Biology Tenth Edition: How Cells Make ATP - PDF

BIOLOGY tenth edition Topic 6 How Cells Make ATP: Energy-Releasing Pathways © Cengage Learning 2015 SOLOMON MARTIN MARTIN BER...

BIOLOGY tenth edition Topic 6 How Cells Make ATP: Energy-Releasing Pathways © Cengage Learning 2015 SOLOMON MARTIN MARTIN BERG  Food For Energy Every organism must extract energy from food molecules that it manufactures by photosynthesis or obtains from the environment © Cengage Learning 2015  Metabolism Metabolism has two complementary components: – Catabolism: releases energy by splitting complex molecules into smaller components – Anabolism: synthesis of complex molecules from simpler building blocks Most anabolic reactions are endergonic and require ATP or some other energy source to drive them © Cengage Learning 2015  Three Metabolic Pathways Exergonic metabolic pathways release free energy that is captured by the cell: – Aerobic cellular respiration: catabolic processes convert energy in chemical bonds of nutrients to chemical energy stored in ATP – Anaerobic respiration: does not require oxygen – Fermentation: does not require oxygen © Cengage Learning 2015  8.1 Redox Reactions Cells use aerobic respiration to obtain energy from glucose: C6H12O6 + 6 O2 + 6 H2O → 6 CO2 + 12 H2O + energy Aerobic respiration requires O2 and nutrients are catabolized to CO2 and H2O – A redox reaction: glucose becomes oxidized and oxygen becomes reduced – Electrons are transferred to oxygen in a series of steps © Cengage Learning 2015  Changes in Free Energy © Cengage Learning 2015  Figure 8-1 Changes in free energy The release of energy from a glucose molecule is analogous to the liberation of energy by a falling object. The total energy released (E) is the same whether it occurs all at once or in a series of steps. © Cengage Learning 2015  8.2 The Four Stages Of Aerobic Respiration Glycolysis Formation Citric acid Electron transport of cycle and acetyl coenzyme A chemiosmosis Glycolysis Acetyl Citric Electron transport coenzyme acid and A cycle chemiosmosis Pyruvate 2 ATP 2 ATP 32 ATP © Cengage Learning 2015  Figure 8-2 The four stages of aerobic respiration The stages of aerobic respiration occur in specific locations. Glycolysis, the first stage, occurs in the cytosol. Pyruvate, the product of glycolysis, enters a mitochondrion, where cellular respiration continues with the formation of acetyl CoA, the citric acid cycle, and electron transport and chemiosmosis. Most ATP is synthesized by chemiosmosis. © Cengage Learning 2015  Reactions Involved in Aerobic Respiration Dehydrogenations: two hydrogen atoms are removed from the substrate and transferred to NAD+ or FAD Decarboxylations: part of a carboxyl group (COOH) is removed from the substrate as a molecule of CO2 Preparation reactions: molecules are rearranged so they can undergo further dehydrogenations or decarboxylations © Cengage Learning 2015  In Glycolysis, Glucose Yields Two Pyruvates Glycolysis: – Takes place in the cytosol – Metabolizes the 6-carbon sugar glucose into two 3-carbon molecules of pyruvate – Does not require oxygen – Net yield: 2 ATP and 2 NADH molecules – Two major phases: Endergonic reactions that require ATP Exergonic reactions that yield ATP and NADH © Cengage Learning 2015  First Phase of Glycolysis Phosphate groups are transferred from ATP to glucose in two separate phosphorylation reactions – The phosphorylated sugar is broken enzymatically into two three-carbon molecules, yielding 2 glyceraldehyde-3- phosphate (G3P) glucose + 2 ATP → 2 G3P + 2 ADP © Cengage Learning 2015  Second Phase of Glycolysis The “energy capture phase” – G3P is converted to pyruvate G3P is oxidized by removal of 2 electrons, which combine with NAD+ NAD+ + 2 H → NADH + H+ – ATP is formed by substrate-level phosphorylation 2 G3P + 2 NAD+ + 4 ADP → 2 pyruvate + 2 NADH + 4 ATP © Cengage Learning 2015  Glycolysis © Cengage Learning 2015  Pyruvate is Converted to Acetyl CoA Undergoes oxidative decarboxylation – A carboxyl group is removed as CO2, which diffuses out of the cell – Occurs in mitochondria of eukaryotes The two-carbon fragment is oxidized and is attached to coenzyme A, yielding acetyl coenzyme A (acetyl CoA) 2 pyruvate + 2 NAD+ + 2 CoA → 2 acetyl CoA + 2 NADH + 2 CO2 © Cengage Learning 2015  Formation of Acetyl CoA Carbon dioxide Pyruvate Coenzyme A Acetyl coenzyme A © Cengage Learning 2015  The Citric Acid Cycle Oxidizes Acetyl Groups Derived From Acetyl CoA Also known as tricarboxylic acid (TCA) cycle and Krebs cycle – Takes place in the matrix of the mitochondria – Specific enzyme catalyzes each of the eight steps – Begins when acetyl CoA transfers its two- carbon acetyl group to the four-carbon acceptor compound oxaloacetate, forming citrate, a six-carbon compound: oxaloacetate + acetyl CoA → citrate + CoA © Cengage Learning 2015  The Citric Acid Cycle (cont’d.) Citrate goes through a series of chemical transformations, losing two carboxyl group as CO2 One ATP is formed (per acetyl group) by substrate-level phosphorylation – Most of the oxidative energy is transferred to NAD+, forming 3 NADH Electrons are also transferred to FAD, forming FADH2 © Cengage Learning 2015  Overview of the Citric Acid Cycle Acetyl coenzyme A Coenzyme A Oxaloacetate Citrate CITRIC ACID CYCLE 5-carbon compound 4-carbon compound © Cengage Learning 2015  Kreb’s Cycle © Cengage Learning 2015  The Electron Transport Chain is Coupled to ATP Synthesis All electrons removed from a glucose during glycolysis, acetyl CoA formation, and citric acid cycle are transferred as part of hydrogen atoms to NADH and FADH2 – NADH and FADH2 enter the electron transport chain (ETC), where electrons move from one acceptor to another – Some electron energy is used to drive synthesis of ATP by oxidative phosphorylation © Cengage Learning 2015  The ETC Transfers Electrons from NADH and FADH2 to Oxygen In eukaryotes, the ETC is a series of electron carriers embedded in the inner mitochondrial membrane – Electrons pass down the ETC in a series of redox reactions, losing some of their energy at each step along the chain – Members of the ETC include flavin mononucleotide, ubiquinone, iron-sulfur proteins, and cytochromes © Cengage Learning 2015  Transfer of Electrons (cont’d.) The ETC includes four large protein complexes: – Complex I accepts electrons from NADH – Complex II accepts electrons from FADH2 – Complex III accepts electrons from reduced ubiquinone and passes them to cytochrome c – Complex IV (cytochrome c oxidase) accepts electrons from cytochrome c and reduces O2, forming H2O © Cengage Learning 2015  Transfer of Electrons (cont’d.) Oxygen is the final electron acceptor in the ETC Lack of oxygen blocks the entire ETC – No additional ATP is produced by oxidative phosphorylation Some poisons also inhibit normal activity of cytochromes – Example: Cyanide binds to iron in cytochrome, blocking ATP production © Cengage Learning 2015  Inquiring About: Electron Transport And Heat Mitochondria in plants such as skunk cabbage uncouple the ETC from ATP production to generate large amounts of heat © Cengage Learning 2015  The Chemiosmotic Model of ATP Synthesis Peter Mitchell (1961) proposed that electron transport and ATP synthesis are coupled by a proton gradient across the inner mitochondrial membrane in eukaryotes – Experiment: bacterial cells placed in an environment with a high hydrogen ion (proton) concentration synthesized ATP even if electron transport was not taking place © Cengage Learning 2015  Key Experiment: Evidence for Chemiosmosis Bacterial cytoplasm (low acid) Synthesized Acidic environment Plasma membrane © Cengage Learning 2015  The Proton Gradient Cytosol Outer mitochondrial membrane Intermembrane space—low pH Inner mitochondrial membrane Matrix—higher pH © Cengage Learning 2015  Synthesis of ATP Protons diffuse from the intermembrane space to the matrix through the enzyme complex ATP synthase Central structure of ATP synthase rotates, catalyzing the phosphorylation of ADP to form ATP Chemiosmosis allows exergonic redox reactions to produce ATP by oxidative phosphorylation © Cengage Learning 2015 Cytosol Outer mitochondrial membrane Intermembrane space Complex I Complex Complex Complex III IV II Outer mitochondrial membrane Matrix of Complex V: mitochondrion ATP synthase  Aerobic Respiration of One Glucose Yields a Maximum of 36 to 38 ATPs Aerobic respiration of one glucose molecule: 1. Glycolysis: glucose + 2 ATP → 2 pyruvates + 2 NADH + 4 ATPs 2. Pyruvate conversion: 2 pyruvates → 2 acetyl CoA + 2 CO2 + 2 NADH 3. Citric acid cycle: 2 acetyl CoA → 4 CO2 + 6 NADH + 2 FADH2 + 2 ATPs Total = 4 ATP + 10 NADH + 2 FADH2 © Cengage Learning 2015  ATP Production Oxidation of NADH in the electron transport chain yields up to 3 ATPs per molecule Oxidation of FADH2 yields 2 ATPs per molecule Certain eukaryotic cells expend energy to shuttle NADH across the mitochondrial membrane, so maximum number of ATPs formed from NADH varies from 28 to 30 © Cengage Learning 2015  Energy Yield from Oxidation of Glucose by Aerobic Respiration © Cengage Learning 2015  Cells Regulate Aerobic Respiration Glycolysis is partly controlled by feedback regulation of the enzyme phosphofructokinase Phosphofructokinase has two allosteric sites: – An inhibitor site that binds ATP (at very high ATP levels) – An activator site to which AMP binds (when ATP is low) © Cengage Learning 2015  8.3 Energy Yield Of Nutrients Other Than Glucose Other nutrients are transformed into metabolic intermediates that enter glycolysis or the citric acid cycle – For amino acids, the amino group (NH2) is removed and the carbon chain is used in aerobic respiration – For lipids, glycerol is converted to a compound that enters glycolysis and fatty acids are converted by β-oxidation to acetyl CoA, which enters the citric acid cycle © Cengage Learning 2015  PROTEINS CARBOHYDRATES FATS Amino Glycerol Glycolysis Fatty acids acids Glucose G3P Pyruvate CO2 Acetyl coenzyme A Citric acid cycle Electron transport and chemiosmosis End products: NH3 H2O CO2  8.4 Anaerobic Respiration and Fermentation Anaerobic respiration does not use oxygen as the final electron acceptor – Used by prokaryotes in environments such as waterlogged soil, stagnant ponds, and animal intestines – Electrons from glucose pass from NADH down an ETC coupled to ATP synthesis by chemiosmosis – An inorganic substance such as nitrate or sulfate is the final electron acceptor © Cengage Learning 2015  Products of Anaerobic Respiration End products of this anaerobic respiration are CO2, one or more reduced inorganic substances, and ATP Example: Anaerobic respiration in the nitrogen cycle C6H12O6+ 12 KNO3 → 6 CO2+ 6 H2O + 12 KNO2 + energy © Cengage Learning 2015  Fermentation Fermentation is an anaerobic pathway that does not involve an ETC – Only two ATPs are formed per glucose (by substrate-level phosphorylation during glycolysis) – NADH molecules transfer H atoms to organic molecules, regenerating NAD+ needed for glycolysis © Cengage Learning 2015  Alcohol Fermentation and Lactate Fermentation are Inefficient Highly inefficient because of partially oxidized fuel – Yeasts are facultative anaerobes: can switch to alcohol fermentation when deprived of oxygen – Enzymes decarboxylate pyruvate, forming acetaldehyde – NADH produced during glycolysis transfers hydrogen atoms to acetaldehyde, reducing it to ethyl alcohol © Cengage Learning 2015  Alcohol and Lactate Fermentation (cont’d.) Certain fungi and bacteria perform lactate fermentation - NADH produced during glycolysis transfers hydrogen atoms to pyruvate, reducing it to lactate Vertebrate muscle cells produce lactate when oxygen is depleted during exercise © Cengage Learning 2015  Fermentation Glycolysis Glycolysis Glucose Glucose 2 NAD+ 2 NADH 2 NAD+ 2 NADH 2 ATP 2 ATP 2 Pyruvate 2 Pyruvate CO2 2 Ethyl alcohol 2 Lactate © Cengage Learning 2015  Comparing Aerobic and Anaerobic Respiration, and Fermentation Aerobic respiration: – Transfers electrons to ETC – Terminal electron acceptor is O2 – Final product is water – ATP synthesized by oxidative phosphorylation, chemiosmosis, substrate-level phosphorylation © Cengage Learning 2015  A Comparison (cont’d.) Anaerobic respiration: – Transfers electrons to ETC – Terminal electron acceptor is inorganic substances – Final product is reduced inorganic substances – ATP synthesized by oxidative phosphorylation, chemiosmosis, substrate-level phosphorylation © Cengage Learning 2015  A Comparison (cont’d.) Fermentation: – Transfers electrons to organic molecule – There is no ETC – Final product is alcohol or lactate – ATP synthesized by substrate-level phosphorylation during glycolysis © Cengage Learning 2015