The (metabolic) 'cost of life': New method quantifies hidden energy costs of maintaining metabolic pathways
A new thermodynamic framework quantifies the hidden energetic costs required to maintain specific metabolic pathways and suppress alternatives, beyond direct metabolic energy use. This method ranks pathways by their maintenance and restriction costs, revealing that nature often selects the least dissipative routes, providing insights into the evolution and selection of metabolic processes.

There are "costs of life" that mechanical physics cannot calculate. A clear example is the energy required to keep specific biochemical processes active—such as those that make up photosynthesis, although the examples are countless—while preventing alternative processes from occurring.

In mechanics, no displacement implies zero work, and, put simply, there is no energetic cost for keeping things from happening. Yet careful stochastic thermodynamic calculations show that these costs do exist—and they are often quite significant.

A paper published in the Journal of Statistical Mechanics: Theory and Experiment (JSTAT) proposes a way to calculate these costs from a thermodynamic perspective and thus to offer a new tool for understanding the selection and evolution of metabolic pathways at the root of life.

When, in an ancient ocean, a handful of organic molecules formed an external boundary—the first cell membrane—a sharp distinction between an inside and an outside appeared for the first time.

From that moment on, that primordial system had to invest energy to maintain this compartmentalization and to select, among the many chemical reactions that could occur, only a few metabolic pathways capable of exploiting valuable substances taken from the "outside" and transforming them into new products. Life was born together with this effort of compartmentalization and choice.

Metabolic processes have a direct energetic cost, but they also require an "extra cost" to keep steering chemical flows into a preferred pathway rather than letting them disperse into all physically possible alternatives.

Part 1

Yet from the viewpoint of classical mechanics, compartmentalization and reaction selection—the "constraints" imposed at a system's boundaries—should have no cost at all, as they are treated as fixed external conditions that do not contribute to entropy production.
Researchers now developed a method to calculate these overlooked costs to rank the pathways. This allows researchers to assess their biological efficiency—valuable information for evolutionary studies exploring how life emerged on our planet.
devised a general method to estimate the thermodynamic costs of metabolic processes systematically. In their framework, the cell is imagined as a system crossed by a constant flow, where, for instance, one molecule (a nutrient) enters and another (a product or waste) exits.

Given the underlying chemistry, one can generate all chemically possible pathways that convert the input into the output. Each pathway has its own "thermodynamic cost." Instead of calculating energy in the classical sense, the method estimates how improbable it would be—in a world driven solely by spontaneous chemistry—to see the network (the set of molecules and reactions that convert input to output) behave in exactly that way.

This improbability has two components. The first is the maintenance cost, meaning how unlikely it is to sustain a constant flow through a certain pathway. The second is the restriction cost, which measures how unlikely it is to block all the alternative reactions in the network while keeping only the pathway of interest active.

The calculated improbability represents the cost of that process, which can then be used to classify metabolic pathways according to how "expensive" it is for the cell to keep one pathway active and silence the others.
Part 2

In nature we usually see that one process is favoured over many. Why is this?

It's true, but in biological systems, catalysis often intervenes—the action of facilitating molecules, enzymes—which accelerate reactions and make them less costly, achieving the same effect as having multiple pathways in parallel. This evolutionary choice happens because maintaining many pathways can have other drawbacks, such as producing many potentially toxic molecules.

 Thermodynamic ranking of pathways in reaction networks, Journal of Statistical Mechanics Theory and Experiment (2025). DOI: 10.1088/1742-5468/ae22eb.

Part 3

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