01Core principlesThe concepts and mechanisms needed to understand the subject.
Cell membranes are dynamic barriers built from amphipathic lipids, proteins and carbohydrates. Hydrophobic lipid tails form a core that excludes charged particles, while cholesterol alters fluidity and membrane proteins create selective routes, receptors, enzymes and anchors. Lipid composition and protein distribution differ between the apical and basolateral surfaces of epithelial cells, allowing directional transport. Junctional complexes preserve this polarity and control paracellular movement. The membrane therefore does more than contain cytoplasm: it separates electrical and chemical environments and couples external signals to intracellular responses.
Simple diffusion depends on concentration difference, permeability and surface area, and falls as diffusion distance increases. Channels create aqueous pores with selectivity filters and gates controlled by voltage, ligands, stretch or phosphorylation. Carriers bind solute and alternate exposure to each side; because binding sites are finite, carrier-mediated flux approaches a maximum. A uniporter moves one species, a symporter moves coupled species in the same direction, and an antiporter exchanges them. Primary pumps such as Na+/K+-ATPase hydrolyse ATP; secondary transporters use the resulting sodium or other gradient, so their apparent energy source is indirect.
Osmolality counts dissolved particles per mass of solvent, while tonicity describes the sustained effect of extracellular solution on cell volume. A permeant solute may raise osmolality yet equilibrate across the membrane and fail to maintain water movement. An impermeant extracellular solute draws water out, shrinking cells; a hypotonic environment drives water into cells. Plasma proteins create colloid osmotic pressure across capillary walls, a related but distinct scale of fluid movement. Cell volume regulation then recruits ion channels and transporters, demonstrating that osmotic behaviour is a dynamic physiological process rather than a one-time dilution calculation.
Membrane transport connects to organelles. Nuclear pores regulate traffic between cytosol and nucleus. Ribosomes beginning secreted or membrane proteins are directed to rough endoplasmic reticulum, where folding and modification start; the Golgi sorts cargo for membranes, lysosomes or secretion. Endocytosis internalises membrane and ligands, while exocytosis can be constitutive or triggered by calcium. Lysosomes use an acidic lumen for degradation, mitochondria maintain their own electrochemical gradient for ATP synthesis, and proteasomes remove tagged cytosolic proteins. Severe energy failure collapses pumps, gradients and volume control, linking molecular transport to tissue injury.
Key points
- The phospholipid bilayer is selectively permeable: small non-polar molecules cross readily, while ions and most polar solutes require channels, carriers or vesicular transport.
- Passive flux follows an electrochemical gradient and does not directly consume metabolic energy; facilitated diffusion is saturable because a finite carrier population changes conformation.
- Primary active transport couples solute movement directly to an energy source, whereas secondary active transport uses a gradient previously created by another pump.
- Water moves toward higher effective osmole concentration; tonicity predicts sustained cell-volume change and differs from measured osmolality when solutes cross the membrane.
- Membrane potential reflects unequal ion distributions and relative permeability, with potassium leak dominating many resting cells and sodium permeability driving rapid depolarisation.
- Organelle targeting, vesicle trafficking and regulated protein degradation are essential to cell function; failure can produce mislocalised proteins, storage, stress responses and apoptosis.
02Mechanisms and patternsImportant relationships and how to distinguish them.
Flux rises with permeability, surface area and concentration difference and falls with distance; no binding site means no classic carrier saturation.
A carrier moves solute down its gradient without direct energy consumption, but competition and finite turnover create specificity and a transport maximum.
The transporter directly couples uphill movement to ATP hydrolysis or another primary energy reaction, establishing gradients used by other processes.
Coupled movement uses favourable flow of one ion to drive another solute uphill; inhibition of the primary gradient eventually impairs both.
Voltage, ligand, mechanical force or intracellular signals alter open probability, allowing rapid flux without binding and cycling for every transported ion.
Sustained cell swelling or shrinkage depends on effectively impermeant solutes and water access, not simply the total osmolality printed on a solution.
03Interpreting evidenceInformation, measurements and their limitations.
Consider the information, its meaning and its limitations before deciding what follows.
- 01
Gradient accounting - Why
- Determine the chemical direction predicted for each transported species.
- Interpretation and limitations
- Write intracellular and extracellular concentrations separately, then add membrane voltage for ions; concentration alone can misstate net electrochemical drive.
- 02
Energy-coupling classification - Why
- Distinguish passive, primary active and secondary active movement.
- Interpretation and limitations
- Identify whether the protein directly consumes ATP, uses another solute's downhill movement or permits downhill flux without coupling.
- 03
Saturation analysis - Why
- Decide whether finite binding and turnover constrain transport rate.
- Interpretation and limitations
- A plateau as substrate rises supports carrier limitation; a channel's conductance can also be finite, but its mechanism does not require alternating access for each ion.
- 04
Osmole and tonicity calculation - Why
- Predict extracellular osmolality and sustained cell-volume direction.
- Interpretation and limitations
- Account for particle number and permeability separately; dissociation changes osmole count, while rapid membrane crossing reduces effective tonicity.
- 05
Membrane-potential reasoning - Why
- Predict how altered permeability or gradient changes voltage.
- Interpretation and limitations
- A shift toward an ion's equilibrium potential follows increased selective permeability, provided the underlying concentration gradient remains available.
- 06
Trafficking localisation - Why
- Infer where a protein enters, is modified and reaches its destination.
- Interpretation and limitations
- Signal sequences and sorting motifs direct compartments; a mutation may preserve synthesis but cause disease through misfolding, retention or misdelivery.
04Applied reasoningWorked examples connecting principles to decisions.
01Worked examplePredict cell volume after changing extracellular soluteA model cell contains 300 mOsm/kg of effectively non-diffusible intracellular solute. It is transferred from isotonic fluid into 250 mOsm/kg fluid whose added solutes cannot cross the membrane; water can equilibrate freely.+
- 1Compare effective extracellular and intracellular particle concentrations at the moment of transfer: 250 outside versus 300 inside creates an inward water-driving gradient.
- 2Because the extracellular solutes are non-diffusible, they maintain the osmotic difference rather than rapidly entering and abolishing it.
- 3Water moves into the cell until the concentrations approach equilibrium, so cell volume increases; no new intracellular particles are required for swelling.
- 4The final qualitative outcome is cellular swelling, with lysis possible if volume reserve and regulatory mechanisms are exceeded.
- 5Verify by reversing the premise: if the external solute were freely permeant, osmolality could still be 250 initially but sustained tonicity and final volume prediction would differ.
02Transport reasoningClassify sodium-glucose cotransportGlucose enters an epithelial cell uphill while sodium enters downhill; Na+/K+-ATPase on the opposite membrane preserves low intracellular sodium.+
- 1Identify that the cotransporter itself does not hydrolyse ATP during each glucose movement.
- 2Recognise sodium's inward electrochemical gradient as the immediate energy source coupled to uphill glucose entry.
- 3Trace that gradient back to ATP consumption by the basolateral sodium pump.
- 4Classify glucose entry as secondary active symport and predict that pump inhibition will eventually reduce it.
03Electrical reasoningIncrease potassium permeability at restA model resting membrane is at −70 mV and the potassium equilibrium potential is −90 mV. Additional selective potassium channels open without changing ion concentrations immediately and without a larger opposing conductance change.+
- 1Identify potassium's equilibrium potential as more negative than the starting membrane potential.
- 2Increased potassium conductance gives that equilibrium potential greater influence on membrane voltage.
- 3Predict membrane hyperpolarisation toward the potassium equilibrium potential rather than unlimited negativity.
- 4Check that the conclusion assumes the potassium gradient remains and no larger opposing conductance change occurs.
05Checking understandingVerify the reasoning, revisit uncertainties and apply feedback.
- Check transport answers by naming both direction and energy source; 'active' without the coupled gradient or ATP-consuming step is incomplete.
- Verify osmotic calculations by separating particle count from permeability and by stating whether the predicted volume change is initial or sustained.
- Revisit membrane-potential predictions with the relevant equilibrium potential rather than using 'positive ion enters' as a universal shortcut.
- Map trafficking defects from synthesis through folding, modification, sorting and degradation to identify the first failed compartment.
- Use errors in worked calculations to decide whether the gap lies in units, dissociation, gradient direction, permeability or transport classification.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Equilibrium is not equal concentration
For an ion, electrical work can balance chemical diffusion while concentrations remain unequal; the equilibrium potential describes that balance.
Pump current and pump gradient differ
Na+/K+-ATPase is electrogenic, but much of its influence on resting voltage comes from maintaining gradients used by leak channels.
Water follows effective particles
A membrane-permeant solute can transiently move water yet does not provide the same sustained volume effect as an impermeant osmole.
Polarity creates vector transport
Different transporters on apical and basolateral membranes allow an epithelium to move solute across the whole tissue rather than merely equilibrating one cell.
Protein quality control is causal
Misfolding can cause loss of surface expression, toxic aggregation or endoplasmic-reticulum stress even when the gene is transcribed normally.
07Common pitfallsFrequent interpretation and management errors.
- 01
Calling every protein-mediated movement active transport even when the solute moves downhill and no energy coupling occurs.
- 02
Using osmolality and tonicity as synonyms without considering whether each extracellular solute crosses the cell membrane.
- 03
Predicting membrane voltage from ion concentration alone while ignoring selective permeability and electrical force.
- 04
Assuming pump inhibition changes every secondary transport flux instantaneously before the driving gradient has time to dissipate.
- 05
Treating organelles as a list of functions instead of tracing the route and failure point of a specific protein.