8.7:
Chemiosmosis
Chemiosmosis is the movement of ions, like protons, across a membrane down their electrochemical gradient.
During mitochondrial cellular respiration, the electron transport chain establishes a proton gradient by pumping hydrogen ions into the intermembrane space.
This electrochemical gradient is then utilized by ATP synthase, a complex embedded within the inner membrane to generate ATP— a process called chemiosmosis.
The ATP synthase complex has a stator, a channel that enables ions to enter a membrane-embedded rotor. The rotor begins to turn, and once it completes a 360° full rotation, the protons dissociate one by one and exit back into the matrix through another stator channel.
The spinning rotor further turns the central stalk bringing conformational changes in the globular head. The globular head subunits then catalyze the conversion of ADP and inorganic phosphate into ATP.
Overall, oxidative phosphorylation, which includes the electron transport chain and chemiosmosis, produces 32 to 34 ATP molecules from one glucose molecule, making it the major energy-contributing stage of cellular respiration.
8.7:
Chemiosmosis
Oxidative phosphorylation is a highly efficient process that generates large amounts of adenosine triphosphate (ATP), the basic unit of energy that drives many cellular processes. Oxidative phosphorylation involves two processes— the electron transport chain and chemiosmosis.
Electron Transport Chain
The electron transport chain involves a series of protein complexes on the inner mitochondrial membrane that undergo a series of redox reactions. At the end of this chain, the electrons reduce molecular oxygen to produce water.
The shuttling of electrons between complexes is coupled with proton transfer from the mitochondrial matrix to the intermembrane space against their concentration gradient. Eventually, the high concentration of protons in the intermembrane space drives ATP synthase, a protein complex embedded within the inner membrane, to produce ATP in a process called chemiosmosis. It was biochemist Peter Mitchell who discovered the chemiosmotic mechanism required in respiring cells for ATP synthesis. Similarly, plants also use chemiosmosis to convert energy from sunlight into chemical energy in the form of ATP.
ATP Synthase
ATP synthase is a multi-subunit complex. It consists of a stator—the channel through which protons enter and leave the complex, a multi-unit rotor (F0) embedded within the membrane, and a knob of catalytic proteins (F1) located in the mitochondrial matrix. The binding of the incoming protons to the F0 rotor makes it spin. The spinning rotor then turns the internal stalk called γ-subunit, which passes through the center of the F1 subunits. The rotation of the γ-subunit facilitates changes in the conformation of F1 sub-units such that they can catalyze the synthesis of ATP from ADP and inorganic phosphate.
ATP Production
The process of aerobic respiration can produce a total of 30 or 32 ATPs per molecule of glucose consumed. Four ATP are produced during glycolysis, but two are consumed in the process, resulting in a net total of two ATP molecules. One ATP molecule is produced per round of the Krebs cycle, and two cycles occur for every glucose molecule, producing a net total of two ATP. Finally, 32 to 34 ATP are produced in the electron transport chain through oxidative phosphorylation, depending on whether NADH or FADH2 is used as the electron carrier.
Suggested Reading
- Xu, Ting, Vijayakanth Pagadala, and David M. Mueller. "Understanding Structure, Function, and Mutations in the Mitochondrial ATP Synthase." Microbial Cell 2, no. 4 (March 24, 2015): 105–25. [Source]
- Turner, Nigel, Gregory J. Cooney, Edward W. Kraegen, and Clinton R. Bruce. "Fatty Acid Metabolism, Energy Expenditure and Insulin Resistance in Muscle." Journal of Endocrinology 220, no. 2 (February 1, 2014): T61–79. [Source]