Chapter 9 Overview of Cellular Respiration Inside the Mitochondria Fermentation 2 Light energy ECOSYSTEM CO2 + H2O Photosynthesis in chloroplasts Cellular respiration in mitochondria ATP Heat energy Organic O molecules + 2 ATP powers most cellular work The breakdown of organic molecules is exergonic Releases HEAT Aerobic respiration consumes organic molecules and O2 and yields ATP Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O2 Fermentation is a partial degradation of sugars that occurs without O2 Cellular respiration includes BOTH aerobic and anaerobic respiration but is often used to refer to aerobic respiration Although carbohydrates, fats, and proteins are ALL consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy (ATP + heat) Cellular respiration is a cellular process that breaks down nutrient molecules produced by photosynthesizers with the concomitant production of ATP. It consumes oxygen and produces carbon dioxide (CO2). Cellular respiration is an aerobic process. It usually involves the complete breakdown of glucose to CO2 and H2O. Energy is extracted from the glucose molecule: Released step-wise Allows ATP to be produced efficiently Oxidation-reduction enzymes include NAD+ and FAD as coenzymes. 6 Oxidation C6H12O6 + 6O2 6CO2 + 6H2O + energy glucose Reduction OIL RIG Oxidation is losing of electrons Reduction is gaining of electrons Electrons are removed from substrates and received by oxygen, which combines with H+ to become water. Glucose is oxidized and O2 is reduced. 7 becomes oxidized (loses electron) becomes reduced (gains electron) becomes oxidized becomes reduced Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. O2 and glucose enter cells, which release H2O and CO. CO2 H2O intermembrane space cristae Mitochondria use energy from glucose to form ATP from ADP + P . ADP + P ATP © E. & P. Bauer/zefa/Corbis 10 Cellular Respiration • NAD+ (nicotinamide adenine dinucleotide) A coenzyme of oxidation-reduction, it is: • Oxidized when it gives up electrons (NAD+) • Reduced when it accepts electrons (NADH) Each NAD+ molecule used over and over again • FAD (flavin adenine dinucleotide) Also a coenzyme of oxidation-reduction Sometimes used instead of NAD+ Accepts two electrons and two hydrogen ions (H+) to become FADH2 11 Cellular respiration includes Three phases: 1. Glycolysis is the breakdown of glucose into two molecules of pyruvate. It occurs in the cytoplasm. ATP is formed. It does not utilize oxygen (anaerobic). 2. Pyruvate Oxidation and the Citric Acid Cycle a. Pyruvate Oxidation Both molecules of pyruvate enter the matrix of the mitochondria and are oxidized to become acetyl CoA. Electron energy is stored in NADH. Two carbons are released as CO2 (one from each pyruvate). 12 2. Pyruvate Oxidation and the Citric Acid Cycle b. Citric acid cycle Occurs in the matrix of the mitochondrion and produces NADH and FADH2 A series of reactions, releases 4 carbons as CO2 Turns twice per glucose molecule (once for each pyruvate) Produces two immediate ATP molecules per glucose molecule 3. Electron transport chain (ETC) A series of carriers on the cristae of the mitochondria Extracts energy from NADH and FADH2 Makes NAD+ and FAD Produces 32 or 34 molecules of ATP 13 1. GLYCOLYSIS (color-coded blue) 2. PYRUVATE OXIDATION and the CITRIC ACID CYCLE (color-coded orange) 3. OXIDATIVE PHOSPHORYLATION: Electron transport and chemiosmosis (color-coded purple) Electrons via NADH GLYCOLYSIS Glucose Pyruvate CYTOSOL ATP Substrate-level MITOCHONDRION Electrons via NADH GLYCOLYSIS Glucose Electrons via NADH and FADH2 PYRUVATE OXIDATION CITRIC ACID CYCLE Pyruvate Acetyl CoA CYTOSOL MITOCHONDRION ATP ATP Substrate-level Substrate-level Electrons via NADH GLYCOLYSIS Glucose Electrons via NADH and FADH2 PYRUVATE OXIDATION CITRIC ACID CYCLE Pyruvate Acetyl CoA CYTOSOL MITOCHONDRION ATP ATP Substrate-level Substrate-level OXIDATIVE PHOSPHORYLATION (Electron transport and chemiosmosis) ATP Oxidative Overview of Cellular Respiration Inside the Mitochondria Fermentation 13 Energy yield from glucose metabolism Net yield per glucose From glycolysis – 2 ATP From citric acid cycle – 2 ATP From electron transport chain – 32 or 34 ATP Energy content Reactant (glucose) 686 kcal Energy yield (36 ATP) 263 kcal Efficiency is 34% Rest of energy from glucose is lost as heat 19 Electron shuttles span membrane CYTOSOL 2 NADH 6 NADH 2 NADH GLYCOLYSIS Glucose MITOCHONDRION 2 NADH or 2 FADH2 2 Pyruvate PYRUVATE OXIDATION 2 Acetyl CoA + 2 ATP Maximum per glucose: 2 FADH2 CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION (Electron transport and chemiosmosis) + 2 ATP + about 26 or 28 ATP About 30 or 32 ATP Glycolysis (“sugar splitting”) breaks down glucose into two molecules of pyruvate Glycolysis occurs in the cytoplasm and has two major phases Energy investment phase Energy payoff phase Glycolysis occurs whether or not O2 is present GLYCOLYSIS ATP PYRUVATE OXIDATION CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION Energy Investment Phase Glucose 2 ATP used 2 ADP + 2 P Energy Payoff Phase 4 ADP + 4 P 2 NAD+ + 4 e− + 4 H+ 4 ATP formed 2 NADH + 2 H+ 2 Pyruvate + 2 H2O Net Glucose 4 ATP formed − 2 ATP used 2 NAD+ + 4 e− + 4 H+ 2 Pyruvate + 2 H2O 2 ATP 2 NADH + 2 H+ GLYCOLYSIS: Energy Investment Phase Glyceraldehyde 3-phosphate (G3P) Glucose Fructose ATP 6-phosphate ADP Glucose 6-phosphate ATP ADP Fructose 1,6-bisphosphate Isomerase Hexokinase 1 Phosphoglucoisomerase 2 Phosphofructokinase 3 Aldolase 4 5 Dihydroxyacetone phosphate (DHAP) GLYCOLYSIS: Energy Payoff Phase 2 NADH + 2 NAD + 2 H+ 2 Triose phosphate 2 dehydrogenase Glyceraldehyde 3-phosphate (G3P) 6 2 ATP 2 ADP 2 Phosphoglycerokinase 1,3-Bisphosphoglycerate 7 2 Phosphoglyceromutase 8 3-Phosphoglycerate 2-Phosphoglycerate 2 H2O 2 Enolase 9 2 ATP 2 ADP 2 Pyruvate kinase 10 Phosphoenolpyruvate (PEP) Pyruvate Before the citric acid cycle can begin, pyruvate must be converted to acetyl Coenzyme A (acetyl CoA), which links glycolysis to the citric acid cycle This step is carried out by a multienzyme complex that catalyses three reactions In the presence of O2, pyruvate enters the mitochondrion (in eukaryotic cells) where the oxidation of glucose is completed GLYCOLYSIS PYRUVATE OXIDATION CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION MITOCHONDRION CYTOSOL CO2 Coenzyme A 3 1 2 Pyruvate Transport protein NAD+ NADH + H+ Acetyl CoA The citric acid cycle, also called the Krebs cycle, completes the break down of pyruvate to CO2 The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 PER TURN Because there are 2 pyruvates for each glucose, the Total yield is: 2 ATP 6 NADH 2 FADH2 GLYCOLYSIS PYRUVATE OXIDATION CITRIC ACID CYCLE ATP OXIDATIVE PHOSPHORYLATION The citric acid cycle has eight steps, each catalyzed by a specific enzyme The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain Acetyl CoA CoA-SH NADH + H+ H2O 1 NAD+ Oxaloacetate 8 2 Malate H2O Citrate Isocitrate NAD+ CITRIC ACID CYCLE 7 Fumarate CO2 CoA-SH 4 6 NADH + H+ 3 -Ketoglutarate CoA-SH FADH2 5 NAD+ FAD Succinate Pi GTP GDP ADP ATP Succinyl CoA NADH + H+ CO2 Acetyl CoA CoA CoA CITRIC ACID CYCLE FADH2 2 CO2 3 NAD+ 3 NADH FAD + 3 H+ ADP + P i ATP Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation GLYCOLYSIS PYRUVATE OXIDATION CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION ATP The electron transport chain is in the inner membrane (cristae) of the mitochondrion Most of the chain’s components are proteins, which exist in multiprotein complexes The carriers alternate reduced and oxidized states as they accept and donate electrons Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O Protein complex of electron carriers H+ H+ H+ Cyt c IV Q III I II FADH2 FAD NADH (carrying electrons from food) 2 H+ + ½ O 2 NAD+ 1 Electron transport chain H 2O Electrons are transferred from NADH or FADH2 to the electron transport chain Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to O2 The electron transport chain generates no ATP directly Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space H+ then moves back across the membrane, passing through the protein complex, ATP synthase ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work ATP synthase H+ ADP + P i ATP H+ 2 Chemiosmosis Protein complex of electron carriers H+ H+ Cyt c IV Q III I II FADH2 FAD NADH (carrying electrons from food) ATP synthase H+ H+ 2 H+ + ½ O2 NAD+ H2O ADP + P i ATP H+ 1 Electron transport chain Oxidative phosphorylation 2 Chemiosmosis Overview of Cellular Respiration Inside the Mitochondria Fermentation 21 Enable cells to produce ATP without the use of oxygen Most cellular respiration requires O2 to produce ATP Without O2, the electron transport chain will cease to operate In that case, glycolysis couples with anaerobic respiration or fermentation to produce ATP Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis Two common types are: Alcohol fermentation Lactic acid fermentation In alcohol fermentation, pyruvate is converted to ethanol in two steps The first step releases CO2 The second step produces ethanol Alcohol fermentation by yeast is used in brewing, winemaking, and baking 2 ADP + 2 P i Glucose 2 ATP GLYCOLYSIS 2 Pyruvate 2 NAD+ 2 Ethanol (a) Alcohol fermentation 2 NADH + 2 H+ 2 CO2 2 Acetaldehyde In lactic acid fermentation, pyruvate is reduced by NADH, forming lactate as an end product, with no release of CO2 Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce 2 ADP + 2 P i Glucose 2 ATP GLYCOLYSIS 2 NAD+ 2 Lactate (b) Lactic acid fermentation 2 NADH + 2 H+ 2 Pyruvate