After the large molecules in food are broken down into their smaller subunits during digestion, the small molecules can be further broken down to transfer their energy to ATP. During cellular respiration, enzymes slowly rearrange the atoms in food molecules. Each rearrangement produces a new molecule in the pathway and can also produce other useful molecules for the cell. Some reactions
NAD+ and FAD act like electron shuttle buses for the cell. The empty buses, NAD+ and FAD, drive up to oxidation reactions and collect electron passengers. When the electrons get on the bus, the driver puts up the H sign to show that the bus is full. Then the full buses, NADH and FADH2, drive over to reactions that need electrons and let the passengers off. The buses are now empty again, so they drive back to another oxidation reaction to collect new passengers. During cellular respiration, the electron shuttle buses drive a loop between the reactions of glycolysis and the Krebs cycle (where they pick up passengers) to the electron transport chain (where they drop off passengers).
In both plants and animals, the process of — which releases stored energy for use — occurs in the mitochondria inside each cell.Chemically speaking, respiration is photosynthesis in reverse, as you can see in this equation:
C6H12O6 + 6O2 → 6H2O + 6CO2+ energyRespiration consists of a complicated series of chemical reactions.
Cellular respiration is also important in the movement of matter through living systems: As living things break down food molecules using cellular respiration, they release the atoms from the food molecules back out into the environment as carbon dioxide and water. Photosynthesis brings in carbon dioxide and water from the environment, and cellular respiration sends them out again, forming a circular matter pathway that scientists call the carbon cycle.