Summary
Highlights
A mountain biker's breakfast is burned to power his ride. Cellular respiration converts energy from fuel into ATP (adenosine triphosphate) within cells. Most ATP is generated in the mitochondria and powers various cellular functions, such as muscle contraction.
Glycolysis is the first step in ATP production from glucose, occurring outside the mitochondria. Energy is initially invested, and the glucose molecule is split. Electron carriers, like NAD+, pick up electrons and hydrogen atoms, becoming NADH. Glycolysis produces a small amount of ATP (2 net ATPs) and pyruvic acid, which still contains significant energy.
Pyruvic acid enters the mitochondrion. One carbon is removed, forming carbon dioxide as a byproduct, and electrons are stripped, forming NADH. Coenzyme A attaches to the two-carbon fragment, creating acetyl Co-A. This enters the citric acid cycle, where it combines with a four-carbon molecule. The six-carbon chain is broken down, releasing more carbon dioxide and capturing electrons in carriers. The carbon dioxide we exhale comes from this process. The citric acid cycle produces 2 ATPs for each glucose molecule.
Electron carriers like NADH deliver their electrons to the electron transport chain, located in the inner membrane of the mitochondrion. Electrons move through a series of carriers, releasing energy. Oxygen pulls electrons from the chain, forming water as a byproduct. The released energy pumps hydrogen ions across the membrane, creating a high concentration. These ions flow back through a turbine-like structure, activating ATP production. This process generates most of the ATP from food, with cells producing millions of ATPs per second to power various bodily functions.