Figure 1. Mitochondrial ATP and heat production.

(A) General diagram of energy conversion within mitochondria. The chemical energy of nutrients is converted into a mitochondrial voltage, which is then converted into ATP and heat. (B) Mechanism of mitochondrial ATP and heat production. Mitochondria have two membranes, the inner mitochondrial membrane (IMM) and outer mitochondrial membrane (OMM). The OMM is freely permeable for ions and small metabolites with a molecular weight less than 2000 Da but presents a barrier for transport of proteins. In contrast, the IMM permeability is tightly controlled for all ions, metabolites and proteins. The IMM contains the machinery for ATP synthesis and confines the enzymes of the Krebs cycle within the mitochondrial matrix. The electron transport chain (ETC) uses the energy of electrons supplied by the Krebs cycle (in the form of electron donors NADH and FADH₂) to pump H⁺ outside the IMM and generates membrane potential (ΔΨ) across the IMM. ΔΨ is then used by the ATP synthase (AS) to generate ATP and by uncoupling proteins (UCPs) to generate heat.