The sodium–potassium pump: the energy cost of electrical readiness

Work that continues without visible movement

A resting person is not a collection of resting chemistry. Cells maintain conditions that make later activity possible. The sodium–potassium pump is one concrete example: it uses ATP to maintain unequal ion concentrations across the cell membrane.

This fits the theme developed in Consistent Eating that food supports ongoing physiological work as well as exercise. For EPK—energy per kilogram—the connection is useful, but it must not become an invented calorie allowance for a process the app does not directly measure.

The central limitation: constructing a system is not explaining a living cell

Reconstitution begins by choosing the protein, lipids, concentrations and experimental conditions. It therefore builds some assumptions of the proposed mechanism into the preparation. A successful result shows what that selected system can do; treating it as a complete explanation of a living cell would exceed what was tested.

Three claims need separating: the components are sufficient for transport in the preparation; the mechanism operates in a living cell; and it accounts for a particular share of that cell’s ion regulation or energy expenditure. Reconstitution directly addresses the first. The second and third require further evidence.

The design is not automatically circular: a constructed system can fail a quantitative prediction. But it is not a decisive comparison with Ling’s account if the features his explanation depends on have been removed and no competing predictions are tested. Demonstrating one workable mechanism is not the same as excluding every other contribution.

What the pump does

The sodium–potassium ATPase normally exchanges three sodium ions from inside the cell for two potassium ions from outside during each ATP-hydrolysis cycle. The resulting gradients contribute to electrical signalling and transport. The pump is a membrane protein with ion-binding sites and changing conformations, not a literal miniature plumbing device. [1]

The pump and an ion channel perform different jobs. Channels provide pathways for ions to move down electrochemical gradients; the pump helps maintain the gradients on which that movement depends. Its contribution to electrical readiness should not be confused with claiming it directly produces the entire rapid waveform of each nerve impulse. [1,4]

What Ling disputed

Ling proposed that interactions among proteins, ions and water could explain intracellular ion distribution without the conventional membrane-pump account. His website presents this as a challenge to the necessity of the sodium pump. [2]

That alternative raises several separate questions: whether a cell boundary exists, what its molecular structure is, and how much ion distribution depends on active transport rather than binding or other processes. Evidence addressing one question does not automatically settle the others.

Ling’s energy challenge: what the calculation establishes

Ling’s 1997 rebuttal argues that sodium pumping would exceed the energy available in poisoned frog muscle. This article now includes a reproducible audit of its printed summary and selected upstream calculations. It is not an independent replication of the experiments. [12]

The distinction is between reproducing a calculation and validating the physical interpretation of its inputs. A large discrepancy deserves investigation; artificial-membrane transport alone does not resolve an energy budget for muscle.

The main arithmetic reproduces

Recalculating Table 8.9 gives requirements of 353.24, 343.21 and 368.55 cal/kg/hour, compared with printed values of 353, 343 and 368. Dividing the printed requirements by the reported supplies gives 30.51, 15.42 and 17.98. These are small calories, not dietary kilocalories. [12]

The calculation uses sodium flux multiplied by the electrochemical potential difference and the Faraday constant, converting joules to calories. Multiplying displayed averages closely reproduces the summary, although it does not reconstruct the original time integration. No order-of-magnitude arithmetic error was found in this table.

Checks further back in the tables

The 9.1% tracer correction and flux conversions reproduce closely. Linear interpolation over 4.5 hours gives 0.13122 mol/kg/hour, matching 0.131. Two displayed voltage totals differ from their component sums by approximately 8 mV; straightforward addition would increase the requirements. One ATP entry gives 1.00 rather than approximately 1.98 using the coefficients applied to other rows. [12]

These discrepancies need explanation, but they do not erase the reported shortfall. The interpolation is an audit convention, not confirmation of the original curve-fitting method. The audit has not recovered every intermediate calculation or established that measurements from different preparations can be combined without adjustment.

The unresolved question is the energetic assignment

A tracer can exchange while total sodium content stays unchanged. That does not mean pumping is unnecessary: an active outward flux can balance an inward leak. Equally, it does not establish that every measured exchange incurs the full outward electrochemical cost. The pathway and its coupling must be identified.

Kennedy and De Weer measured increased glycoside-sensitive sodium influx and efflux after metabolic poisoning in frog muscle, interpreting the result as sodium–sodium exchange. This provides a concrete reason to separate exchange from gradient-restoring transport. Their conditions were not an exact replication of Ling’s, so their findings cannot supply an assumed numerical correction to his table. [14]

Ling’s later exclusion of ATP and phosphocreatine raises a separate issue. [12] Breaking a bond alone and the free-energy change of a complete hydrolysis reaction are different quantities. Rejecting a simplistic high-energy-bond description does not justify assigning zero usable free energy to the full reaction. Mechanistic work distinguishes the steps involved; physiological spectroscopy estimates reaction free energy from measured metabolites, with explicit modelling assumptions. [13,15]

The finding is therefore specific: the main summary arithmetic substantially reproduces, while some upstream values and the assignment of energy cost remain unresolved. This audit has not demonstrated that Ling’s entire theory is correct or that sodium pumping is impossible. It also has not found a numerical correction that, by itself, makes his original reported shortfall disappear.

Transport rebuilt in artificial membranes

A 1977 study placed kidney Na+,K+-ATPase preparations into liposomes—small artificial membrane vesicles. The preparations carried out ATP-dependent sodium and potassium transport at an approximate three-to-two ratio, inhibited by ouabain. [3]

Artificial membranes are not living cells. Reconstitution shows that the preparation can perform ATP-dependent transport under the experimental conditions. It does not, by itself, establish how much that process contributes to ion distribution or energy expenditure inside an intact cell. Preparation quality, permeability, orientation and residual components remain relevant controls.

What reconstruction changes or leaves out

The lipid environment is selected rather than inherited intact from the cell. Primary work on reconstituted proteorhodopsin showed that lipid composition and surface charge could change protein orientation. Another study of lactose permease demonstrated lipid-dependent changes in topology after assembly. These are examples involving other proteins, not proof of a defect in the sodium-pump experiment; they establish why the environment cannot be treated as an incidental container. [10] [11]

Purification and reconstruction also separate the chosen components from much of the surrounding cellular organisation. The simplified vesicle does not reproduce the complete cytoplasm, interactions with neighbouring structures, ongoing metabolic regulation or the full mixture of transport pathways. That simplification helps isolate a mechanism, but restricts extrapolation to the intact system.

Orientation, sealing and background permeability must be measured. A mixed population of correctly oriented, inverted and inactive preparations can complicate the relationship between added protein and observed transport. ATP and ion concentrations imposed by the investigator also need comparison with the cellular conditions to which the result is being applied. These are questions for examining a study’s controls, not allegations that every preparation failed them.

Why a positive result does not settle the whole dispute

If both a conventional account and an alternative allow an isolated protein to transport ions in an artificial vesicle, observing that transport does not distinguish them. A stronger test requires predictions on which the accounts disagree: for example, quantitative changes in ion distribution and flux during a controlled perturbation, while measuring the binding, metabolic and structural variables each explanation invokes.

Reconstruction can establish a component’s capability without establishing its necessity or dominance in living physiology. It cannot on its own allocate the cell’s energy budget or show that protein–ion binding is negligible. Equally, those limitations do not demonstrate that pumps are absent or that Ling’s alternative is correct.

The conclusion here is therefore claim-specific. The cited artificial-membrane experiment is not a disproof of Ling’s entire association–induction hypothesis. His claims about ion binding, water organisation, membrane function and ATP must each be assessed against relevant evidence. Leaving a wider theory unresolved is not the same as having validated it.

Structure provides a separate line of evidence

Morth and colleagues determined a 3.5 Å crystal structure of the pig kidney pump, including bound rubidium used as a potassium analogue. Structural observations were accompanied by functional characterisation. [1]

Structural and reconstitution evidence support the existence and transport capability of the protein. They are not a complete account of the living cell. To assess its physiological contribution, these findings must be considered alongside cellular measurements and perturbations, with the limitations of each method kept visible.

What Hillman, Pollack and Peat actually disputed

Hillman and Sartory argued in 1977 that the trilaminar appearance of membranes in electron micrographs was an artefact, while explicitly distinguishing that claim from denying the membranes themselves. Their argument therefore concerns interpretation of structure, not a blanket claim that cell boundaries do not exist. [5]

Pollack’s gel-based account questions whether membrane integrity is required for particular cell functions and emphasises the physical properties of the cytoplasm. That is a challenge to the explanatory importance assigned to membranes, rather than the same claim as Hillman’s criticism of microscopic appearance. [6]

Peat likewise criticised treating the cell interior as a dilute solution and assuming disrupted-cell preparations faithfully represent living organisation. His discussion is a theoretical critique, not a direct experimental demonstration that membranes are absent. [7]

These objections should be stated individually. Shared criticism of a conventional model does not mean these authors proposed identical alternatives or that evidence for one alternative validates all the others.

Evidence from living cells, with its own limitations

A study in isolated rabbit heart-muscle cells measured current using whole-cell voltage clamp. The investigators identified a potassium-sensitive current that was suppressed by ouabain and responded to intracellular sodium and extracellular potassium. This extends the evidence beyond an artificial vesicle to a cellular preparation. [8]

However, an isolated, voltage-clamped cell is still experimentally manipulated. Whole-cell recording alters access to the intracellular environment, and identifying a pump current depends on controls and how drug-sensitive or ion-sensitive components are separated. It is not an untouched cell operating normally in a living animal.

Live-cell fluorescence nanoscopy has also tracked the motion of labelled lipid molecules at the cell surface. This is evidence obtained without relying solely on the stained, dehydrated electron-microscopy appearance criticised by Hillman. Labels, imaging resolution and interpretation still impose limitations; the experiment does not directly measure sodium-pump transport. [9]

The useful comparison therefore combines methods without conflating them: structural studies locate molecular features, reconstitution tests transport capability, cellular recordings test activity under controlled conditions, and live-cell imaging examines organisation and dynamics. No single preparation should stand in for the entire living system.

The link to brain energy—and its limits

Attwell and Laughlin constructed a physiological energy budget for mammalian grey matter, largely using rodent data. Restoring ion gradients after signalling was a major part of their account. This was a model informed by experimental measurements, not a direct measurement of the calories used in a stressful human meeting. [4]

It explains why the brain needs energy even when the body is seated. It does not show that harder thinking adds a large fixed amount to daily expenditure. Baseline brain expenditure is already included when whole-body resting expenditure is measured; it should not be added a second time.

A practical research question for EPK

The defensible EPK question concerns whether adequate nutrition supports measurable function under defined conditions. Dietary intake, electrolyte balance, pump activity and performance are separate parts of that question. None of these papers validates a five-day wearable baseline or a particular dietary surplus.

A useful protocol would compare a defined nutritional exposure with an appropriate control and measure the outcome of interest, while accounting for relevant disease and medication. It should not infer pump failure from tiredness alone or treat extra salt or potassium as a general solution.

The sodium pump strengthens the explanation that maintenance has an energetic cost. It also demands precision: explaining a cellular mechanism is a foundation for research, not a completed demonstration of a dietary theory.

Related EPK articles

• Mental fatigue, stress and energy needs
• Recovery is more than replacing calories
• ATP does more than supply energy
• Cell water: an active environment, but how structured?
• How do we know mitochondria make ATP?

Related working papers

Read the relevant PDFs: EPK–001 to 007 are the revised version 1.2 papers; EPK–008 to 010 are version 1.0 supplements. Earlier PDFs and DOI records remain on the paper archive.