Chapter 11 – Catabolism: Energy Release and Conservation PDF

Chapter 11 – Catabolism: Energy Release and Conservation Hairy-Chested...

Chapter 11 – Catabolism: Energy Release and Conservation Hairy-Chested Yeti Crab: A species found in Antarctica 1 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Requirements for Carbon, Hydrogen, and Oxygen Often satisfied together. Carbon source often provides H, O, and electrons. Heterotrophs. Use organic molecules as carbon sources, which often serves as the energy source. Can use a variety of carbon sources, but not CO2. Autotrophs. Use carbon dioxide (CO2) as their sole or principal carbon source. Must obtain energy from other sources. Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Nutritional Types of Organisms Based on energy source: Phototrophs use light. Chemotrophs obtain energy from oxidation of chemical compounds. Based on electron source: Lithotrophs use reduced (higher energy) inorganic substances. Organotrophs obtain electrons from organic compounds. Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Major Nutritional Types Carbon Energy Electron Nutritional Type Source Source Source Representative Microorganisms Photolithoautotroph * CO2 Light Inorganic e− Purple and green sulfur bacteria, donor cyanobacteria, diatoms Photoorganoheterotroph Organic Light Organic e− Purple nonsulfur bacteria, green carbon donor nonsulfur bacteria Chemolithoautotroph* CO2 Inorganic Inorganic e− Sulfur-oxidizing bacteria, hydrogen- chemicals donor oxidizing bacteria, methanogens, nitrifying bacteria, iron-oxidizing bacteria Chemolithoheterotroph Organic Inorganic Inorganic e− Some sulfur-oxidizing bacteria (For carbon chemicals donor example, Beggiatoa) Chemoorganoheterotroph * Organic Organic Organic e− Most nonphotosynthetic microbes, carbon chemicals, donor, often including most pathogens, fungi, and often same same as C many protists and archaea as C source source * This course focuses on these three nutritional types Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Microbial Metabolism Microbes have representatives in all five major nutritional types Contribute to cycling of elements in ecosystems – some cycling reactions performed only by microbes 5 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Features of Nutritional Groups I. Chemoorganoheterotrophs use 3 fueling processes 1. Aerobic respiration (3-step process) 2. Anaerobic respiration (same 3 steps as aerobic) 3. Fermentation (substrate level phosphorylation, no ETC) II. Chemolithoautotrophs – 3 major groups 1. Hydrogen oxidizers 2. Sulfur oxidizers 3. Nitrogen oxidizers III. Photolithoautotrophs – 1 or 2 photosystems 6 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Nutritional Group: Chemoorganotrophs I. Chemoorganotrophs use 3 fueling processes 1. Aerobic respiration (3 steps for glucose catabolism) 1. Glycolysis (glucose to pyruvate; 3 pathway options) 2. Pyruvate to CO2 (Citric Acid Cycle) 3. ETC and Oxidative Phosphorylation make ATP 2. Anaerobic respiration (same 3 steps as aerobic) 3. Fermentation (substrate level phosphorylation, not ETC) II. Catabolism of organic molecules other than glucose 1. Carbohydrates 2. Lipids 3. Proteins and amino acids 7 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. The Highest Energy-Yield Fueling Process AEROBIC RESPIRATION 8 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Aerobic respiration includes four steps 1. Gycolysis - Glucose to pyruvate (3 pathway options) 1. Embden-Meyerhof-Parnas Pathway (EMP), 2. Entner-Duodoroff Pathway -minor variant of EMP 3. Pentose phosphate pathway - important for generating biosynthetic precursors 2. Pyruvate oxidized to acetyl CoA (with release of CO 2) 3. Citric Acid Cycle – each acetyl CoA is oxidized to CO 2 4. Electron transport and oxidative phosphorylation generates the most ATP 9 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Aerobic Respiration Process that can completely catabolize an organic energy source to CO2 using – glycolytic pathways (glycolysis) – Citric Acid Cycle (also called Tricarboxylic Acid Cycle or Krebs Cycle) – electron transport chain with oxygen as the final electron acceptor Produces ATP by substrate level phosphorylation and oxidative phosphorylation. – Most ATP is made by oxidative phosphorylation via the activity of the electron transport chain and chemiosmosis. – High-energy electron carriers, NADH and FADH, drive the energetics of oxidative phosphorylation 10 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Stages of Aerobic Respiration CYTOSOL MITOCHONDRION − − − − Electrons carried by NADH + FADH2 Stage 1 Stage 2 Stage 3 Glycolysis Oxidative Pyruvate Citric Acid Phosphorylation Glucose Pyruvate (electron transport Oxidation Cycle and chemiosmosis) O2 H2O CO2 ATP ATP ATP Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Energy Accounting for Aerobic Respiration CYTOSOL MITOCHONDRION – – – – – – – – 2 NADH 2 NADH 6 NADH + 2 FADH2 Glycolysis Pyruvate Oxidative 2 Oxidation Citric Acid Phosphorylation Glucose Pyruvate 2 Acetyl Cycle (electron transport CoA and chemiosmosis) O2 Maximum H2O per glucose: CO2 2 2 About ATP ATP = About 28 ATP 32 ATP by substrate-level by substrate-level by oxidative phosphorylation phosphorylation phosphorylation Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Step 1: Glycolysis Three common routes – Embden-Meyerhof-Parnas (EMP) pathway is 10 chemical reactions – Pentose phosphate pathway – Entner-Duodoroff pathway Most microorganisms use the EMP pathway and the pentose phosphate pathway 2 NAD+ 2 NADH Glycolysis 2 ADP 2 ATP Glucose 2 Pyruvate (3-carbon molecules) 13 Net yield of 2 / glucose Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Step 2: Pyruvate Processing Pyruvate dehydrogenase complex converts pyruvate to acetyl CoA – 2 Pyruvate molecules (3-carbon) from each glucose (6-carbon) – Yields one molecule of NADH (this is the reduced, high-energy form) – Happens in the mitochondrial matrix (eukaryotes) or the cytoplasm of prokaryotes NAD+ NADH Pyruvate Dehydrogenase CoA + CO2 Pyruvate Acetyl CoA (2-carbon acetyl ) 14 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Step 3: Citric Acid Cycle The Citric Acid Cycle oxidizes Acetyl CoA into CO2 – Each cycle accepts 1 Acetyl CoA and releases 2 CO2 – One cycle yields 1 ATP (or GTP), 3 NADH, 1 FADH2 – 2 Acetyl CoA molecules (2-carbon) enter from each glucose (6-carbon) – Happens in the mitochondrial matrix (eukaryotes) or the cytoplasm of prokaryotes – NADH and FADH2 carry electrons to the electron transport chain (ETC) Biological Sciences by Freeman 6thEd, Pearson Figure 9.10 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Cellular Respiration Oxidizes Glucose to Make A TP Where does it happen? Is Oxygen Required? Biological Sciences By Freeman, 6 th ed. Pearson Figure 9.2 Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Catabolism of Macromolecules Carbohydrates Fats Proteins Sugars Glycerol Fatty acids Amino acids Amine groups converted to waste Citric Glucose G3P Pyruvate Acetyl CoA Acid Oxidative Glycolysis Cycle Phosphorylation ATP Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Oxidative phosphorylation makes ATP efficiently ELECTRON TRANSPORT AND CHEMIOSMOSIS 18 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Energy Accounting for Aerobic Respiration CYTOSOL MITOCHONDRION – – – – – – – – 2 NADH 2 NADH 6 NADH + 2 FADH2 Oxidation of glucose yields: Glycolysis Pyruvate Oxidative 10 NADH Glucose 2 Oxidation 2 Acetyl Citric Acid Phosphorylation (electron transport 2 FADH2 Pyruvate Cycle CoA and chemiosmosis) How is this O2 Maximum electron energy CO2 H2O per glucose: used to make 2 ATP 2 ATP About = About ATP? 28 ATP 32 ATP by substrate-level by substrate-level by oxidative phosphorylation phosphorylation phosphorylation Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Step 4: Electron Transport and Chemiosmosis Electron Transport Chain (ETC) – Reduced electron carriers NADH and FADH2 transfer their electrons to ETC – The ETC complex is embedded in the inner mitochondrial membrane (eukaryotes) or the cytoplasmic membrane of prokaryotes – Energy released by electron transfers is harvested to pump hydrogen ions across the membrane NADH 2H+ Electrochemical gradient 2e- 1 /2O2 -> H2O Electron Transport Chain ADP ATP NAD+ NADH & FADH2 electron carriers Oxidative phosphorylation 20 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Electron Transport Chain (ETC) Mitochondrial ETC Bacterial ETC 21 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  How Does the Electron Transport Chain Work? Analogous compartments in Bacteria and Archaea Periplasmic space Plasma membrane Cytoplasm Biological Sciences by Freeman 6thEd, Pearson Figure 9.15 Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Electron Transport Chain The complexes that oxidize NADH and FADH2 are called the electron transport chain (ETC): – Most are proteins with prosthetic “helper” groups that are readily reduced or oxidized – One component of the ETC is a lipid-soluble, nonprotein called ubiquinone or coenzyme Q or “Q” – They have different ability to accept electrons, called their redox potential – Some accept only electrons; others accept electrons plus protons – Those that accept protons pump the protons across the mitochondrial inner membrane and into the intermembrane space. Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  A Series of Reduction– Oxidation Reactions Happen in an Electron Transport Chain The energy from redox reactions pumps protons from the matrix into the intermembrane space Biological Sciences by Freeman 6thEd, Pearson Figure 9.14 Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  The ETC Pumps Protons Across the Membrane A proton High [H+] electrochemical gradient forms on opposite sides of the inner membrane. This proton gradient stores potential Low [H+] energy. Biological Sciences by Freeman 6thEd, Pearson Figure 9.15 Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Just as Dams Convert Potential Energy into Electricity … ATP synthase works like the turbine generators in this dam 26 Image source: Xcel Energy https://tx.my.xcelenergy.com/s/energy-portfolio/hydro Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  The Discovery of ATP Synthase The proton gradient drives synthesis of ATP from ADP and Pi — a process ii called chemiosmosis ATP production is powered by a proton-motive force generated by the proton electrochemical gradient Biological Sciences by Freeman 6thEd, Pearson Figure 9.16 Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  ATP Synthase has a Proton-Driven Rotor and an ATP-Generating Enzyme Chemiosmosis is the process of making ATP from ADP and Pi , using the energy produced by a proton electrochemical gradient. ATP production is powered by a proton-motive force as protons diffuse through the channel in ATP synthase. The proton electrochemical gradient produces the proton motive force that spins the rotor. Biological Sciences by Freeman 6thEd, Pearson Figure 9.18 Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Efficient ATP Production by Cellular Respiration Oxidative phosphorylation makes ATP from a proton gradient, proton motive force and ATP synthase Biological Sciences by Freeman 6thEd, Pearson Figure 9.19 Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  location CYTOSOL location membrane Outer mitochondrial Test yourself: Identify the sub- mitochondrial structures INTERMEMBRANE location H+ SPACE and locations in this H+ H+ H+ ATP diagram that are covered H+ H+ synthase H+ up with the white boxes. Inner H+ H+ Rotor Identify the processes location mitochondrial membrane Cyt c IV related to aerobic III H+ H+ I respiration. Q H+ Internal II – rod Electron – – – flow – – FADH2 FAD NADH 1 O 2 + 2 H+ 2 NAD + H+ H2O ADP + P ATP Complexes or process Electron Transport Chain Chemiosmosis process MITOCHONDRIAL location Process OXIDATIVE PHOSPHORYLATION MATRIX Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  location CYTOSOL location membrane Outer mitochondrial Test yourself: Identify the Gram- negative bacterial INTERMEMBRANE location H+ SPACE structures and locations H+ H+ H+ ATP in this diagram that are location H+ H+ synthase H+ covered up with the Inner H+ H+ Rotor white boxes. location mitochondrial membrane Cyt c IV Identify the processes III H+ H+ I related to aerobic Q respiration. Internal H+ II – rod Electron – – – flow – – FADH2 FAD NADH 1 O 2 + 2 H+ 2 NAD + H+ H2O ADP + P ATP Complexes or process Electron Transport Chain Chemiosmosis process MITOCHONDRIAL location Process OXIDATIVE PHOSPHORYLATION MATRIX Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  A CYTOSOL B Outer mitochondrial membrane Test yourself: Identify the sub- mitochondrial structures INTERMEMBRANE SPACE C H+ and locations in this H+ H+ H+ ATP diagram that are covered H+ H+ synthase H+ up with the white boxes. Inner H+ H+ Rotor Identify the processes D mitochondrial membrane Cyt c IV related to aerobic III H+ H+ I respiration. Q H+ Internal II – rod Electron – – – flow – – FADH2 FAD NADH 1 O 2 + 2 H+ 2 NAD + H+ H2O ADP + P ATP E Electron Transport Chain Chemiosmosis F MITOCHONDRIAL H OXIDATIVE PHOSPHORYLATION MATRIX G Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Bacterial and Archaeal ETCs Located in plasma membrane Protons are pumped into the periplasmic space outside the membrane. Some components resemble mitochondrial ETC, but many are different – different electron carriers – may be branched – may be shorter – may have lower energy yields 33 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Summary: The Central Role of Proton Motive Force 34 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Lower energy yield without oxygen as the terminal electron acceptor ANAEROBIC RESPIRATION 35 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Anaerobic Respiration Uses electron acceptor molecules other than O2 Yields less energy because E0 of electron acceptor is less positive than E0 of O2 Some organisms can switch between aerobic and anaerobic respiration. e.g. Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 36  An Example of Anaerobic Respiration Denitrification is the loss of nitrate (NO3) in the soil when it is reduced to nitrogen gas (N2) 37 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Example Ecological Impact of Anaerobic Respiration Dissimilatory nitrate reduction to ammonium – uses nitrate as the terminal electron acceptor in anaerobic respiration and converts it to ammonia – Part of the terrestrial and oceanic nitrogen cycle. Unlike denitrification, it acts to conserve bioavailable nitrogen in the system, producing soluble ammonium rather than unreactive dinitrogen gas. Denitrification – reduction of nitrate to nitrogen gas – causes loss of soil fertility: plants deprived of nitrate when an area is flooded and anoxic. – commonly used to remove nitrogen from sewage and municipal wastewater. 38 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  When there is no need for Electron Transport FERMENTATION 39 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Fermentation 40 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Fermentation Oxygen is not needed for fermentation ATP is generated by glycolysis The NADH produced by glycolysis is re-oxidized to NAD+ Pyruvate is the endogenous electron acceptor, so it gets reduced to lactate Other molecules and pathways can be used, including the production of alcohols Oxidative phosphorylation does not occur – ATP is formed by substrate-level phosphorylation only 41 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Common Homolactic Alcoholic Microbial fermenters fermentation Fermentatio Heterolactic n fermenters Food Alcoholic spoilage beverages, bread, etc. Yogurt, sauerkraut, Reference info. pickles, etc. Too much detail to memorize. 42 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Microbes build life from inorganic molecules CHEMOLITHOTROPHS 43 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Yeti Crabs Swarm Hydrothermal Vents in Antarctica 8,500 feet (2,600 meters) deep. No Light. What do they eat? Chemolithoautotrophic bacteria farmed on their hairs. Bacteria are epibionts that live on the surface of another organism. 44 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Chemolithoautotrophs in Deep Sea Vents High densities of organisms thrive at the interface where hot, mineral-rich fluids discharge from the seafloor and mix with colder, oxygenated seawater. The hot fluids enriched in reduced gases (e.g. H 2S, CH4, H2) and metals (e.g. Fe2+, Cu, Mn) relative to seawater. Microorganisms oxidize the reduced molecules in vent fluids and use the energy released to fix CO 2 or other single carbon compounds (e.g. CO, CH 4) into cellular material. Microbial chemosynthesis replaces photosynthesis as primary production at the base of the food chain. 45 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Fig 8. Kiwa tyleri sp. nov.; type material. Thatje S, Marsh L, Roterman CN, Mavrogordato MN, Linse K (2015) Adaptations to Hydrothermal Vent Life in Kiwa tyleri, a New Species of Yeti Crab from the East Scotia Ridge, Antarctica. PLOS ONE 10(6): e0127621. https://doi.org/10.1371/journal.pone.0127621 http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0127621 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Chemolithotrophy Chemolithotrophs use reduced inorganic molecules as an e - source e- released from inorganic molecules are transferred to an ETC ATP is synthesized by oxidative phosphorylation Donor redox are more positive than most organic molecule bonds, so the energy yields are low 47 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Three Major Groups of Chemolithotrophs Great ecological importance I. Several bacteria and archaea oxidize hydrogen II. Sulfur-oxidizing microbes hydrogen sulfide (H2S), sulfur (S0), thiosulfate (S2O32-) III. Nitrifying bacteria oxidize ammonia to nitrate 1. Genus Nitrosomonas convert ammonium into nitrite (toxic to plants), 2. Genus Nitrobacter oxidize nitrite to nitrate ions usable by plants. (Most of the nitrogen contained in fertilizer is made available to plants by these bacteria.) 48 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Energy Sources used by Chemolithotrophs Bacterial and archaeal species have specific electron donor/acceptor preferences. Much less energy is available from oxidation of inorganic molecules than glucose due to more positive redox potentials of the donors. 49 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Inorganic Energy Sources Yield Less Energy than Organic Sources Glucose -> CO2 △G◦’ = -686 kcal/mol Main point: Low energy yields 50 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Reverse Electron Flow by Chemolithotrophs How do chemolithotrophs anabolize (build carbon- based molecules) when the energy sources are insufficient to reduce NAD(P)H? Recall the Calvin cycle requires NADPH as e - source for fixing CO2 – many energy sources used by chemolithotrophs have E0 more positive than NAD+(P)/NAD(P)H – Answer: they use reverse electron flow to generate NADH or NADPH – This is an energy-intensive process. How can they afford to do it and still survive? 51 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Reverse Electron Flow by Chemolithotrophs Protons move periplasm to cytoplasm Protons move cytoplasm periplasm 52 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Metabolic Flexibility of Chemolithotrophs Many switch from chemolithotrophic metabolism to chemoorganotrophic metabolism when organic nutrients are present Many switch from autotrophic metabolism (via Calvin cycle) to heterotrophic metabolism 53 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Converting light energy to organic chemical energy PHOTOTROPHY 54 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Phototrophy Phototrophy: – a metabolic mode in which organisms convert light energy into chemical energy for growth. Photosynthesis – energy from light is trapped and converted to chemical energy – 50-85% of the earth’s oxygen is estimated to come from photosynthetic phytoplankton. – 5% comes from cyanobacteria Prochlorococcus 55 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Photosynthesis The light reactions make the high- energy ATP and NADPH that the Calvin cycle uses to fix CO2 into sugars. Light-dependent reactions trap light energy to make ATP and NADPH Light-independent reactions use ATP and NADPH to fix CO2 and make organic sugars. We will cover the Calvin cycle in the next chapter on anabolism. 56 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Light Reactions in Oxygenic Photosynthesis Photosynthetic eukaryotes and cyanobacteria Oxygen is generated and released into the environment Most important pigments are chlorophylls 57 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Chlorophyll-Based System Properties Chlorophylls: major light-absorbing pigments. Different chlorophylls have different absorption peaks Accessory pigments: transfer light energy to chlorophylls (carotenoids and phycobiliproteins) accessory pigments absorb different wavelengths of light than chlorophylls 58 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Organization of Pigments Antennas – highly organized arrays of chlorophylls and accessory pigments – captured light transferred to special reaction-center chlorophyll directly involved in photosynthetic electron transport – Accessory pigments expand the wavelengths of absorbed light Photosystems are the antenna and its associated reaction-center chlorophyll Electron flow: H2O ETC O2 (PMF ATP) 59 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. Chloroplasts and Light Reactions 60 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Oxygenic Photosynthesis Noncyclic electron flow makes ATP + NADPH Cyclic electron flow makes ATP only Photophosphorylation is the term for making ATP with light energy. 61 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.  Bacterial Anoxygenic Photosynthesis H2O not used as an electron source; therefore O 2 is not produced. Instead, uses electron donors such as H2, H2S, S, or organic matter. Only one photosystem involved Makes ATP directly, but does not reduce NAD(P)H Uses bacteriochlorophylls and mechanisms to generate reducing power Used by phototrophic green bacteria, phototrophic purple bacteria, and heliobacteria 62 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. An Example of Bacterial Anoxygenic Photosynthesis 63 Copyright (c) 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.