Explanation To a Friend

When you eat carbohydrates they go into you digestive system and are ultimately broken down into simple sugars, the most important being glucose. Glucose is sent off to individual cells which take the glucose into their innards, the cytosol. In the cell every one molecule of glucose is transformed into two molecules of pyruvate by an energy creating process called glycolysis. The pyruvates are handed off to these things inside the cell called mitochondria which convert glucose into Acetyl CoA.

Inside the mitochondria Acetyl CoA is put through a process called the citric acid cycle where even more energy is created. During both glycolysis and the citric acid cycle hydrogen atoms are primarily handed off to a molecule called nicotinamide adenine dinucleotide (NAD+) to form NADH. There is another molecule with the same function abbreviated FAD+ that gets changed to FADH₂ when hydrogen is added. The NADH and FADH₂ are responsible for handing those hydrogen atoms off to the electron transport chain.

The hydrogen atoms, aka: protons are pumped across a membrane in the mitochondria by the reactions of the electron transport chain and are allowed to travel back across the membrane though an enzyme called ATP Synthase. These protons passing through stimulate ATP Synthase to create energy. The currency of energy in the cell is in the form of molecules called ATP when we say energy is created it means ATP is made by adding a phosphate to ADP and conversely when we say energy is used it means ATP is changed back into ADP by removal of a phosphate. The whole process from glycolysis to the electron transport chain creates about 38 ATP for the cell to use as energy.