Selective membranes and transport proteins regulate movement between a cell and its surroundings.
Key knowledge
Cell membranes are selectively permeable. Passive transport allows net movement down the relevant gradient, through the membrane or transport proteins. Active transport couples movement against a gradient to an energy source, directly or through another gradient. Osmosis is passive net water movement across a selectively permeable membrane.
Conditions and scope
For ions, both concentration and electrical potential affect the electrochemical gradient.
Permeability and transport mechanisms depend on the substance and membrane; not all active transport directly hydrolyses ATP.
Sources · 3
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Published selective-permeability, passive/active and primary/secondary transport accounts; no permeability or electrical measurement was made.
2026-10-10 · English transport distinctions checked against sections 5.2–5.3; medical examples and universal membrane-potential values excluded.
Foundational review
2026-10-10 · Fresh primary-source inspection of the complete compact record, including every statement, equation, quantity and material condition, followed by independent AI editorial review. This check does not certify all linked lessons or every passage of every cited publication.
Sources from other places
Scope or convention differs
Selective membranes, diffusion, osmosis and energy-dependent transport.
The introductory concentration model is narrower than ion electrochemical gradients and indirect energy coupling.
Cell membranes are selectively permeable. Passive transport allows net movement down the relevant gradient, through the membrane or transport proteins. Active transport couples movement against a gradient to an energy source, directly or through another gradient. Osmosis is passive net water movement across a selectively permeable membrane.
Metabolism includes chemical pathways that break down substances and build biological materials. Catabolic pathways break complex molecules into simpler ones, often releasing usable energy; anabolic pathways build complex molecules and require energy input. Enzymes catalyse many of these reactions.
Cell metabolism can supply energy for active membrane transport.
Cells couple ATP hydrolysis to otherwise energetically unfavourable processes through shared reaction steps or conformational changes. The Gibbs-energy change belongs to the overall reaction under cellular conditions; breaking an ATP bond alone is not the source of released free energy.
The source's ATP-coupled ion pump is a membrane transport example.
In many neurons, voltage-dependent ion conductances generate action potentials that propagate along excitable membranes. At chemical synapses, released neurotransmitter acts on receptors of a receiving cell. Signal integration and channel dynamics determine whether further electrical activity occurs.
Ion conductances across membranes generate electrical activity.
In mammalian kidneys, glomerular filtration forms a filtrate that renal tubules modify through reabsorption and secretion. Regulated transport of water and solutes contributes to extracellular-fluid composition and waste excretion. Other animal groups use different osmoregulatory structures.
Tubular transport regulates water and solute reabsorption and secretion.
In the mammalian nephron model, selective tubular transport modifies filtered fluid. Water permeability and salt transport differ along the loop of Henle, supporting a medullary osmotic gradient; collecting-duct water permeability helps determine how much water returns to the body. Filtration, reabsorption, secretion and excretion are distinct flows.
Selective permeability and transport modify filtrate.
In the eukaryotic secretory pathway, many membrane and exported proteins enter the rough endoplasmic reticulum, undergo processing and move in vesicles through the Golgi toward selected destinations. Endomembrane compartments coordinate synthesis, modification, sorting and degradation. Mitochondria and chloroplasts are not simply downstream secretory vesicles.
A respiratory electron-transport chain couples favourable electron transfers to an electrochemical proton gradient across a membrane. ATP synthase can use proton return to synthesize ATP. In the mitochondrial aerobic example oxygen is the terminal acceptor; electron transfer, proton pumping and ATP production have distinct steps.
An intact selective membrane supports the gradient.
For an ideal dilute solution separated from pure solvent by a solvent-permeable membrane, osmotic pressure is approximately the molar concentration of osmotically active solute particles times RT. Dissociation, association and nonideal interactions can change the particle accounting and pressure.
Osmosis depends on selective membrane permeability.
Characteristic archaeal membrane lipids contain branched isoprenoid chains ether-linked to a glycerol-1-phosphate backbone, compared with the common bacterial/eukaryotic fatty-acyl ester-linked glycerol-3-phosphate model. Membrane-spanning bipolar tetraether lipids can form a monolayer. This is a characteristic chemical distinction, not an assertion that every membrane is a bilayer or that ether lipids occur only in Archaea.
Lipid organization supports a permeability boundary.
Diatoms provide a photosynthetic eukaryotic example with silica-rich cell walls. In a representative Paramecium model, cilia direct food into the oral region, food vacuoles support intracellular digestion and contractile vacuoles expel excess water. In the human–Anopheles malaria cycle, Plasmodium undergoes asexual liver/blood multiplication in the vertebrate host and gamete fusion in the mosquito host. These mechanisms illustrate distinct nutritional and lifecycle strategies rather than one protist organization.
Water molecules are polar and can form hydrogen bonds with each other and suitable solutes. Their interactions support cohesion and help water solvate many ions and polar substances; nonpolar surfaces interact differently. Water's relatively high heat capacity buffers temperature changes, and ordinary ice is less dense than liquid water near its melting point at atmospheric pressure.
Water's solvent behavior and nonpolar interactions supply chemical context for aqueous membranes; they do not by themselves specify membrane permeability.
A spherical cell's surface-to-volume ratio is 3/r. Increasing radius therefore decreases membrane area per unit internal volume. If exchange capacity per area and demand per volume were otherwise comparable, this creates a transport constraint. Shape, membrane folds, intracellular transport and demand can change the outcome; it is not a universal maximum cell-size law.
Membrane transport is constrained by available exchange area relative to internal volume, subject to permeability and gradients.
Osmolarity counts osmotically active solute particles per solution volume. Tonicity describes a solution's lasting effect on cell volume relative to a specified membrane and timescale, principally through effectively nonpenetrating solutes. In the passive wall-less-cell model, a hypotonic external solution causes swelling, a hypertonic one shrinking, and an isotonic one no lasting volume change. Equal osmolarity alone need not imply equal tonicity.
Tonicity interprets osmotic exchange through a specified selectively permeable membrane.
A fatty acid has a carboxyl group and a hydrocarbon portion. Saturated fatty-acid chains lack carbon–carbon double bonds; unsaturated chains contain one or more. A triacylglycerol has three fatty-acyl groups ester-linked to glycerol. A representative glycerophospholipid has two fatty-acyl tails and a phosphate-containing polar head region: its amphipathic structure supports bilayers with tails shielded from water. Steroids instead have four fused rings, illustrating that lipid structures are diverse.
A lipid bilayer supplies a membrane matrix; channels, carriers and other proteins specify many transport pathways.
The plasma membrane separates a cell's interior from its environment and regulates exchange. Cytosol is the aqueous intracellular medium; in a eukaryotic cell, cytoplasm conventionally includes cytosol and structures outside the nucleus. A cell wall, where present, lies outside the plasma membrane and supplies support; it is not a substitute for that membrane. Typical cells contain DNA and ribosomes, while eukaryotic organelles partition functions into compartments. Specialized mature cells can lose components, so a generic diagram is not a checklist for every cell.
A plasma membrane is the selective exchange boundary even where a supporting external cell wall is present.