2.2 Cellular Respiration: The Details

Energy Carriers NAD+ and FAD+ are low energy, oxidized coenzymes that act as electron acceptors. When an electron(s) are added to these molecules, they become reduced to NADH and FADH2. In this case, reducing a molecule gives it more energy.

Aerobic Respiration: Overview Occurs in Four Distinct Stages: Glycolysis: 10-step process in the cytoplasm. Pyruvate Oxidation: 1-step process in the mitochondrial matrix. Krebs Cycle: 8-step cyclical process in the mitochondrial matrix. Electron Transport Chain & Chemiosmosis: Multi-step process in the inner mitochondrial membrane. Krebs Cycle also known as the tricarboxylic acid cycle, TCA cycle or the citric acid cycle. Chemiosmosis is also known as oxidative phosphorylation.

Energy Transfer Terminology Substrate-level Phosphorylation: ATP forms directly in an enzyme-catalyzed reaction. Oxidative Phosphorylation: ATP forms indirectly through a series of enzyme-catalyzed redox reactions involving oxygen as the final electron acceptor. Need to cover this terminology before moving onto aerobic cellular respiration. Substrate-level phosphorylation: a phosphate-containing compound transfers a phosphate group directly to ADP, forming ATP. For each glucose molecule processed, 4 ATP are produced this way in glycolysis and 2 in the Krebs cycle.

Glycolysis 2 ATPs are used in steps 1 & 3 to prepare glucose for splitting. F 1,6-BP splits into DHAP and G3P. DHAP converts to G3P. 2 NADH are formed in step 6. 2 ATP are formed by substrate-level phosphorylation in both steps 7 and 10. 2 pyruvates are produced in step 10. Glucose is phosphorylated in step 1 so that it becomes trapped in the cell and is more chemically reactive. Glucose 6-phosphate is converted to its isomer, fructose 6-phosphate. Glucose is phosphorylated again in step 3, so that there is one phosphate group on either end of the molecule…the sugar is now ready to split.

Glycolysis Energy Yield & Products: 4 ATP produced – 2 ATP used = 2 net ATP 2 NADH 2 pyruvates Further processing in aerobic cellular respiration (if oxygen is available) NADH is shuttled to the electron transport chain if there is oxygen available.

Folds of the inner membrane Fluid-filled intermembrane space Mitochondria Smooth Highly folded Folds of the inner membrane Protein-rich liquid Fluid-filled intermembrane space Must go over the structure of the mitochondria before moving onto aerobic cellular respiration.

Pyruvate Oxidation (if oxygen is present…) The following occurs for each pyruvate: CO2 removed. NAD+ reduced to NADH and the 2-carbon compound becomes acetic acid. Coenzyme A (CoA) attaches to acetic acid to form acetyl-CoA. The CO2 removed was the carboxyl group on the pyruvate. When NAD is reduced it removes an H+ from the compound… NADH is shuttled to the electron transport chain. The fate of acetyl-CoA depends on the levels of ATP in the cell… If ATP levels are low, acetyl-CoA goes into the Krebs cycle to increase ATP production. If ATP levels are high, acetyl-CoA goes on to produce lipids…this is why we gain fat when we consume more food than we require. All nutrients, whether protein, lipid or carbohydrate, are converted to acetyl-CoA and are then chanelled toward fat production or ATP production.

Pyruvate Oxidation

Pyruvate Oxidation Energy Yield & Products: 2 NADH 2 acetyl-CoA 2 CO2 (released as waste) Per glucose

Occurs twice for each molecule of glucose, 1 for each acetyl-CoA. The Krebs Cycle Discovered by Hans Krebs in 1937 8-step cyclic process with oxaloacetate, the product of step 8, being the reactant in step 1 Occurs twice for each molecule of glucose, 1 for each acetyl-CoA.

The Krebs Cycle In step 1, acetyl-CoA combines with oxaloacetate to form citrate. In step 2, citrate is rearranged to isocitrate. NAD+ is reduced to NADH in steps 3, 4 and 8. FAD is reduced to FADH2 in step 6. ATP if formed in step 5 by substrate-level phosphorylation. The phosphate group from succinyl-CoA is transferred to GDP, forming GTP, which then forms ATP. In step 8, oxaloacetate is formed from malate, which is used as a reactant in step 1. CO2 is released in steps 3 and 4. After step 1, CoA released and is available to process another pyruvate molecule. GDP stands for guanosine diphosphate

The Krebs Cycle Energy Yield & Products: 2 ATP 6 NADH 2 FADH2 4 CO2 (released as waste) NADH and FADH2 carry electrons to the electron transport chain for further production of ATP by oxidative phosphorylation.

Electron Transport Chain (ETC) A series of electron acceptors (proteins) are embedded in the cristae. These proteins are arranged in order of increasing electronegativity. The weakest attractor of electrons (NADH dehydrogenase) is at the start of the chain and the strongest (cytochrome oxidase) is at the end. The ETC uses a series of steps so that energy is slowly converted into ATP. If the energy were released all at once, the cell would burn up

ETC These proteins pass electrons from NADH and FADH2 to one another through a series of redox reactions. ETC protein complexes are alternately reduced and oxidized as they accept and donate electrons. Electronegativity – attraction of an atom/compound for electrons

ETC As the electrons pass from one molecule to the next, it occupies a more stable position. The free energy released is used to pump protons (H+) to the intermembrane space. 3 for every NADH and 2 for every FADH2. This creates an electrochemical gradient, creating potential difference (voltage) similar to a battery. The inner mitochondrial membrane is essentially impermeable to protons, so the intermembrane space becomes a proton reservoir. This is known as the chemiosmotic theory…the process for synthesizing ATP using the energy of an electrochemical gradient (produces large amounts of potential energy) and the ATP synthase enzyme.

ETC Protons enter the matrix through proton channels associated with ATP synthase (ATPase). For every H+ that passes through, enough free energy is released to create 1 ATP from the phosphorylation of ADP. Conditions must be aerobic because oxygen acts as the final electron and H+ acceptor (water is formed as a byproduct).

Cytochrome b-c1 complex ETC NADH dehydrogenase Cytochrome b-c1 complex FADH2 FAD+

ATP Yield from Aerobic Respiration Less ATP for FADH2 since it enters the chain at the second component.

Controlling Aerobic Respiration Regulated by feedback inhibition and product activation loops. Phosphofructokinase (PFK) is an allosteric enzyme that catalyzes the third reaction in glycolysis and is inhibited by ATP and stimulated by ADP. If citrate accumulates, some will enter the cytoplasm and inhibit PFK to slow down glycolysis. As citrate is used up, its concentration will decrease and the rate of glycolysis will increase.

Controlling Aerobic Respiration A high concentration of NADH indicates that the ETCs are full of electrons and ATP production is high. NADH allosterically inhibits an enzyme that reduces the amount of acetyl-CoA that is shuttled to the Krebs cycle, reducing the amount of NADH produced.