1989: Explain the Krebs cycle (citric acid) cycle and electron transport by describing the following
- The location of the Krebs cycle and ETC in the mitochondria
- The cyclic nature of the reactions in the Krebs cycle
- The production of ATP and reduced coenzymes during the cycle
- The chemiosmotic production of ATP during electron transport
Krebs cycle occurs after glycolysis, the breaking down of glucose into two molecules of a pyruvate, in the mitochondrial matrix. Krebs cycle completes the job after glycolysis by decomposing a derivative of pyruvate to CO2. Krebs cycle begins once pyruvate enters the mitochondrion. As soon as pyruvate enters, it converts into acetyl coenzyme A (acetyl CoA). This step occurs because of a multienzyme complex that catalyzes three reactions: Pyruvates carboxyl group is removed and given off as a molecule of CO2, the remaining two-carbon fragment is oxidized to form a compound named acetate, and then coenzyme A is attached to the acetate by an unstable bond that makes the acetyl group reactive. The Krebs cycle has 8 steps. The first step is acetyl CoA adding its two-carbon fragment to oxaloacetate which then produces citrate. After this, citrate is converted to its isomer, isocitrate, by one water molecule being removed and another being added. Citrate then loses a CO2 molecule and this results in the oxidation of NAD+ turning it into NADH. Alpha-Ketoglutarate is the molecule formed. Another CO2 molecule is lost and again NAD+ is reduced and turned into NADH. The remaining molecule is attached to CoA by an unstable bond. Now CoA is displaced by a phosphate group which is transferred to GDP, forming GTP, and then to ADP, forming ATP. Two hydrogens are then transferred to FAD and forms FADH2. After, a water molecule is added and this rearranges the bonds in the substrate. In the 8th and last step of the cycle a substrate is oxidized and NAD+ is reduced to NADH and regenerates oxaloacetate.
The Krebs cycle generates 1 ATP per turn by substrate phosphorylation but most of the chemical energy is transferred during the redox reactions to NAD+ and FAD. The reduced coenzymes, NADH and FADH2, shuttle their cargo of high energy electrons to the electron transport chain, which uses the energy to synthesize ATP by oxidative phosphorylation. The electron transport chain eases the fall of electrons from food to oxygen. It breaks a large free energy drop into a series of smaller steps that release a certain amount of energy. ETC also generates and maintains an H+ gradient. This chain is an energy converter that uses exergonic flow of electrons to pump H+ across the membrane, from the matrix into the intermembrane space. The H+ goes back across the membrane, diffusing down its gradient. The ions pass through ATP synthase, an enzyme that makes ATP from ADP and inorganic phosphate, which uses the exergonic flow of H+ to move the oxidative phosphorylation of ADP. H+ gradient across a membrane couples the redox reactions of the ETC to ATP synthesis. Chemiosmosis is the coupling mechanism.