surface and internal structures that carry out motility, attachment, and the basic chemistry of life.

Definition

Prokaryotic cell structure is the cellular organization of bacteria characterized by the absence of a membrane-bounded nucleus and of membrane-bounded organelles, with the chromosome organized as a nucleoid and protein synthesis carried out by 70S ribosomes.

Scope

This topic describes the components common to bacterial cells, the nucleoid and plasmids, cytoplasm and 70S ribosomes, the plasma membrane, the cell wall and envelope, and surface appendages such as flagella, pili, and capsules, and contrasts the prokaryotic plan with the eukaryotic cell. It treats these as reference anatomy; the envelope, shape, and staining are covered in detail by sibling topics.

Core questions

• Which structures are common to essentially all bacterial cells?
• How is the bacterial chromosome organized and spatially arranged without a nucleus?
• What distinguishes the prokaryotic cell plan from the eukaryotic one?

Key concepts

• Nucleoid and plasmids
• 70S ribosomes
• Plasma membrane
• Cell wall and envelope
• Cytoskeletal proteins (e.g. MreB, FtsZ)
• Surface appendages: flagella, pili, capsule
• Absence of membrane-bounded organelles

Mechanisms

A bacterial cell encloses its cytoplasm within a plasma membrane that performs energy transduction and transport; in most species an external wall provides shape and protection. The chromosome is a typically circular DNA molecule condensed into a nucleoid and is not separated from the cytoplasm by a nuclear membrane, so transcription and translation can occur together; additional genes may reside on plasmids. Protein synthesis uses 70S ribosomes. Although long regarded as structureless, bacteria possess cytoskeletal proteins, such as the actin homologue MreB and the tubulin homologue FtsZ, that organize the cell, position the chromosome, and direct division and shape. Surface structures, flagella for motility, pili for attachment and conjugation, and capsules or slime layers, mediate interaction with the environment.

Clinical relevance

Several prokaryote-specific structures, the wall, the 70S ribosome, and unique membrane and division machinery, are points at which prokaryotic and eukaryotic biology diverge and are therefore central to how microbiologists describe and distinguish bacterial cells. This entry is descriptive reference anatomy and is not a basis for individual diagnostic or treatment decisions.

Evidence & guidelines

The structural account is consolidated in standard microbiology textbooks, while the molecular distinction of the prokaryotic domain and the discovery of a bacterial cytoskeleton rest on the primary literature in molecular phylogeny and cell biology.

History

The prokaryote-eukaryote distinction was formalized in the mid-twentieth century on the basis of the absence of a nucleus and organelles, and the molecular phylogeny of the 1970s redefined the prokaryotic world. The later discovery that bacteria possess cytoskeletal proteins overturned the older picture of the bacterial cell as an unstructured bag of enzymes.

Key figures

• Carl Woese
• Lucy Shapiro
• Zemer Gitai

Seminal works

• woese-1977
• silhavy-2010
• gitai-2005

Frequently asked questions

What is the main difference between a prokaryotic and a eukaryotic cell?

Prokaryotic cells lack a membrane-bounded nucleus and membrane-bounded organelles; their chromosome lies in the cytoplasm as a nucleoid and they use 70S rather than 80S ribosomes.

Do bacteria have a cytoskeleton?

Yes. Bacteria possess cytoskeletal proteins such as MreB (an actin homologue) and FtsZ (a tubulin homologue) that organize cell shape, chromosome position, and division.