Cellular respiration is the process by which cells obtain energy from food through chemical reaction with an inorganic electron acceptor, usually oxygen. The principal product is adenosine triphosphate (ATP), a high energy compound used for a wide variety of energy-requiring processes in the cell. Cellular respiration occurs in three main stages: glycolysis, Krebs cycle, and oxidative phosphorylation. (In some textbooks, cellular respiration refers only to the Krebs cycle and oxidative phosphorylation, while in others, it refers only to oxidative phosphorylation). Cellular respiration is contrasted with physiologic respiration, which refers to the mechanisms of gas exchange at the lungs.
Sugars and fatty acids are the primary food sources for cells. Each contains large numbers of C-C and C-H bonds, which are relatively weak compared to C-O and H-O bonds. During cellular respiration, these weaker bonds are broken, while the stronger bonds with oxygen are formed, thus releasing energy. This energy is used to form the weakly bonded ATP molecule from its constituents, adenosine diphosphate (ADP) and inorganic phosphate (Pi). The formation of ATP absorbs energy, which is thus stored and available for driving reactions elsewhere in the cell.
Glycolysis, the first stage of cellular respiration, occurs in the cytosol of the cell. Only sugars undergo glycolysis. During glycolysis, a glucose molecule (C6H12O6) is split to form two molecules of pyruvic acid (C3H8O3). Hydrogens from glucose are removed by the carrier molecule nicotinamide adenine dinucleotide (NAD+), forming NADH. The bond joining the H to the NAD+ is weak, meaning the electrons of the bond are still high-energy electrons. In this way, NADH serves as a transporter of high-energy electrons from the cytosol into the mitochondria, where the rest of cellular respiration takes place. Two NADH are formed for each glucose reacted, along with two molecules of ATP. Most of the energy of the glucose remains in the pyruvates, however.
Pyruvic acid passes into the inner compartment of the mitochondrion, the cell organelle chiefly responsible for ATP production. In this compartment, called the matrix, the pyruvic acid is decarboxylated to a two-carbon acetyl group, releasing a CO2 molecule, and is enzymatically joined to Coenzyme A, a large carrier molecule. This reaction creates another NADH molecule. Fatty acids are also linked with coenzyme A, two carbons at a time. The product in all cases is acetyl- coA.
Acetyl-coA then enters the Krebs cycle. In this series of reactions, the two- carbon acetyl group is linked to a four-carbon compound, forming citric acid. (The Krebs cycle is also called the citric acid cycle, and, because of the three carboxyl groups in citric acid, it is also known as the tricarboxylic acid cycle.) In a series of transformations, the four-carbon compound is regenerated, carbon dioxide is released, and ATP, NADH, and FADH2 are formed. FADH2 is another high-energy electron carrier.
The final stage of cellular respiration occurs in two steps. In the first step, NADH and FADH2 are stripped of their electrons, regenerating the original carrier molecules, which are recycled to their original locations. The electrons are attracted away from their carriers by NADH-Q reductase, the first in a series of increasingly electronegative proteins that form an electron transport chain in the inner membrane of the mitochondria. Each protein in turn is first reduced when it accepts the electrons, then oxidized as they are removed by the next protein in the chain. In succession, these carriers are ubiquinone, cytochrome reductase, cytochrome c, cytochrome oxidase. The electrons are finally accepted by molecular oxygen, which together join with H+ ions to form water. The energy released during this series of redox reactions is used to transport other H+ ions across the inner mitochondrial membrane, creating an electrochemical gradient.
The second, final step of this stage uses the energy stored in the electrochemical gradient to produce ATP. H+ ions flow through a membrane protein called ATP synthase. The energy released by this flow drives the synthesis of ATP from ADP and Pi. This process is known as chemiosmosis. The combination of electron transport and chemiosmostic ATP synthesis is known as oxidative phosphorylation, sometimes abbreviated as OXPHOS.
The overall ATP harvest from one glucose molecule is either 36 or 38 ATP, depending on the cell type involved. Of these, all but four are formed by ATP synthase.
In some instances, electron transport can be "uncoupled" from ATP synthesis, so that food molecules are consumed, but no ATP is created. Instead, the mitochondria releases the energy harvested as heat. This occurs naturally in brown fat, a mitochondria-rich tissue found in human babies and in adults of some other species that hibernate. This fat tissue is brown due to the high numbers of mitochondria present. In these mitochondria, the inner membrane is porous, so that the H+ ions transported during the electron transport chain flow back through, without ATP production. In this way, heat is generated directly, without requiring creation and consumption of ATP. Fat-soluble proton carriers such as dinitrophenol act as chemical uncouplers. Such compounds have been tried as diet drugs, but the effective dose is too close to the lethal dose to allow them to be used safely.
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