Summary
Highlights
Just as burning a potato releases energy, eating a potato allows the body to slowly break it down into nutrients to fuel various activities like doing math, running, or lifting weights. This process is called cellular respiration.
Cellular respiration is the multi-step process where cells convert food into usable energy, ATP. The general equation is C6H12O6 + 6O2 → ATP + 6CO2 + 6H2O. Most organisms use oxygen for this (aerobic respiration), but some can produce ATP without oxygen (anaerobic respiration).
ATP (Adenosine Triphosphate) is the cell's energy currency. It stores energy in the bonds between its three negatively charged phosphate groups. When an enzyme breaks the bond between two phosphates, it releases energy for the cell to do work, forming ADP. ADP is then recycled back into ATP.
Cells primarily use glucose (carbohydrates) for energy, though they can use fats and proteins if glucose is scarce. One glucose molecule can yield approximately 32 ATP molecules. Cellular respiration occurs in three main steps: Glycolysis, the Krebs Cycle, and Oxidative Phosphorylation, all facilitated by enzymes.
Glycolysis breaks down a six-carbon glucose molecule into two three-carbon pyruvic acid (pyruvate) molecules, producing a net of 2 ATP molecules and 2 NADH molecules. This process requires an initial investment of 2 ATP.
Pyruvates enter the mitochondria, where they are converted into Acetyl CoA. This Acetyl CoA enters the Krebs cycle, reacting with oxaloacetate to form citric acid. Through 8 reactions, the cycle produces byproducts like carbon dioxide and water, and regenerates oxaloacetate. Most importantly, it yields NADH and FADH2, which are electron carriers like 'energized electrons'.
NADH and FADH2 donate their electrons to the electron transport chain, a series of protein complexes in the inner mitochondrial membrane. As electrons pass, protons are pumped into the intermembrane space, creating an electrochemical gradient. This gradient's potential energy is harnessed by ATP synthase, a molecular motor that spins to produce ATP from ADP. Oxygen is the final electron acceptor; without it, ATP synthesis halts.
Cellular respiration yields approximately 32 ATP molecules per glucose. It also produces intermediate chemicals that can be used to build other molecules like proteins and nucleic acids. Byproducts include carbon dioxide (exhaled), water, and heat, which some animals use for warmth.