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An action potential is a brief regenerative excursion of membrane potential, produced by changing electrical resistance and conductance through voltage-gated ion channels. In a typical fast neuronal action potential, sodium entry raises membrane potential, activating further sodium channels. This feedback produces the rapid rising phase. Inactivation of sodium channels and increased conductance to potassium then lower membrane potential again; persisting potassium conductance can produce an undershoot below the resting level. See Purves and colleagues: the ionic basis of action potentials.

For an illustrative electrical calculation, write inward sodium electric current as $I_{\mathrm{Na}}=g_{\mathrm{Na}}(E_{\mathrm{Na}}-V)$. At $V=-50\mathrm{mV}$, taking $E_{\mathrm{Na}}=60\mathrm{mV}$ and $g_{\mathrm{Na}}=10\mathrm{nS}$ gives $I_{\mathrm{Na}}=1.1\mathrm{nA}$ inward. A potassium current $I_{\mathrm{K}}=g_{\mathrm{K}}(E_{\mathrm{K}}-V)$ with $E_{\mathrm{K}}=-80\mathrm{mV}$ and $g_{\mathrm{K}}=20\mathrm{nS}$ gives $-0.6\mathrm{nA}$, outward. These illustrative values expose opposing driving forces; a waveform requires the conductances to evolve with membrane potential and time, as in the Hodgkin-Huxley model.

An action potential propagates along an axon because local electric currents bring neighboring regions of cell membrane into the regime where their voltage-gated ion channels regenerate the signal. It is a traveling pattern of excitation, not a packet of sodium ions traversing the entire axon. Regeneration allows an action potential to travel without the passive attenuation of a small local voltage change. See Ion Channels and the Electrical Properties of Membranes.

The neuronal refractory period constrains how soon another action potential can follow. Under comparable conditions, successive action potentials have roughly stereotyped shapes, making event count and timing useful for neural coding; waveform details can nevertheless change with channel state and conditions. The leaky integrate-and-fire model abstracts each action potential to a threshold event and reset. Its reset jump is a mathematical rule, not the actual regenerative waveform produced by sodium and potassium currents.

Animals are multicellular organisms with eukaryotic cellular organization. A representative animal cell has a cell membrane, cell nucleus and mitochondria, but no cell wall or chloroplasts. A neuron illustrates how this common plan can acquire a specialized shape and signaling function. See OpenStax Biology 2e, 4.3: animal cell organization.

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 biological cell is a membrane-bounded basic unit of living organization. In a cell-level experiment it can be an experimental unit, provided that treatment assignment and relevant interactions are handled at that level.

The shared architecture of cellular life combines a selective cell membrane, internal cytoplasm, inherited information in DNA, and ribosomes that assemble proteins. Metabolism supplies transformations of matter and energy needed for maintenance and growth. These components form a coupled system: a boundary retains useful molecules, enzymes organize chemical reactions, and inherited information supports rebuilding the machinery. See Clark, Douglas and Choi, Biology 2e: prokaryotic cells.

A worked structural comparison: in a typical prokaryote, DNA occupies a region without a surrounding cell nucleus; in a typical nucleated eukaryote, much of the DNA lies inside a membrane-bounded cell nucleus. Both still have a cell membrane, cytoplasm and ribosomes. The extra boundary compartmentalizes processes; it does not create a different basic principle of living organization. See Clark, Douglas and Choi, Biology 2e: eukaryotic cells.

Shared architecture describes cellular life and its developmental machinery, rather than requiring every specialized biological cell to retain every component. A mature mammalian red blood cell, for example, lacks a cell nucleus, although it develops from nucleated precursors. See Clark, Douglas and Choi, Biology 2e: components of blood. This distinction makes cellular unity compatible with striking specialization.

Biology studies living systems: how biological cells maintain themselves, how organisms develop and reproduce, and how evolution generates diversity. Its scales connect molecules, genes, biological cells, organisms and the interactions studied by ecology. These are complementary viewpoints: a protein's chemistry can explain a cellular mechanism, while evolution explains why inherited variants of that mechanism persist. See Clark, Douglas and Choi, Biology 2e: the science of biology.

Two organizing ideas make this immense subject navigable: shared cellular organization and descent with inherited modification. A neuron and a gliding bacterium differ dramatically in shape and behavior, yet both depend on a cell membrane and metabolism. Their differences become intelligible through specialization and evolution.

Keep observations separate from the mathematical models used to explain them. A measured membrane potential is evidence; an idealized two-state ion-channel gating equation proposes a mechanism. A mathematical model earns trust through predictions and comparisons with measurements. Its assumptions delimit its usefulness, and disagreement can reveal missing biology rather than a faulty measurement.

The cell envelope comprises a biological cell's surrounding boundary layers, including its cell membrane and a cell wall or additional outer structures when present. The term is especially useful when discussing bacteria, where the layers must be distinguished.

The cell membrane, also called the plasma membrane, is the selective boundary separating a biological cell's cytoplasm from its surroundings. Both prokaryotes and eukaryotes have a cell membrane. Where a cell wall is present, it is a distinct structure outside the cell membrane; the two names are not synonyms. This separation lets a biological cell maintain internal conditions while exchanging material through membrane transport. See NHGRI: cell membrane.

The familiar architecture is a lipid bilayer containing membrane proteins. In a phospholipid-based lipid bilayer, two membrane leaflets face opposite ways: hydrophilic heads face the water on each side, while hydrophobic tails face one another in the interior. The heads are therefore exposed both outside and inside the biological cell. This arrangement combines a water-facing surface with a barrier to many water-soluble solutes; the schematic represents organization rather than an atomistic structure. See Alberts et al., Molecular Biology of the Cell: the lipid bilayer.

Selective permeability means that different substances cross at different rates under comparable conditions. Small nonpolar molecules can pass through a lipid bilayer by diffusion, while ions encounter a strong barrier without a suitable route. An ion channel supplies a selective water-filled pathway when open; a carrier protein binds a solute and changes shape to expose it to the other side. These membrane proteins therefore add specific routes to the barrier. An open ion channel permits passive transport, whereas a carrier protein can mediate either passive transport or active transport. See Alberts et al.: principles of membrane transport.

The useful combination is retention plus controlled exchange: a completely impermeable boundary would prevent uptake, whereas unrestricted mixing would erase differences in concentration and membrane potential. The fluid mosaic model connects these functions to an organized but mobile lipid bilayer containing proteins.

A cell-surface receptor detects an extracellular signal at the cell membrane. Its changed state affects the intracellular side, coupling reception to signal transduction or directly to an ion channel's opening.

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]

Cholesterol is a lipid with four fused rings and a small hydrophilic region. It helps regulate the fluidity of animal cell membranes; it is structurally distinct from a phospholipid. See Biology 2e: cholesterol.

The cytoplasm is the material inside a cell membrane but outside the cell nucleus, when one is present. It includes the cytosol, the organelles suspended in it, and other structures; it is not just a name for the liquid. In a prokaryote, which has no cell nucleus, the nucleoid and ribosomes are within the cytoplasm. See Clark, Douglas and Choi, Biology 2e, 4.3: cytoplasm and cytosol and Clark, Douglas and Choi, Biology 2e, 4.2: prokaryotic organization.

Try classifying four positions in the diagram. A point in the pale blue region belongs to both cytosol and cytoplasm. A point inside the mitochondrion belongs to an organelle included in the cytoplasm, but not to the surrounding cytosol. A point on a free ribosome belongs to the cytoplasm, but the ribosome itself is not cytosol. A point inside the cell nucleus belongs to neither cytoplasm nor cytosol. These distinctions prevent the misleading inference that everything outside the cell nucleus shares one undivided liquid environment.

The usefulness of the cytoplasm is this combination of shared surroundings and specialized structures: enzymes in the cytosol carry out many steps of metabolism, while organelles maintain their own internal conditions. A soluble protein made by a free ribosome may work in the cytosol or be transported elsewhere, including into the cell nucleus. Cellular organization therefore combines separation with exchange, rather than assigning each protein permanently to its place of synthesis. See Alberts et al., Molecular Biology of the Cell (2002): traffic between nucleus and cytosol.