The Relational Motive Force

There is a machine inside you, right now, spinning at roughly 9,000 revolutions per minute.

It is smaller than a wavelength of visible light. It has no brain, no nervous system, no awareness of your ambitions or your heartbreaks. And yet, without it, you would be dead in seconds. Every thought you are having as you read this sentence, every beat of your heart, every flicker of recognition when you see someone you love — all of it is powered by this spinning thing, this molecular turbine called ATP synthase, buried deep in the inner membrane of your mitochondria.

I came to this machine through Nick Lane, the British biochemist whose books — Power, Sex, Suicide and The Vital Question — read less like textbooks and more like dispatches from the edge of the knowable. Lane has spent his career asking a question so fundamental it sounds almost naive: Why are we alive at all? 

Prof Nick Lane

His answer, arrived at through decades of research, keeps returning to the same place: a membrane, a gradient, and a tiny spinning motor.

The more I sat with this idea, the more I found myself thinking not about cells, but about people. About the five or so individuals in any life who constitute what evolutionary psychologist Robin Dunbar calls the innermost circle — the ones you would call at 3 a.m. About what it actually takes to build and sustain those relationships. And about whether the same logic that powers every living cell on Earth might also illuminate why some human bonds endure while others quietly collapse.

I. The Dam That Runs the World

To understand what I mean, you need to spend a moment with the proton motive force.

Here is the basic story, stripped to its essentials. Inside your mitochondria, a series of protein complexes — the electron transport chain — acts like a pump, pushing positively charged hydrogen ions (protons) from one side of the inner mitochondrial membrane to the other. This creates a gradient: a high concentration of protons on one side, a low concentration on the other. The membrane itself is nearly impermeable to protons, so they cannot simply leak back. They are held there, under pressure, like water behind a dam.

Then comes ATP synthase. Protons, desperate to flow back down their concentration gradient, are funnelled through this enzyme. As they pass through, they drive the rotation of a molecular rotor — literally a spinning shaft — and this mechanical rotation catalyses the synthesis of ATP, the universal energy currency of life. As Nick Lane wrote in Nature Education, the process "works much like a hydroelectric dam. The energy released by the oxidation of food is used to pump protons across a membrane — the dam — creating, in effect, a proton reservoir on one side of the membrane. The flow of protons through amazing protein turbines embedded in this membrane powers the synthesis of ATP in much the same way that the flow of water through mechanized turbines generates electricity."

Source: DOI:10.1186/2046-2395-3-3

The term for this stored electrochemical potential is the proton motive force — or, more precisely, the protonmotive force (pmf). It is not merely a metaphor. It is a measurable voltage, typically around 150–200 millivolts across the inner mitochondrial membrane, composed of both a chemical gradient (the difference in proton concentration, ΔpH) and an electrical gradient (the membrane potential, Δψm). Together, they constitute a form of potential energy that, when discharged through ATP synthase, does the work of life.

What makes this so staggering is its universality. The proton motive force is not a quirk of animal cells. It is found in bacteria, in archaea, in the chloroplasts of plants. It appears to be as ancient as life itself. Lane and his colleagues have argued, compellingly, that natural proton gradients at alkaline hydrothermal vents may have powered the very first metabolic reactions on Earth, four billion years ago — that life did not begin despite the need for a gradient, but because of one. The gradient is not a feature of life. It may be the precondition for it.

You are, in the most literal sense, a gradient-powered machine.

II. The Inner Circle and Its Demands

Now consider a different kind of gradient.

In the early 1990s, Robin Dunbar, then a primatologist at the University of Liverpool, was puzzling over a graph. He had plotted the average social group sizes of various primate species against the relative size of their neocortex — the brain region associated with complex cognition. The correlation was striking. The bigger the neocortex, the larger the stable social group the animal could maintain. When Dunbar plugged human brain size into the equation, the number that came out was 150.

He then spent weeks in the library, cross-referencing ethnographic data on hunter-gatherer societies, Neolithic villages, military units, and Christmas card lists. The number kept appearing. As Dunbar himself has written, "The community level of organisation turned out to be almost exactly 150. Thus was born the 'social brain hypothesis' and 'Dunbar's number.'"

But 150 is only the outer boundary. Nested within it are smaller, more intimate layers. According to Dunbar's research, the tightest circle contains just five people — the loved ones, the ones you would call in a crisis. Then 15 (good friends), then 50 (friends), then the full 150 (meaningful contacts). Each layer outward involves less emotional investment, less frequency of contact, less of what Dunbar calls the "grooming" that holds primate societies together.

In humans, that grooming has largely been replaced by language, laughter, shared meals, and — crucially — acts of consistent care. The five people in your innermost circle are not there by accident. They are there because someone, at some point, built something. They invested. They showed up. They created, through repeated acts of attention and kindness, a kind of stored potential — a reservoir of goodwill and trust that could be drawn upon when needed.

Which brings me back to the dam.

III. The Relational Motive Force

What if the logic of the proton motive force applies, by analogy, to the architecture of close relationships?

I want to be careful here. I am not claiming that human bonds are reducible to electrochemistry, or that love is merely a gradient. The analogy is structural, not literal. But structural analogies, when they are good ones, illuminate something real.

Consider: the proton motive force works because of asymmetry. Protons are pumped against their natural tendency to diffuse. Energy is expended to create a difference — a high side and a low side. That difference is the source of all the work that follows. Without the gradient, there is no force. Without the force, there is no ATP. Without ATP, there is no life.

Now consider what John Gottman, the psychologist who has spent four decades studying couples in his "Love Lab" at the University of Washington, calls the Emotional Bank Account. Gottman's research found that couples who remained married turned toward their partner's bids for emotional connection 86% of the time, while those who eventually divorced averaged just 33%. The difference was not the absence of conflict. It was the presence — or absence — of accumulated positive investment. Couples who had built up a reserve of goodwill could weather disagreements. Those who hadn't, couldn't.

Gottman's "magic ratio" is 5:1 — five positive interactions for every negative one, during conflict. In everyday life, the ratio needs to be even higher: closer to 20:1. The relationship, in other words, requires constant pumping. Constant investment against the natural tendency of things to drift apart, to become indifferent, to equilibrate.

This is the relational motive force: the stored potential that accumulates when we consistently invest in the people closest to us — when we listen without distraction, remember what matters to them, show up when it costs us something, extend kindness in the small unrecorded moments that no one else sees. Like protons accumulating behind a membrane, these acts of care build a gradient. And like the proton motive force, this gradient is what powers the real work of a relationship — the repair after conflict, the support through loss, the quiet sustaining presence that makes a life feel less arbitrary.

The gradient does not build itself. It requires energy. It requires intention. And — this is the part that the mitochondrial analogy makes vivid — it requires a membrane: a boundary that prevents the potential from simply leaking away.

IV. The Membrane Problem

In mitochondria, the inner membrane is almost impermeable to protons. This impermeability is not incidental — it is the whole point. If protons could freely diffuse back across the membrane, the gradient would collapse, ATP synthesis would stop, and the cell would die. The membrane is not a passive wall. It is an active resistance — the thing that holds the potential in place long enough for it to do something useful.

In relationships, the membrane has a name, or rather, several names. It is distance — the friend who moved cities, the parent in a different time zone, the slow drift of two people whose lives have simply stopped overlapping. It is time, or the lack of it: the colleague who became a stranger not through any falling-out but through the quiet accumulation of busy weeks. It is competing interests, the gravitational pull of work and obligation and the thousand small urgencies that fill a day and leave no room for the people who matter most. It is emotional distance too — the unspoken grievance, the habit of guardedness, the way two people can share a home and still be separated by a membrane of their own making.

These barriers are not always bad. Some distance is necessary. A relationship with no membrane — no separateness, no space between two people — has no gradient to speak of. But when the membrane becomes too thick, when the barriers accumulate faster than the investments, the potential leaks away. The protons dissipate. The gradient collapses. And what remains is the form of a relationship without its force.

This is where ATP synthase enters the picture — and in the relational analogy, ATP synthase is not a metaphor for anything vague or abstract. It is the frank conversation you finally had after months of circling around something difficult. It is the weekend away with no agenda, no phones, no performance of connection — just two people actually present to each other. It is what we call, with a phrase that sounds almost embarrassingly simple, quality time: the specific, irreplaceable moments when the accumulated potential of a relationship is channelled through the right machinery and converted into something real.

The biology here is precise in a way that matters. Protons do not produce ATP simply by existing on one side of the membrane. They must pass through ATP synthase — through the turbine — and it is the passage itself, the controlled flow through the right channel, that drives the synthesis. The gradient is necessary but not sufficient. Without the turbine, the potential just sits there, or leaks away through other routes, producing heat but no work.

The same is true of relationships. You can accumulate years of shared history, of small kindnesses, of loyalty quietly demonstrated — and still find that the potential never quite converts into the intimacy you were hoping for, because the turbine was never engaged. The frank conversation never happened. The quality time was always deferred. The membrane held the gradient in place, but nothing was ever channelled through the right opening.

The five people in Dunbar's innermost circle are, I suspect, the ones with whom we have managed to do both: build the gradient and engage the turbine. Not just invest, but invest in the right way, at the right moments, through the machinery that actually converts potential into connection.

V. Uncoupling and the Slow Death of Relationships

There is a phenomenon in mitochondrial biology called uncoupling. It occurs when protons are allowed to leak back across the inner membrane through pathways other than ATP synthase — through proteins called uncoupling proteins, or through chemical agents that make the membrane permeable. When this happens, the proton gradient is dissipated, but no ATP is produced. The energy is lost as heat. The cell is respiring, burning fuel, but producing nothing useful.

As Berry et al. noted in the Journal of Molecular Biology, uncoupling "dissipates the pmf and energy is lost as heat." Some degree of uncoupling is normal and even beneficial — it regulates temperature, modulates reactive oxygen species. But chronic, uncontrolled uncoupling is pathological. It is the cellular equivalent of running an engine with no load: all consumption, no output.

We recognise this pattern in relationships. There are relationships that consume enormous amounts of emotional energy — time, attention, vulnerability, care — without producing anything that sustains either person. The investment is real, but it is not being channelled through the right turbine. It leaks away in resentment, in misunderstanding, in the exhausting performance of connection without its substance.

Conversely, there are relationships that have become so tightly coupled — so enmeshed, so mutually dependent — that they have lost the flexibility to adapt. In mitochondria, a pmf that is too high actually slows the electron transport chain, because the complexes must pump protons against an increasingly steep gradient. The system becomes rigid, inefficient, prone to generating reactive oxygen species — the cellular equivalent of toxic byproducts.

The healthiest relationships, like the healthiest mitochondria, maintain a dynamic equilibrium: enough gradient to do real work, enough flexibility to adapt, enough uncoupling to prevent rigidity, and a membrane intact enough to prevent the whole thing from simply dissipating into noise.

VI. Building the Gradient

So what does this mean, practically, for the five people in your innermost circle?

It means that the work of close relationship is not primarily the work of grand gestures. It is the work of consistent, small investments — the cellular equivalent of pumping protons, one at a time, against the gradient. It is the 3 a.m. call answered. The opinion asked for and genuinely considered. The memory of what someone told you six months ago, retrieved and honoured. The willingness to be inconvenienced.

Dunbar's research suggests that maintaining the innermost circle of five requires a disproportionate investment of time and emotional energy. His data shows that what determines these layers, in the face-to-face world, is the frequency at which you see people. But frequency alone is not enough — it is the quality of that frequency, the degree to which each encounter actually builds potential rather than merely consuming it.

The social brain hypothesis also implies something sobering: the cognitive and emotional resources required to maintain five genuinely close relationships are substantial. We are not built for unlimited intimacy. The neocortex that enables us to track the emotional states, histories, and needs of our closest people is a finite resource. This is why the innermost circle is five, not fifty. This is why the gradient is hard to build and easy to lose.

It is also why the people who manage to sustain deep relationships over decades are not, in my observation, the most charismatic or the most socially gifted. They are the most consistent. They are the ones who keep pumping, even when the gradient is already high, even when the return is not immediately visible, even when life makes it inconvenient. They understand, perhaps intuitively, what the mitochondria have known for four billion years: that the gradient does not maintain itself. It requires continuous investment of energy against the natural tendency of things to equilibrate.

But that picture — five people, a shared history, the slow accumulation of reciprocal care — assumes a particular arrangement of life. It assumes there are other people reliably on the far side of the membrane. For many, that assumption does not hold.

VII. Solitude and Alternate Gradients

Living alone — whether by choice, by divorce, by bereavement, or by circumstance — complicates the dam-and-turbine picture, but it does not break it. The relational motive force I have described so far assumes there are other people standing on the far side of the membrane, pumping, withholding, turning the turbines. For many who live alone, the picture looks different: the pumps are distributed, the membrane is partly internal, and the turbines may be elsewhere.

First, an uncomfortable fact: chronic social isolation and loneliness carry real health costs. Large-scale reviews identify social isolation and perceived loneliness as risk factors for poorer physical and mental health; the absence of reliable, reciprocal social ties can shorten lives and amplify disease risk. (See work by Julianne Holt-Lunstad and colleagues on loneliness and mortality, and the decades of research by John Cacioppo on the physiology of loneliness.) Those findings remind us that gradients built across other people are not merely pleasant extras; they are, for many, part of what keeps a life metabolically and emotionally healthy.

But solitude is not a single thing. It is a spectrum from chosen solitude — the restorative, creative hours alone — to enforced isolation. And the mitochondrial analogy helps us see why those two kinds of solitude can feel so different. Chosen solitude can act like an internal membrane and turbine: it preserves potential while giving you space to convert it into meaningful energy. It is the afternoon spent writing that clarifies a relationship; the single long walk that lets a hard conversation resolve itself in your head; the ritual of calling one friend every Sunday. These practices are, for people who live alone, the work of pumping and the careful design of channels through which their emotional energy can be spent productively.

When solitude is enforced, however, the membrane is thin and leaky. There are few reliable pathways by which goodwill, help, or consolation returns. The gradient — the reservoir of reciprocal care — cannot be sustained. The result is the slow uncoupling of effort and reward: invitations unanswered, birthday gestures that go unnoticed, support that has no pipeline back. The energy is consumed without useful output.

That observation points to two practical implications, neither of which is simply "spend more time with people." The first concerns the architecture of the network itself. People who live alone often rely on more diverse constellations of connection — neighbours, colleagues, hobby groups, volunteer projects, religious communities, therapists, and yes, pets. Each of these can act as a small proton pump or a specialised turbine. The point is not to replicate a nuclear family exactly, but to create multiple, reliable channels for investment and reciprocation: predictable rituals, appointments, and commitments that prevent the potential from simply dissipating into the air.

The second implication is more interior. Solitude forces a question that communal living tends to defer: can one serve, at least in part, as a functional membrane and an effective turbine for oneself? Practices of honest self-reflection, purposeful rest, and deliberate creative work can act like internal pumps that keep affective reserves from going flat. This is not an argument for self-sufficiency as an ideal — humans are not built for emotional autarky, and the research is unambiguous on that point — but it is an argument for designing one's inner life with the same intentionality one might bring to a close friendship.

The gradient, to be maintained, needs tending from both sides of the membrane.

There is also a social answer, and it operates at a different scale entirely. The costs of isolation are not borne equally: widows, newly single parents, migrants, and the bereaved often face structural barriers to the kinds of repeated small investments that build gradients. Interventions that create predictable social contact — community centres, neighbourhood outreach, social prescribing, affordable shared spaces — function like public infrastructure for relational gradients. They lower the activation energy required for people who live alone to find the channels that convert potential into connection.

Finally, there is something worth saying plainly: a life lived alone need not be a life bereft of relational motive force. The gradient can be distributed across civic ties, creative communities, long-distance friendships maintained with ritual, and the quieter machinery of a well-tended inner life. The measure of a sustaining life is not only how many people sleep under the same roof, but how many reliable channels exist for investment and return — and whether those channels are intentionally kept open.

The mitochondrial lesson, after all, is not that any single pump or membrane or turbine is indispensable. It is that the gradient must be maintained by whatever means are available. For people who live alone, that lesson asks for imaginative engineering — and for the recognition that the engineering, done carefully, is entirely possible. Which brings us, finally, to the question underneath all of this: why does any of it matter as much as it does?

VIII. The Oldest Force

There is something almost vertiginous about the thought that the same principle — build a gradient, maintain a membrane, let the potential do the work — underlies both the spinning turbine in your mitochondria and the five relationships that make your life feel worth living. Or the distributed web of connections that sustains a person living alone. Or the quiet interior architecture that keeps any of us functional when the external pumps run dry.

Nick Lane has argued that the proton motive force is not merely a feature of life but a precondition for it — that the first living systems were, in essence, gradient-exploiting machines, and that everything that followed, from the first bacterium to the last human thought, has been an elaboration of that original trick. Life is what happens when you build a difference and refuse to let it collapse.

Perhaps the same is true of love, in its broadest sense. Perhaps what we call intimacy — the particular quality of being truly known by another person, and truly knowing them — is also what happens when you build a difference and refuse to let it collapse. When you invest, consistently, in the asymmetry between two people: the asymmetry of their histories, their perspectives, their needs, their gifts. When you maintain the membrane that keeps that asymmetry from simply diffusing away into the ambient noise of modern life. And when no single relationship holds all of that weight, the same logic applies to the whole network — the neighbours, the Sunday phone calls, the volunteer group that meets on Thursdays — each one a small difference, held carefully in place.

The five people in your innermost circle are not there because you were lucky. They are there because someone — you, them, both of you — kept pumping. Kept building the gradient. Kept the membrane intact. And for those whose innermost circle looks different from Dunbar's tidy diagram, the same is true: the connections that hold are the ones that were tended.

That is the relational motive force. And like its molecular cousin, it is not a metaphor for life.

It is life.

Sources cited:

• Lane, N. (2010). Why Are Cells Powered by Proton Gradients? Nature Education 3(9):18

• Lane, N., Allen, J.F., & Martin, W. (2010). How did LUCA make a living? Chemiosmosis in the origin of life. BioEssays

• Berry, B.J. et al. (2018). Use the protonmotive force: mitochondrial uncoupling and reactive oxygen species. Journal of Molecular Biology

• Dunbar, R. (2021). Dunbar's number: why my theory has withstood 30 years of scrutiny. The Conversation

• Dunbar, R. (2019). Dunbar's number: Why we can only maintain 150 relationships. BBC Future

• Gottman, J. Invest in Your Relationship: The Emotional Bank Account. The Gottman Institute