My BIO 101 book states that when human cells convert glucose to ATP, the process is only approx 35% efficient, and much of the potential energy is lost as heat. However, that heat is useful to us in maintaining our overall body temperature.

Cold-blooded animals (reptiles, for example) are not as effective at maintaining their own body temperature. Does that imply that their cellular metabolism (conversion of glucose to ATP) is more efficient than humans, and thus doesn't produce as much heat?

Or is cold-blooded- / warm-blooded-ness completely unrelated to heat released during cellular metabolism?

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    $\begingroup$ As far as I know the difference is not in the efficiency but rather the simple amount of catabolism of nutrients. Cold-blooded animals ingest less but also burn much less and generally expend less as well. I'm not entirely sure about this though. The central metabolic pathways are pretty much the same in most animals. $\endgroup$
    – Armatus
    Jun 5, 2012 at 14:29

1 Answer 1


While heat is lost during glycolysis, a lot more heat can be lost during chemiosmosis. This is the movement of H+ (protons) across the inner mitochondrial membrane. The energy potential of having a greater concentration of H+ on one side of the membrane usually powers ATP synthesis through ATP synthase. In mammals, however, special proteins called uncoupling proteins (UCPs) can make the membrane 'leaky' to H+, so the energy is lost as heat, instead of being used to make ATP. This is called non-shivering thermogenesis, and usually happens in brown adipose tissue (because fatty acids are used to enable UCP production).

It is not that glycolysis in ectotherms is less efficient (as far as I'm aware), but that ectotherms have different levels of different types of UCPs. Some UCP genes may have their expression altered in response to colder conditions (e.g. UCP2 and 3 may be upregulated) or the genes may not be present at all (e.g. UCP1 is not present in birds and crocodiles).

A better summary of UCPs in reptiles etc. can be found in this abstract:



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