How do we mechanically burn calories into body heat.
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In one sentence – Mitochondria take the chemical energy from nutrients and convert it into an electrical voltage, which they then convert into ATP and heat.
Nutrients enter the Krebs cycle. This produces NADH and FADH, which are high-energy electron donors. Those high energy electrons get passed along the electron transport chain in the mitochondria and eventually end up given to oxygen, where they have less energy.
The energy ‘lost’ is used to power pumps that push H+ ions out of the mitochondria. This creates a electro-chemical gradient of H+ ions (more outside than inside the mitochondria) and because they’re ions, they’re electrically charged – so it’s a *voltage;* electrical potential difference. They want to diffuse back into the mitochondria, so they do, along an enzyme called ATP Synthase.
The electrical potential energy is converted back into chemical energy in the form of ATP, and a little bit is ‘lost’ as heat, because the enzyme is not 100% efficient.
The mitochondria can also intentionally increase heat production by ‘uncoupling’. It gets a new protein called Uncoupling Protein to make a hole in the membrane for the H+ ions to flow through. This means that instead of ‘some energy goes to ATP, some is heat’ *all* of the energy from the H+ ions moving turns into heat, but you can’t make ATP.
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