The cell,
kept deliberately out of equilibrium.

A living cell maintains low intracellular sodium and calcium, high intracellular potassium, a characteristic pH, controlled water content and a negative intracellular voltage in many excitable tissues. None of these states occurs at equilibrium. Passive leak constantly tends to dissipate them; metabolism continuously restores them — which is precisely why a cell must spend energy simply to remain itself.

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Two corrections to read alongside this diagram. Its numbered trafficking list mislabels the final steps — the correct order is 7 secretory vesicle → 8 fusion with the plasma membrane → 9 exocytosis, set out in full below. Its mitochondrion box also compresses ATP production to “pyruvate + O₂ → oxidative phosphorylation”; oxygen does not react with pyruvate directly. The real sequence is pyruvate → acetyl-CoA → TCA cycle → NADH/FADH₂ → electron-transport chain, with oxygen as the final electron acceptor and the proton gradient driving ATP synthase — developed in §03.

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The protein-trafficking route, in the order it actually happens

A membrane receptor, channel or secreted protein cannot affect cellular physiology until this full route has run. Some proteins (those staying in the cytosol) skip the rough-ER/Golgi portion entirely.

1. 1 · DNA transcription in the nucleus produces messenger ribonucleic acid (mRNA).
2. 2 · mRNA exits through a nuclear pore into the cytosol.
3. 3 · Translation begins on a free cytosolic ribosome; a signal sequence in the growing chain directs the ribosome-mRNA complex to the rough endoplasmic reticulum (ER).
4. 4 · Folding and initial processing occur inside the rough ER as translation continues.
5. 5 · A transport vesicle buds from the ER and carries the folded protein to the Golgi apparatus.
6. 6 · Golgi modification and sorting (for example glycosylation) address the protein to its final destination.
7. 7 · A secretory vesicle buds from the Golgi’s trans face, carrying the protein toward the plasma membrane.
8. 8 · The vesicle fuses with the plasma membrane.
9. 9 · Exocytosis releases the cargo, or — for a channel or receptor — inserts the protein into the membrane itself, where it can finally do its physiological job.

1. Glycolysis in cytosol converts glucose to pyruvate and supplies a small ATP yield without directly requiring oxygen.
2. With oxygen available, pyruvate enters mitochondria and becomes acetyl-coenzyme A (acetyl-CoA).
3. The citric-acid (Krebs/TCA) cycle oxidises acetyl-CoA and generates the reduced electron carriers NADH and FADH₂, plus a small amount of ATP/GTP directly.
4. The electron-transport chain in the inner mitochondrial membrane accepts electrons from NADH and FADH₂ and passes them down a series of carriers, pumping protons (H⁺) across the inner membrane and creating a proton electrochemical gradient. Oxygen is the final electron acceptor, combining with electrons and H⁺ to form water — it never reacts with pyruvate directly.
5. The proton gradient drives protons back through ATP synthase, which phosphorylates ADP to ATP. This coupling of electron transport to phosphorylation is oxidative phosphorylation, and it is what oxygen delivery is actually for.

When oxygen supply is inadequate, pyruvate is converted to lactate instead of entering mitochondria, so glycolysis can continue regenerating the NAD⁺ it needs. This preserves a limited ATP supply but cannot replace mitochondrial oxidative phosphorylation in highly aerobic tissues such as brain and myocardium.

Where the ATP is spent

Membrane ion pumps — above all the Na⁺/K⁺-ATPase, which builds the sodium and potassium gradients Sections 05–07 depend on — are among the largest continuous ATP consumers in most cells, alongside protein synthesis, the sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) and cytoskeletal/motor-protein work. This is why pump failure is the first functional consequence of ATP depletion, not a late or incidental one — the cell is already spending heavily on these pumps at rest.

With the energy budget established, the next three sections build the structure that budget maintains: the membrane itself, the transport mechanisms it hosts, and the water movement those mechanisms control.

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Phospholipids are amphipathic: a hydrophilic phosphate head faces water on either side, while two hydrophobic fatty-acid tails pack together to form the bilayer core, so the membrane is a barrier to water-soluble and charged solutes at the same time as it stays fluid enough for proteins to move within it.

Cholesterol's effect on fluidity is temperature-dependent, and the direction matters. Above the membrane's phase-transition temperature, cholesterol's rigid ring system wedges between disordered phospholipid tails and reduces fluidity. Below that temperature, it disrupts the tight packing tails would otherwise adopt and increases fluidity — narrowing the temperature range over which membrane fluidity changes sharply, relevant across deliberate intraoperative hypothermia and fever alike.

Only small, lipid-soluble molecules and water cross the bilayer directly at a physiologically useful rate; everything else — ions, glucose, amino acids — depends entirely on the channel, carrier and pump proteins this structure hosts, which the next section works through in turn.

Simple diffusion needs no protein and does not saturate — flux keeps rising as the gradient steepens, unlike every carrier-mediated process below. Facilitated diffusion uses a carrier but is still passive: glucose moves down its electrochemical gradient through GLUT transporters without ATP. Because carrier number is finite, every carrier-mediated process shares three properties: specificity, saturation at a transport maximum (Tmax), and competition from structurally similar solutes.

Primary active transport: the Na⁺/K⁺-ATPase builds the gradients everything else spends

The sodium–potassium adenosine triphosphatase moves three sodium ions out and two potassium ions in for each ATP hydrolysed. It is electrogenic (the unequal 3:2 exchange contributes a small direct current), but its dominant importance is maintaining the Na⁺/K⁺ gradients that cell-volume regulation, the resting membrane potential (Section 07) and secondary active transport all depend on. Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) and the plasma-membrane Ca²⁺-ATPase use the same primary-active principle to keep resting cytosolic calcium roughly four orders of magnitude below extracellular calcium — the gradient Section 09 relies on for calcium signalling.

Secondary active transport spends, rather than creates, a gradient. The sodium-glucose cotransporter (SGLT) uses the inward electrochemical pull on Na⁺ to drag glucose uphill — a symporter. The Na⁺/Ca²⁺ exchanger uses the same stored gradient to extrude calcium — an antiporter. Neither hydrolyses ATP itself; both stop working if the Na⁺/K⁺-ATPase is poisoned, which is why "active transport" is defined by movement against a gradient, not by which step consumes ATP.

Predict what happens if: extracellular glucose is pushed far above the physiological range — facilitated GLUT-mediated entry approaches its transport maximum and plateaus, unlike simple diffusion of a lipid-soluble gas, which keeps rising linearly with the gradient.

Hypotonic extracellular fluid drives net water entry and cell swelling — worsening cerebral oedema, which is why hypotonic intravenous fluids are avoided in neurological injury. Hypertonic fluid (hypertonic saline, mannitol) draws water out and is used to reduce intracranial pressure. With water movement and cell volume now explained by the same gradients the Na⁺/K⁺-ATPase builds, the next section turns those same gradients into a voltage.

The Nernst equation: one ion’s equilibrium potential

Goldman-Hodgkin-Katz: the actual membrane potential with several permeant ions

Chloride's terms are inverted because it is an anion. Use Nernst for one ion's equilibrium potential; use Goldman-Hodgkin-Katz for the actual membrane potential of a membrane permeable to several ions at once — the two answer different questions, and naming either without that distinction leaves it unclear which was being calculated.

The pump builds the gradients; permeability decides the instant voltage

Without the Na⁺/K⁺-ATPase, the concentration gradients Nernst and Goldman-Hodgkin-Katz depend on would run down within minutes. But at any single instant, Vm is set by which channels are open and how permeable the membrane currently is to each ion — not by the pump acting directly on voltage. This is why a channel-blocking drug or a change in extracellular K⁺ can alter Vm within milliseconds, while pump inhibition changes the gradients only gradually.

The action potential is regenerative channel behaviour

Conduction velocity is set by axial resistance (larger diameter is faster) and membrane properties (myelination is faster, via saltatory conduction between nodes of Ranvier). Local-anaesthetic Na⁺-channel block and demyelination both slow or stop conduction by reducing available regenerative current or increasing leak — not by changing the resting potential itself. With the membrane now able to generate and propagate a voltage signal, the next section turns to how a chemical signal — a receptor and its ligand — uses that same membrane to communicate.

Ligand-gated ion channels: the fastest family

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Effect depends on selectivity, not the receptor alone. Which ion moves, and which direction, depends on the pore's selectivity and the electrochemical gradient at the time. Chloride influx causing hyperpolarisation is the usual mature-neuron case (because mature neurons keep intracellular Cl⁻ low) — not a universal rule.

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Intracellular and nuclear receptors: the slowest family

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Not every type I receptor is cytosolic before ligand binds — this is the general pattern, not an absolute rule. The oestrogen receptor, formally type I, is predominantly nuclear even without ligand; state the mechanism for the specific receptor asked about.

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Signal termination matters as much as generation: ligand removal, receptor desensitisation/internalisation, GTP hydrolysis, phosphatases and calcium re-sequestration all prevent an amplified response from becoming persistent noise — the G-protein branch of that termination machinery is next.

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"Functionally separate" is more accurate than "dissociate." Both Gα-GTP and the freed Gβγ dimer stay membrane-anchored and can each regulate their own effector — activation is a conformational rearrangement, not necessarily complete physical separation. Gα's own intrinsic GTPase activity, accelerated by regulator-of-G-protein-signalling (RGS) proteins, makes the cycle self-terminating rather than a one-way switch.

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One receptor can activate many G proteins in turn, and each Gα-GTP or Gβγ can regulate its own effector — the basis of signal amplification, and the reason a handful of bound hormone molecules can produce a large, coordinated cellular response. Which effector responds depends on which Gα subtype is coupled to the receptor:

GsGi/oGq/11

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Golf and Gt2 — specialised extension examples

Two further Gα subtypes use the same activation cycle for sensory transduction, beyond the core MMed Gs/Gi/Gq set above.

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The chloride efflux shown reflects the mature olfactory-receptor-neuron chloride gradient specifically — the opposite direction to the usual mature-neuron GABA-A/glycine case, because these neurons hold intracellular Cl⁻ high enough for its efflux to be depolarising.

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The opposite sign of change to Golf above — channel closure and hyperpolarisation, not opening and depolarisation — useful for showing the same GDP-GTP cycle can be coupled to either outcome depending on the effector.

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Calcium is a signal only because resting calcium is low

Resting cytosolic free Ca²⁺ is held roughly four orders of magnitude below the extracellular concentration by continuous, energy-consuming work (Section 05): the plasma-membrane Ca²⁺-ATPase, the Na⁺/Ca²⁺ exchanger, and SERCA. Because the resting level is so low, a signal only has to open a channel — voltage-gated, or IP₃-/ryanodine-gated on the sarcoplasmic reticulum — to produce a large, rapid, easily detected relative rise. Calcium-dependent vesicle fusion is the final trigger for neurotransmitter release, and the same rise-and-clearance cycle underlies excitation-contraction coupling.

Predict what happens if: a Gs-coupled β1-adrenoceptor agonist and a phosphodiesterase inhibitor are given together. Both raise cAMP through different steps of the same pathway, so their positive inotropic and chronotropic effects would be expected to add.

Meyer and Overton observed, around 1900, that anaesthetic potency correlates closely with lipid solubility across chemically unrelated agents (Section 04). This correlation is real and durable, but the mechanistic interpretation has changed: stereoselective differences between anaesthetic enantiomers of equal lipid solubility, together with direct structural evidence of anaesthetic binding within specific ion-channel proteins, now support direct protein binding rather than a bulk lipid-membrane effect.

These are not redundant descriptions of one effect: agents that potentiate GABA-A (most volatile agents, propofol, etomidate, benzodiazepines) and agents that inhibit NMDA instead (ketamine, nitrous oxide, xenon) produce recognisably different clinical anaesthetic states — evidence the two actions are mechanistically separate, not converging on one common pathway. No single receptor or theory fully explains general anaesthesia.

Immobility and unconsciousness are produced at different anatomical sites

Minimum alveolar concentration (MAC) — the concentration preventing movement in 50% of subjects at surgical incision — measures immobility specifically, generated largely at the spinal cord. Sedation, hypnosis and amnesia depend instead on supraspinal (cortical and subcortical) mechanisms. A technique that abolishes movement does not, by that fact alone, guarantee unconsciousness.

Cellular injury: the same system, failing

1. Reduced oxygen delivery impairs oxidative phosphorylation (Section 03).
2. ATP depletion impairs ion pumps and protein synthesis.
3. Sodium and water entry (Section 05/06 in reverse) cause cell and organelle swelling; membrane potential collapses.
4. Calcium accumulation (Section 09’s gradient, lost) activates phospholipases, proteases, endonucleases and ATPases.
5. Mitochondrial and oxidative damage worsens ATP failure and injures membranes, proteins and DNA — reversible or irreversible depending on how far the sequence has run.

Autophagy — a cell recycling its own damaged organelles via the lysosome (Section 02) — is a survival response under moderate stress, distinct from both necrosis and apoptosis, and only worth invoking where it clarifies a specific question rather than as a third category to memorise alongside them.

Take-home points for this lesson. A cell spends energy to stay away from equilibrium; organelles build and deliver the proteins that do that work; oxidative phosphorylation pays for it and fails first when oxygen delivery fails; the Na⁺/K⁺-ATPase and its secondary transporters build the gradients that set water movement and membrane voltage; receptor family determines signalling speed; G proteins amplify a single ligand-binding event; general anaesthetics act through direct, saturable binding to a small number of confirmed protein targets; and cell injury is this entire system failing in a fixed causal order, with the point of no return set by how far that order has run.

Active recall: without looking back, state the six-step G-protein activation cycle, the Na⁺/K⁺-ATPase stoichiometry, the difference between Nernst and Goldman-Hodgkin-Katz, and the effect of hyperkalaemia on excitability. Retrieving them cold is what fixes them; if any comes out reversed or incomplete, that is the section to read again.