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Archaea are a major evolutionary lineage distinct from bacteria. The cellular comparison here concerns the prokaryotic members of archaea; see two-domain system for broader clade usage. Their biological cells have a nucleoid rather than a membrane-bounded cell nucleus, yet aspects of DNA replication, transcription and biological translation resemble those of eukaryotes. Their characteristic lipids differ chemically from the usual bacterial membrane lipids, and their cell walls, when present, lack peptidoglycan. See OpenStax Biology 2e, 22.2: archaeal distinctions.

Some archaea perform methanogenesis, producing methane as part of their metabolism. Archaea also inhabit ordinary soils and oceans: extreme habitats do not define the lineage. See OpenStax Biology 2e, 22.1: moderate and extreme environments. Woese and Fox's 1977 analysis of ribosomal RNA distinguished the methanogenic group they called archaebacteria from typical bacteria. Their proposed three primary lines of descent were a historical interpretation, rather than the final resolution of deep phylogenetics. See Woese and Fox, 1977: Phylogenetic structure of the prokaryotic domain.

Bacteria are a major evolutionary lineage of organisms with prokaryotic cellular organization. Escherichia coli and cyanobacteria illustrate its variety: the same broad cellular plan supports different kinds of metabolism. Most bacteria have a cell wall containing peptidoglycan; archaea do not have peptidoglycan walls. A cell wall and a cell membrane are distinct structures, and some bacteria lack a cell wall. See OpenStax Biology 2e, 22.2: bacterial envelopes.

The usefulness of recognizing bacteria extends beyond naming: it identifies a shared evolutionary background for comparing genes, proteins and cellular mechanisms. A familiar shape or the absence of a cell nucleus alone cannot distinguish bacteria from archaea; molecular comparisons, including ribosomal RNA, supply additional evidence.

A bacterial outer membrane is an additional membrane outside the peptidoglycan in typical Gram-negative bacteria. It adds a barrier while the inner cell membrane remains the boundary of the cytoplasm.

A cell wall is a supporting structure outside a cell membrane. It helps preserve shape and bears the load when osmosis raises internal pressure. The cell membrane supplies the selective permeability described in its own article; the cell wall supplies mechanical reinforcement. These functions cooperate: a porous supporting layer can restrain expansion without being the boundary that retains dissolved solutes. Turgor pressure makes this cooperation visible in a firm leaf; plasmolysis and a lysozyme wall-removal experiment separate the two functions experimentally.^[ref]

The material is not universal. Most bacteria use peptidoglycan, whose linked sugar chains and amino acid cross-links are explained below. A plant cell wall uses cellulose; a fungal cell wall contains chitin and glucans. Animals lack a cell wall, some bacteria are naturally wall-free, and archaea have different wall chemistry. Thus a cell wall is neither a universal cellular component nor one chemical recipe.^{[ref][ref][ref]}

The bacterial outer membrane in typical Gram-negative bacteria is an additional membrane outside the peptidoglycan, not another name for that supporting mesh. The cell envelope includes the boundary layers together. An extra barrier can change access to the peptidoglycan, so response to lysozyme cannot be inferred from the mere presence of a cell wall.^[ref]

A fungus is a eukaryote that obtains nutrients by absorption rather than photosynthesis. Fungi include unicellular yeasts and multicellular forms. Their cell walls differ from bacterial peptidoglycan walls; having a cell wall does not make a fungus a plant. See OpenStax Biology 2e, 24.1: fungal nutrition and organization.

Typical Gram-negative bacteria have a relatively thin peptidoglycan layer between the cell membrane and a bacterial outer membrane.

Typical Gram-positive bacteria have a relatively thick peptidoglycan layer outside the cell membrane, without the bacterial outer membrane of typical Gram-negative bacteria. This is a representative cell envelope pattern, not a complete description of every member of bacteria.

Lysozyme is a protein enzyme that attacks the peptidoglycan of bacterial cell walls. Its active site is a cleft in the folded protein structure, where a suitable substrate can interact with positioned chemical groups. This illustrates why possessing the right amino acid residues is insufficient if their spatial arrangement is lost. See Alberts et al.: lysozyme as a worked example of protein binding and enzyme function.

A concrete structural example is Cheetham, Artymiuk and Phillips's 1992 study of hen egg-white lysozyme bound to a short carbohydrate that inhibits its action. The bound carbohydrate occupies part of the active site; it is not a reacting substrate in this complex. The study describes specific interactions and changes of shape accompanying binding. Thus a real binding site has chemical interactions and flexibility as well as a geometric outline. See Cheetham, Artymiuk and Phillips, 1992: structure of a lysozyme complex with bound inhibitor and RCSB PDB entry 1HEW: the authors' structure and bound three-unit carbohydrate. The diagrams above illustrate the idea; their twelve-residue strings are not lysozyme sequences or coordinates.

Water enters when the intracellular osmotic environment favours uptake, generating turgor pressure. A lipid membrane alone cannot generally sustain the large pressure difference of a normal walled bacterium. The covalent peptidoglycan mesh provides tensile resistance: glycan strands are connected by peptide crosslinks, distributing load over a continuous sacculus. Gram-positive bacteria generally possess a thick wall outside the cytoplasmic membrane. Gram-negative bacteria have a thinner peptidoglycan layer within a multilayer envelope, whose outer membrane and connections can also contribute to mechanical integrity. These architectures should not be treated as identical thin bags.

A simple mechanical model explains the need for reinforcement. For a thin cylindrical wall of radius $r$ and thickness $t$ under pressure difference $P$, circumferential stress scales aswhile axial stress is about half that value. This idealization neglects the molecular mesh and envelope heterogeneity, but shows why a larger radius or a thinner load-bearing layer increases tensile demand. No pressure, radius or thickness values are supplied, so a numerical stress cannot be inferred from this paper.

Steady growth requires adding wall while preserving connectivity. Cytoplasmic precursor synthesis supplies glycan–peptide building blocks; membrane-associated transport presents them to polymerizing and crosslinking enzymes outside the cytoplasm. Penicillin-binding proteins and associated synthases connect the new material into the sacculus. Controlled cleavage by autolysins permits expansion and insertion, but cutting must be coordinated with synthesis and repair. Unregulated hydrolysis or failure of crosslinking can make turgor destructive. The wall is therefore renewed through spatially and temporally limited reactions, rather than opened wholesale and rebuilt afterward.

In many rod-shaped bacteria, the Rod complex and MreB organize lateral growth. Imaging linked circumferential motion of MreB-associated machinery to ongoing cell-wall synthesis, consistent with coordinated insertion around the cell rather than a static cytoskeletal scaffold alone.^[ref] Species with other shapes use different arrangements; MreB is not a universal component of all bacteria.

Division requires a second local remodeling programme. FtsZ polymers help organize the divisome at an appropriate site, where septal synthesis accompanies inward envelope growth. Daughter-cell separation must be delayed until enough new wall protects each daughter. Local hydrolases then cleave selected septal connections. Spatial regulation prevents a division-associated cleavage reaction from becoming a catastrophic breach across the cell sidewall. Membrane constriction, wall synthesis and separation are coupled processes; attributing all division force or protection to one cytoskeletal protein would miss this coordination.

Osmoregulation adjusts the load imposed on this structure. Under high external osmolarity, cells can accumulate ions and compatible solutes to retain water. Upon sudden transfer into dilute medium, the existing solute pool instead drives rapid water entry. MscS and MscL mechanosensitive ion channels open in response to membrane tension and release solutes, reducing the osmotic imbalance before slower transcriptional responses can act. Genetic loss-of-function and downshock experiments demonstrated that these channels protect against extreme turgor.^[ref]

Their emergency role differs from routine growth control. A constitutively open large pore would lose valuable metabolites, so gating must be sensitive and reversible. Conversely, emergency release cannot replace a missing wall during sustained growth in an ordinary environment. Envelope-stress sensing, balanced precursor supply, repair and correctly timed hydrolysis remain necessary.

A useful experimental test measures lysis and survival after a defined downshock while independently measuring wall integrity and growth. Rescue with a functional channel distinguishes emergency relief from nonspecific osmotic adaptation, while perturbing a synthase or autolysin reveals the wall-remodeling contribution. Protection is an integrated system: maintain the barrier, regulate its internal pressure, and coordinate the cuts and additions required to grow and divide.

Peptidoglycan is a mesh of linked sugar chains cross-linked by short chains of amino acids in the cell walls of most bacteria. Its absence from the cell walls of archaea helps distinguish envelope chemistry without confusing either cell wall with a cell membrane. See OpenStax Microbiology, 3.3: cell walls and peptidoglycan.

A protoplast is the living contents of a biological cell bounded by its cell membrane after removal of the cell wall. Removing the cell wall need not immediately destroy the cell membrane; stability depends on the preparation and surroundings. Lysozyme acts on susceptible bacterial peptidoglycan, not on cellulose or chitin, so it is not a universal wall-removal enzyme.