Cellular Respiration: Harvesting Chemical Energy PDF

th Campbell & Reece, 8 Ed 1 Chapter 9 Cellular Respiration: Harvesting Chemical Energy I. Introduction Organic compounds store energy in their arrangements of atoms. Catabolic pathways systematically degrade complex organic molec...

th Campbell & Reece, 8 Ed 1 Chapter 9 Cellular Respiration: Harvesting Chemical Energy I. Introduction Organic compounds store energy in their arrangements of atoms. Catabolic pathways systematically degrade complex organic molecules (rich in energy) to simpler waste products, which store less energy. In the process, energy is transferred from the energy rich nutrient molecules to energy rich molecules that can be used directly by the cell to perform work. Cellular Respiration Organic compounds are degraded in the presence of oxygen to produce carbon dioxide, water and usable forms of energy (i.e., ATP) Organic compounds carbohydrates (e.g., glucose) fats proteins C6H12O6 + 6 O2 ®®®®® 6 CO2 + 6 H2O + Energy (ATP + Heat) ∆G = - 686 kcal/mol of glucose Where does cellular respiration occur? II. What is Cellular Respiration? Cellular respiration is how cells transfer the energy stored in complex organic molecules (“food”) to ATP Cellular respiration is a controlled stepwise oxidation of organic molecules in a cell Enzymes catalyze oxidation via a series of small steps. Free energy is transferred to carrier molecules (most often but not always ATP and NADH). What is Oxidation? During catabolism of organic molecules, electrons are relocated. This releases stored energy that is used to synthesize ATP and generate heat. Transfer of one or more electrons (e-) from one reactant to another occurs in many chemical reactions. These e- transfers are oxidation-reduction reactions or Redox reactions. Note: Redox reactions always occur together (i.e., reduction can’t occur without oxidation also occurring!) oxidation - loss of electrons (e-) by a substance reduction - gain of e- by a substance Xe- + Y ® X + Ye- reducing agent ® electron donor (X) oxidizing agent ® electron acceptor (Y) oxidation C6H12O6 + 6 O2 ----> 6 CO2 + 6 H2O + Usable form of Energy reduction III. Cellular Respiration Cellular respiration of glucose is the cumulative function of three metabolic stages: 1. Glycolysis 2. Citric Acid Cycle (also known as Krebs cycle, Tricarboxylic acid cycle – TCA cycle) 3. Oxidative Phosphorylation: Electron Transport and chemiosmosis 1. Glycolysis (“splitting of sugar”) occurs in the cytosol oxygen is not required and CO2 is not released!!! 10 enzymatic steps (Figures 9.8 and 9.9 – You are not expected to memorize the steps in Fig 9.9 – just have a look at the steps) For 1 molecule of glucose (C6H12O6) entering glycolysis, it goes through 10 steps (reactions) A. energy investment phase  first 5 steps uses 2 ATP B. energy payoff phase last 5 steps produces 4 ATP 2 NADH (nicotinamide adenine dinucleotide) 2 pyruvate 2 H2O How are the 4 ATP produced during glycolysis? Substrate-level phosphorylation:. ATP produced in glycolysis is generated by substrate level phosphorylation: when an enzyme (i.e., a kinase) transfers a phosphate group from a substrate molecule to ADP (Figure 9.7) How are the 2 NADH produced? + Dehydrogenases remove 2 H+ and 2 e- from the substrate (fuel molecule) and transfer 1 H+ and 2 e- to NAD (a coenzyme). The other proton is released as H+ into the surrounding solution. (Figure 9.4) dehydrogenase | | H-C-OH + NAD+ C=O + NADH + H+ | | If oxygen is present then energy stored in NADH can be used to generate ATP by oxidative phosphorylation (Electron transport and chemiosmosis) 2. Citric Acid Cycle (also known as Krebs cycle and tricarboxylic acid cycle) described by Hans Krebs - 1930s If oxygen is present then pyruvate is actively transported into the mitochondrion and oxidation is completed. Junction step between Glycolysis and Citric Acid Cycle (Figure 9.10) Conversion of pyruvate ® acetyl CoA (catalyzed by a multi-enzyme pyruvate dehydrogenase complex) Citric Acid Cycle (Figure 9.11, 9.12)  8 enzyme catalyzed steps (all enzymes are found in the mitochondrial matrix, except for the enzyme that catalyzes step 6, it is found in the inner mitochondrial membrane) Overall reaction for 2 pyruvate molecules going through the junction step and Citric Acid cycle 2 pyruvate ® 2 (3 CO2 + 4 NADH + 4 H+ + 1 FADH2 + 1 ATP) How does the cell use NADH and FADH2? 3. Oxidative Phosphorylation (Electron Transport and Chemiosmosis) The bulk of ATP is synthesized during this phase of aerobic respiration How is ATP synthesized? Oxidative phosphorylation Electron transport chain (ETC): a series of carrier molecules that are built into the inner membrane of the mitochondrion (Figure 9.16). 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 It creates an Electrochemical Gradient with (H+) How is the energy stored in the proton gradient used to make ATP? ATP synthase (Figure 9.14) also found in the inner mitochondrial membrane ATP synthase is a reverse proton pump which uses the proton motive force established across the inner mitochondrial membrane by the electron transport chain ATP synthase is a multi-subunit complex made up of many polypeptides that synthesizes ATP from ADP and Pi. It + takes about 3 to 4 H flowing through the ATP synthase (from the intermembrane space to the mitochondrial matrix) to make 1 ATP. How is the proton gradient established? Recall: The electron transport chain uses the exergonic flow of electrons to pump H+ across the inner membrane into the intermembrane space!!! The energy released as protons diffuse down the electrochemical gradient is captured by ATP synthase and used to perform the chemical work of ATP synthesis. ATP synthase ADP + Pi ® ATP + H2O Most energy in cellular respiration flows Glucose ® NADH ® ETC ® Proton Motive Force ® ATP Cellular Respiration Net ATP Production C6H12O6 + 6 O2 ® 6 CO2 + 6 H2O + 36 ATP