Biology Made Easy - Guide to Cellular Respiration

Cellular respiration is the process that turns food into ATP, a form of energy that our cells can use. Cellular respiration is separated into three stages. The stages of cellular respiration are glycolysis, the Kreb's Cycle, and the electron transport chain.

The first step of cellular respiration is glycolysis. Glycolysis occurs in the cytosol of the cell and does not need oxygen. An ATP molecule first phosphorylates a glucose molecule. This makes the glucose become its fructose isomer. The 6-carbon fructose molecule is cut into two 3-carbon molecules of pyruvate. These pyruvate molecules will enter the Krebs Cycle. The glycolysis reaction results in a net gain of 2 ATP molecules, 2 NADH molecules, and the two pyruvates.

The second step in cellular respiration is the Krebs cycle. The Krebs cycle, or citric acid cycle, occurs in the mitochondrial matrix. The pyruvate molecules from glycolysis are first changed into the molecule acetyl-CoA. The 2-carbon acetyl group is transferred to a 4-carbon oxaloacetate molecule to form citrate. Each glucose molecule results in two pyruvate molecules, which means two acetyl groups ultimately get transferred; this results in six NADH molecules, 2 FADH2 molecules, and 2 ATP molecules per glucose.

The final step of cellular respiration is the electron transport chain. This is where the NADH and FADH2 molecules are converted into ATPs. The electron transport chain resides in the inner mitochondrial membrane. The chain is actually a series of proteins in the membrane to which electrons are passed. The electrons are initially passed to these proteins from NADH and FADH2 . They then move down the series of proteins because each protein has a higher affinity for electrons than its predecessor. The final electron acceptor is O2, so water is created as an end product. As the electrons move down the chain, protons are pumped by the proteins into the intermembrane space using some of the energy generated by downward movement of the electrons. These protons are pumped from the intermembrane space into the mitochondrial matrix through an ATP synthase enzyme. By using the concentration gradient of the protons, the ATP synthase turns ADP into ATP. Each NADH molecule results in 3 ATPs, and each FADH2 results in 2 ATPs. FADH2 results in one less ATP because it donates its electron to an acceptor further down the chain than the one to which NADH donates.

The end result of cellular respiration for each glucose molecule is a maximum of 38 ATP.