Posts like Jenda's are depressingly unimaginative.

I wish HNers, and people in general, would learn some biology, and use some critical thinking to imagine that when something doesn't fit their very basic mental model, that's probably because there's exponentially more to know/learn(!). Maybe then they would stop thoughtlessly regurgitating such basic misconceptions.

Biology has so much complexity, but so many people want to insist calories in MUST balance calories out, without any exceptions. As if humans are just burning our foods at 100% efficiency in a bomb calorimiter. Biology isn't this boring.

Forgetting digestion and a host of other variably efficient processes, Mitochondria produce ATP from various possible substrates. These substrates are not equally efficient at producing ATP. So just here, in the simplest form, you have a mechanism by which input energy can be wasted or conserved.

Separately, when demand for ATP has suddenly ceased, Mitochondria can change modes to deterministically waste huge amounts of energy to avoid over-producing ROS. So even in the same cell, even provided the same substrates, efficiently can be dialed up and down rapidly.

And we haven't dug deeply into anything. There are so incredibly many processes with highly variable efficiency.

There's much more that we have yet to learn than that we know. Don't underestimate the complexity of biology.