Cellular Respiration and Fermentation 7 Cellular Respiration and Fermentation

Aim: How can we describe the internal mechanisms required in respiration? Do Now: Describe the chemical equation for respiration. Homework: Complete Chapter 7 concept questions due Monday.

Life Is Work Living cells require energy from outside sources. Some animals, such as the giraffe, obtain energy by eating plants, and some animals feed on other organisms that eat plants. Energy flows into an ecosystem as sunlight and leaves as heat. Photosynthesis generates O2 and organic molecules, which are used as fuel for cellular respiration. Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work. Animation: Carbon Cycle © 2014 Pearson Education, Inc. 3

Light energy ECOSYSTEM Photosynthesis in chloroplasts Organic Figure 7.2 Light energy ECOSYSTEM Photosynthesis in chloroplasts Organic molecules CO2  H2O  O2 Cellular respiration in mitochondria Figure 7.2 Energy flow and chemical recycling in ecosystems ATP powers most cellular work ATP Heat energy 4

Catabolic Pathways and Production of ATP The breakdown of organic molecules is exergonic. Fermentation is a partial break down of sugars that occurs without O. Aerobic respiration consumes organic molecules and O2 and yields ATP. Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O2. © 2014 Pearson Education, Inc. 5

C6H12O6  6 O2  6 CO2  6 H2O  Energy (ATP  heat) 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) © 2014 Pearson Education, Inc. 6

Redox Reactions: Oxidation and Reduction The transfer of electrons during chemical reactions releases energy stored in organic molecules. This released energy is ultimately used to synthesize ATP. In oxidation, a substance loses electrons, or is oxidized. In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced). © 2014 Pearson Education, Inc. 7

Oxidation of Organic Fuel Molecules During Cellular Respiration During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced. Organic molecules with an abundance of hydrogen, like carbohydrates and fats, are excellent fuels. As hydrogen (with its electron) is transferred to oxygen, energy is released that can be used in ATP synthesis. © 2014 Pearson Education, Inc. 8

becomes oxidized becomes reduced Figure 7.UN03 becomes oxidized becomes reduced Figure 7.UN03 In-text figure, cellular respiration redox reaction, p. 137 9

Stepwise Energy Harvest via NAD+ and the Electron Transport Chain In cellular respiration, glucose and other organic molecules are broken down in a series of steps. Electrons from organic compounds are usually first transferred to NAD, a coenzyme. NAD is an electron acceptor. Each NADH represents stored energy that is tapped to synthesize ATP. © 2014 Pearson Education, Inc. 10

NADH passes the electrons to the electron transport chain. Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction. O2 pulls electrons down the chain in an energy-yielding tumble. The energy yielded is used to regenerate ATP. © 2014 Pearson Education, Inc. 11

The Stages of Cellular Respiration: A Preview Harvesting of energy from glucose has three stages: 1. Glycolysis (breaks down glucose into two molecules of pyruvate). 2. Pyruvate oxidation and the citric acid cycle (completes the breakdown of glucose). 3. Oxidative phosphorylation (accounts for most of the ATP synthesis). Animation: Cellular Respiration © 2014 Pearson Education, Inc. 12

Electrons via NADH Glycolysis Glucose Pyruvate CYTOSOL MITOCHONDRION Figure 7.6-1 Electrons via NADH Glycolysis Glucose Pyruvate CYTOSOL MITOCHONDRION Figure 7.6-1 An overview of cellular respiration (step 1) ATP Substrate-level 13

Electrons via NADH and FADH2 Electrons via NADH Pyruvate oxidation Figure 7.6-2 Electrons via NADH and FADH2 Electrons via NADH Pyruvate oxidation Glycolysis Citric acid cycle Glucose Pyruvate Acetyl CoA CYTOSOL MITOCHONDRION Figure 7.6-2 An overview of cellular respiration (step 2) ATP ATP Substrate-level Substrate-level 14

Electrons via NADH and FADH2 Electrons via NADH Oxidative Figure 7.6-3 Electrons via NADH and FADH2 Electrons via NADH Oxidative phosphorylation: electron transport and chemiosmosis Pyruvate oxidation Glycolysis Citric acid cycle Glucose Pyruvate Acetyl CoA CYTOSOL MITOCHONDRION Figure 7.6-3 An overview of cellular respiration (step 3) ATP ATP ATP Substrate-level Substrate-level Oxidative 15

The step that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions (STEP # 3). © 2014 Pearson Education, Inc. 16

Aim: How can we describe glycolysis, pyruvate oxidation, and citric acid cycle? Do Now: Describe the overall goal of cellular respiration. Homework: Exam Friday Chapter 5 and 7.

Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration. A smaller amount of ATP is formed in glycolysis and the citric acid cycle. For each molecule of glucose degraded to CO2 and water by respiration, the cell makes up to 32 molecules of ATP. © 2014 Pearson Education, Inc. 18

Concept 7.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate 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. © 2014 Pearson Education, Inc. 19

Citric acid cycle Oxidative phosphorylation Pyruvate oxidation Figure 7.UN06 Citric acid cycle Oxidative phosphorylation Pyruvate oxidation Glycolysis Figure 7.UN06 In-text figure, mini-map, glycolysis, p. 140 ATP ATP ATP 20

Energy Investment Phase Figure 7.8 Energy Investment Phase Glucose 2 ADP  2 P 2 ATP used Energy Payoff Phase 4 ADP  4 P 4 ATP formed 2 NAD  4 e−  4 H 2 NADH  2 H Figure 7.8 The energy input and output of glycolysis 2 Pyruvate  2 H2O Net Glucose 2 Pyruvate  2 H2O 4 ATP formed − 2 ATP used 2 ATP 2 NAD  4 e−  4 H 2 NADH  2 H 21

Concept 7.3: After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules In the presence of O2, pyruvate enters the mitochondrion (in eukaryotic cells), where the oxidation of glucose is completed. 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. © 2014 Pearson Education, Inc. 24

Citric acid cycle Oxidative phosphorylation Pyruvate oxidation Figure 7.UN07 Citric acid cycle Oxidative phosphorylation Pyruvate oxidation Glycolysis Figure 7.UN07 In-text figure, mini-map, pyruvate oxidation, p. 142 ATP ATP ATP 25

2 molecules per glucose) CYTOSOL Figure 7.10a Pyruvate (from glycolysis, 2 molecules per glucose) CYTOSOL CO2 NAD CoA Figure 7.10a An overview of pyruvate oxidation and the citric acid cycle (part 1: pyruvate oxidation) NADH  H Acetyl CoA MITOCHONDRION CoA 26

Citric acid cycle Acetyl CoA CoA CoA 2 CO2 FADH2 3 NAD 3 NADH FAD Figure 7.10b Acetyl CoA CoA CoA Citric acid cycle 2 CO2 FADH2 3 NAD Figure 7.10b An overview of pyruvate oxidation and the citric acid cycle (part 2: citric acid cycle) 3 NADH FAD  3 H ADP  P i ATP 27

Citric acid cycle Figure 7.10 Pyruvate (from glycolysis, 2 molecules per glucose) CYTOSOL CO2 NAD CoA NADH  H Acetyl CoA MITOCHONDRION CoA CoA Citric acid cycle Figure 7.10 An overview of pyruvate oxidation and the citric acid cycle 2 CO2 FADH2 3 NAD FAD 3 NADH  3 H ADP  P i ATP 28

The citric acid cycle, also called the Krebs cycle, completes the breakdown of pyruvate to CO2. Products of the citric acid cycle: 1 ATP, 3 NADH, and 1 FADH2 per turn. © 2014 Pearson Education, Inc. 29

Citric acid cycle Oxidative phosphorylation Pyruvate oxidation Figure 7.UN08 Citric acid cycle Oxidative phosphorylation Pyruvate oxidation Glycolysis Figure 7.UN08 In-text figure, mini-map, citric acid cycle, p. 143 ATP ATP ATP 30

**OVERVIEW OF STEPS** 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. © 2014 Pearson Education, Inc. 31

Aim: How can we describe the electron transport chain Aim: How can we describe the electron transport chain? Do Now: Does oxygen have to be present in order for respiration to occur?

Concept 7.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis 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. © 2014 Pearson Education, Inc. 33

The Pathway of Electron Transport The electron transport chain is in the inner membrane (cristae) of the mitochondrion. Most of the chain’s components are proteins, which exist in multi-protein 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. © 2014 Pearson Education, Inc. 34

Oxidative phosphorylation: electron transport and chemiosmosis Citric Figure 7.UN09 Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle Pyruvate oxidation Glycolysis Figure 7.UN09 In-text figure, mini-map, oxidative phosphorylation, p. 144 ATP ATP ATP 35

Free energy (G) relative to O2 (kcal/mol) Figure 7.12 NADH 50 2 e− NAD FADH2 Multiprotein complexes 2 e− FAD 40 I FMN II Fe•S Fe•S Q III Cyt b 30 Fe•S Cyt c1 IV Free energy (G) relative to O2 (kcal/mol) Cyt c Cyt a Cyt a3 20 Figure 7.12 Free-energy change during electron transport 10 2 e− (originally from NADH or FADH2) 2 H  ½ O2 H2O 36

Electrons are passed through a number of proteins Electrons are transferred from NADH or FADH2 to the electron transport chain Electrons are passed through a number of proteins The electron transport chain generates no ATP directly It breaks the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts © 2014 Pearson Education, Inc. 37

Chemiosmosis: The Energy-Coupling Mechanism Electron transfer in the electron transport chain causes proteins to pump H from the mitochondrial matrix to the inter-membrane 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. © 2014 Pearson Education, Inc. 38

Video: ATP Synthase 3-D Side View Video: ATP Synthase 3-D Top View © 2014 Pearson Education, Inc. 39

H INTERMEMBRANE SPACE Stator Rotor Internal rod Catalytic knob ADP  Figure 7.13 H INTERMEMBRANE SPACE Stator Rotor Internal rod Catalytic knob Figure 7.13 ATP synthase, a molecular mill ADP  P ATP MITOCHONDRIAL MATRIX i 40

The energy stored in a H gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis. The H gradient is referred to as a proton-motive force, emphasizing its capacity to do work. © 2014 Pearson Education, Inc. 41

Oxidative phosphorylation: electron transport and chemiosmosis Citric Figure 7.UN09 Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle Pyruvate oxidation Glycolysis Figure 7.UN09 In-text figure, mini-map, oxidative phosphorylation, p. 146 ATP ATP ATP 42

Electron transport chain Oxidative phosphorylation Figure 7.14 H H Protein complex of electron carriers H H Cyt c IV Q III I ATP synthase II 2 H  ½ O2 H2O FADH2 FAD NADH NAD Figure 7.14 Chemiosmosis couples the electron transport chain to ATP synthesis ADP  P ATP i (carrying electrons from food) H 1 Electron transport chain 2 Chemiosmosis Oxidative phosphorylation 43

Electron transport chain Figure 7.14a H H Protein complex of electron carriers H Cyt c IV Q III I II 2 H  ½ O2 H2O FADH2 FAD Figure 7.14a Chemiosmosis couples the electron transport chain to ATP synthesis (part 1: electron transport chain) NAD NADH (carrying electrons from food) 1 Electron transport chain 44

2 ATP synthase Chemiosmosis H H ADP  ATP i P Figure 7.14b Figure 7.14b Chemiosmosis couples the electron transport chain to ATP synthesis (part 2: chemiosmosis) ADP  P ATP i H 2 Chemiosmosis 45

An Accounting of ATP Production by Cellular Respiration During cellular respiration, most energy flows in the following sequence: glucose  NADH  electron transport chain  proton-motive force  ATP About 34% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 32 ATP. The exact number of ATP released is not known. © 2014 Pearson Education, Inc. 46

Electron shuttles span membrane MITOCHONDRION 2 NADH CYTOSOL or Figure 7.15 Electron shuttles span membrane MITOCHONDRION 2 NADH CYTOSOL or 2 FADH2 2 NADH 2 NADH 6 NADH 2 FADH2 Glycolysis Pyruvate oxidation 2 Acetyl CoA Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle 2 Pyruvate Glucose  2 ATP Figure 7.15 ATP yield per molecule of glucose at each stage of cellular respiration  2 ATP  about 26 or 28 ATP About 30 or 32 ATP Maximum per glucose: 47

Aim: How can we describe chemiosmosis and fermentation Aim: How can we describe chemiosmosis and fermentation? Do Now: Does every organism on earth require oxygen to respire? Explain.

Concept 7.5: Fermentation and anaerobic respiration 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. If NO OXYGEN is present, glycolysis couples with fermentation or anaerobic respiration to produce ATP. © 2014 Pearson Education, Inc. 49

Anaerobic respiration uses an electron transport chain with a final electron acceptor that is something other than O2, for example, sulfate. Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP. © 2014 Pearson Education, Inc. 50

Types of Fermentation Fermentation consists of glycolysis plus reactions that regenerate NAD, which can be reused by glycolysis. Two common types are: alcohol fermentation lactic acid fermentation © 2014 Pearson Education, Inc. 51

In alcohol fermentation, pyruvate is converted to ethanol in two steps. The first step releases CO2 from pyruvate, and the second step reduces acetaldehyde to ethanol. Alcohol fermentation by yeast is used in brewing, winemaking, and baking. Animation: Fermentation Overview © 2014 Pearson Education, Inc. 52

(a) Alcohol fermentation (b) Lactic acid fermentation Figure 7.16 2 ADP  2 P 2 ADP  2 i 2 ATP P i 2 ATP Glucose Glycolysis Glucose Glycolysis 2 Pyruvate 2 NAD 2 NADH 2 CO2 2 NAD 2 NADH  2 H  2 H 2 Pyruvate Figure 7.16 Fermentation 2 Ethanol 2 Acetaldehyde 2 Lactate (a) Alcohol fermentation (b) Lactic acid fermentation 53

(a) Alcohol fermentation Figure 7.16a 2 ADP  2 P 2 ATP i Glucose Glycolysis 2 Pyruvate 2 NAD 2 NADH 2 CO2  2 H Figure 7.16a Fermentation (part 1: alcohol) 2 Acetaldehyde 2 Ethanol (a) Alcohol fermentation 54

(b) Lactic acid fermentation Figure 7.16b 2 ADP  2 P 2 ATP i Glucose Glycolysis 2 NAD 2 NADH  2 H 2 Pyruvate Figure 7.16b Fermentation (part 2: lactic acid) 2 Lactate (b) Lactic acid fermentation 55

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. © 2014 Pearson Education, Inc. 56

Comparing Fermentation with Anaerobic and Aerobic Respiration All use glycolysis (net ATP  2) to oxidize glucose and harvest chemical energy of food. In all three, NAD is the oxidizing agent that accepts electrons during glycolysis. The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration. Cellular respiration produces 32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule. © 2014 Pearson Education, Inc. 57

Obligate anaerobes carry out only fermentation or anaerobic respiration and cannot survive in the presence of O2. Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration. In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes. © 2014 Pearson Education, Inc. 58