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IB HL #7 Cell respiration involves the production of ATP using energy released by the oxidation of glucose, fat or other substrates. If glucose is the substrate, the first stage of cell respiration is a metabolic pathway called glycolysis. GLYCOLYSIS SUMMARY: • The pathway is catalyzed by enzymes in the cytoplasm. • Glucose is partially oxidized & a small amount of ATP is produced. • This partial oxidation is achieved without the use of oxygen, so glycolysis can form part of both aerobic and anaerobic respiration. COMPARISON OF OXIDATION & REDUCTION Oxidation reaction Reduction reaction • Addition of oxygen • Removal of oxygen atoms to a substance from a substance • Removal of hydrogen • Addition of hydrogen atoms from a substance atoms to a substance • Loss of electrons from • Addition of electrons a substance to a substance OXIDATION & REDUCTION IN A CELL • Hydrogen carriers accept atoms removed from substrates in cell respiration. • The most commonly used hydrogen carrier is NAD. (nicotinamide adenine dinucleotide) • Hydrogen atoms consist of one proton & one electron. • When two hydrogen atoms are removed from a respiratory substrate, NAD+ accepts the electrons from both atoms & the proton from one of them. NAD+ + 2H NADH + H+ Examples of oxidation & reduction in cell respiration Fe3+ + electron Fe2+ Fe2+ Fe3+ + electron succinate + FAD malate + NAD+ fumarate + FADH2 oxaloacetate + NADH + H+ pyruvate + NADH + H+ lactate + NAD+ CONVERTING GLUCOSE TO PYRUVATE IN GLYCOLYSIS THERE ARE FOUR MAIN STAGES STAGE #1 • Two phosphate groups are added to a molecule of glucose to form hexose biphosphate • Adding a phosphate group is called phosphorylation . • Two molecules of ATP provide the phosphate groups. • The energy level of the hexose is raised by phosphorylation making the subsequent reactions possible. STAGE #2 • The hexose biphosphate is split to form two molecules of triose phosphate. • Splitting molecules is called lysis. STAGE #3: • Two atoms of hydrogen are removed from each triose phosphate molecule. • This is an oxidation. • The energy released by this oxidation is used to link on another phosphate group, producing a 3-carbon compound. • NAD+ is the hydrogen carrier that accepts the hydrogen atoms. STAGE #4: • Pyruvate is formed by removing the two phosphate groups and passing them on to ADP. • This results in ATP formation. SUMMARY OF GLYCOLYSIS • ONE GLUCSOE IS CONVERTED INTO TWO PYRUVATES. • TWO ATP MOLECULES ARE USED PER GLUCOSE BUT FOUR ARE PRODUCED SO THERE IS A NET YIELD OF TWO ATP’S. • TWO NAD+ ARE CONVERTED INTO TWO NADH + H+. Enzymes in the matrix of the mitochondria catalyze a cycle of reactions called the Krebs Cycle. These reactions can only occur if oxygen is available. (aerobic cell respiration) One turn of the Krebs cycle yields: • 2 CO2 • 3 x NADP + H+ • 1 x FADH2 • 1 ATP C2 + C4 = C6 C5 CO2 C4 CO2 Acetyl groups (CH3CO) are the substrate used in the Krebs Cycle. A carrier called CoA (Coenzyme A) accepts acetyl groups produced in metabolism & brings them for use in the cycle. acetyl group + CoA acetyl CoA Acetyl CoA is formed from the metabolism of carboydrates and fats. • Carbohydrates are converted into pyruvate & the pyruvate is converted into acetyl CoA by a reaction that is called the the link reaction. •Fats are broken down into fatty acids & glycerol and then oxidized to form acetyl CoA. • It’s called the link reaction because it links glycolysis & the Krebs Cycle. THE LINKS REACTION •Pyruvate from glycolysis is absorbed by the mitochondrion. • Enzymes in the matrix of the mitochondria remove hydrogen & carbon dioxide from the pyruvate. • The hydrogen is accepted by NAD+ . Removal of hydrogen is called oxidation. • Removal of carbon dioxide is decarboxylation. • Therefore the whole conversion is called: oxidative decarboxylation. • The product of o. d. is acetyl group which is accepted by CoA. Summary of the Link Reaction NAD+ NAD + H+ Pyruvate acetyl CoA CoA CO2 KREBS CYCLE: • An acetyl group is transferred from acetyl CoA to a four-carbon compound (oxaloacetate) to form a six-carbon compound (citrate) • Citrate is converted back into oxaloacetate at the end of the cycle. • Carbon dioxide is removed in two of the reactions. These reactions are called decarboxylations. CO2 is a waste product. • Hydrogen is removed in four of the reactions. These reactions are oxidations. In three of oxidations H is accepted by NAD+. In the other oxidation FAD accept it. These oxidative reactions release energy, which is stored by the carriers when they accept hydrogen. This energy is later released by the electron transport chain and used to make ATP. • ATP is produced directly in one of the reactions. This reaction is substrate-level phosphorylation. fats Summary of metabolic Pathways involving Acetyl CoA carbohydrates glucose glycerol pyruvate fatty acids acetyl CoA Krebs Cycle The Electron Transport Chain: The electron transport chain is a series of electron carriers, located in the inner membrane of the mitochondria. • NADH supplies two electrons to the first carrier in the chain. The two electrons come from oxidative reactions in the early stages of cell resp. • The two electrons pass along the chain of carriers & give up energy each time they pass from one carrier to the next. • At three points along the chain enough energy is given up for ATP to be made by ATP synthase. • ATP synthase is located in the inner mitochondria membrane. • This ATP production relies on energy released by oxidation so it is called oxidative phosphorylation. • FADH2 also feeds electrons into the transport chain. THE ROLE OF OXYGEN • At the end of the electron transport chain the electrons are given to oxygen. • At the same time oxygen accepts hydrogen ions to form water. • This happens on the matrix • This is the only stage at which oxygen is used in cell respiration. • If oxygen is not available, electron flow stops & NADH + H+ can’t be converted to NAD+ . Supplies of NAD+ run out & the link reaction & Krebs Cycle can’t continue. • Glycolysis can continue. • Aerobic cell resp. gives a yield of about 36 ATP’s per glucose mol. • Glycolysis produces only two ATP’s. The coupling of electron transport to ATP synthesis • Going down the energy gradient, the electrons release energy. • The energy released from the electrons is used to pump protons (H+), from the matrix to the intermembranal space. Due to the small volume of this space, it becomes highly concentrated with protons very quickly. • This creates two compartments in the mitochondria with different proton conc. The matrix with low conc. & the intermembranal space with high conc. • This results in the protons moving down their . gradient from high conc. to low. However the only path they can pass through is channels provided by the enzyme ATPsynthase embedded in the inner membrane of the mitochondria. • Protons diffuse quickly through the ATPsynthase channels activating the enzyme & releasing lots of energy. • The active enzyme catalyzes the phosphorylation of ADP into ATP. Carbon from the glucose are released as a waste (CO2). The coupling of ATP synthesis to electron tranport via a conc. gradient of protons is called chemiosmosis. MITOCHONDRIA Relationship between structure & function. • Cristae form a large surface area for oxidative phosphorylation • The small space between inner & outer membrane allows for accumulation of protons. • The fluid matrix contains enzymes for the Krebs cycle. • The inner membrane contains electron transport chains & ATP synthase to carry out oxidative phosphorylation.