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added a bit more because i was skipping too much awesome
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karpathy committed Jun 21, 2020
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Expand Up @@ -57,7 +57,7 @@ Since I am a computer scientist it is hard to avoid a comparison of this "energy

**Combustion**. Let's now turn to the chemical process underlying weight loss. You know how you can take wood and light it on fire to "burn" it? This chemical reaction is *combustion*; You're taking a bunch of organic matter with a lot of C-C and C-H bonds and, with a spark, providing the activation energy necessary for the surrounding voraciously electronegative oxygen to react with it, stripping away all of the carbons into carbon dioxide (CO2) and all of the hydrogens into water (H2O). This reaction releases a lot of heat in the process, thus sustaining the reaction until all energy-rich C-C and C-H bonds are depleted. These bonds are referred to as "energy-rich" because energetically carbon reeeallly wants to be carbon dioxide (CO2) and hydrogen reeeeally wants to be water (H2O), but this reaction is gated by an activation energy barrier, allowing large amounts of C-C/C-H rich macromolecules to exist in stable forms, in ambient conditions, and in the presence of oxygen.

**Cellular respiration: "slow motion" combustion**. Remarkably, your body does the exact same thing as far as inputs (organic compounds), outputs (CO2 and H2O) and stoichiometry are concerned, but the burning is not explosive but slow and controlled, with plenty of molecular intermediates that torture biology students. This biochemical miracle begins with fats/carbohydrates/proteins (molecules rich in C-C and C-H bonds) and goes through stepwise, complete, slow-motion combustion via glycolysis / beta oxidation, citric acid cycle, oxidative phosphorylation, and finally the electron transport chain and the whoa-are-you-serious molecular motor - the [ATP synthase](https://en.wikipedia.org/wiki/ATP_synthase), imo the most incredible macromolecule not DNA. Okay potentially a tie with the Ribosome. Even better, this is an exceedingly efficient process that traps almost 40% of the energy in the form of ATP (the rest is lost as heat). This is much more efficient than your typical internal combustion motor at around 25%.
**Cellular respiration: "slow motion" combustion**. Remarkably, your body does the exact same thing as far as inputs (organic compounds), outputs (CO2 and H2O) and stoichiometry are concerned, but the burning is not explosive but slow and controlled, with plenty of molecular intermediates that torture biology students. This biochemical miracle begins with fats/carbohydrates/proteins (molecules rich in C-C and C-H bonds) and goes through stepwise, complete, slow-motion combustion via glycolysis / beta oxidation, citric acid cycle, oxidative phosphorylation, and finally the electron transport chain and the whoa-are-you-serious molecular motor - the [ATP synthase](https://en.wikipedia.org/wiki/ATP_synthase), imo the most incredible macromolecule not DNA. Okay potentially a tie with the Ribosome. Even better, this is an exceedingly efficient process that traps almost 40% of the energy in the form of ATP (the rest is lost as heat). This is much more efficient than your typical internal combustion motor at around 25%. I am also skipping a lot of incredible detail that doesn't fit into a paragraph, including how food is chopped up piece by piece all the way to tiny acetate molecules, how their electrons are stripped and loaded up on molecular shuttles (NAD+ -> NADH), how they then quantum tunnel their way down the electron transport chain (literally a flow of electricity down a protein complex "wire", from food to oxygen), how this pumps protons across the inner mitochondrial membrane (an electrochemical equaivalent of pumping water uphill in a hydro plant), how this process is brilliant, flexible, ancient, highly conserved in all of life and very closely related to photosynthesis, and finally how the protons are allowed to flow back through little holes in the ATP synthase, spinning it like a water wheel on a river, and powering its head to take an ADP and a phosphate and snap them together to ATP.

<div class="imgcap">
<img src="/assets/bio/combustion.jpeg" style="width:57%">
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