Cellular Respiration and Fermentation 7 Cellular Respiration and Fermentation 1

Overview: 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 © 2014 Pearson Education, Inc.

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 © 2014 Pearson Education, Inc.

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 ATP powers most cellular work ATP Heat energy

Concept 7.1: Catabolic pathways yield energy by oxidizing organic fuels Several processes are central to cellular respiration and related pathways © 2014 Pearson Education, Inc.

Catabolic Pathways and Production of ATP The breakdown of organic molecules is exergonic Fermentation is a partial degradation of sugars that occurs without O2 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.

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.

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 © 2014 Pearson Education, Inc.

The Principle of Redox Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions 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.

becomes oxidized (loses electron) becomes reduced (gains electron) Figure 7.UN01 becomes oxidized (loses electron) becomes reduced (gains electron)

becomes oxidized becomes reduced Figure 7.UN02 becomes oxidized becomes reduced

The electron donor is called the reducing agent The electron acceptor is called the oxidizing agent Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds An example is the reaction between methane and O2 © 2014 Pearson Education, Inc.

Reactants Products becomes oxidized becomes reduced Methane (reducing Figure 7.3 Reactants Products becomes oxidized becomes reduced Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water

LEO says GER LEO GER Lose Electrons Oxidized Gain Electrons Reduced © 2014 Pearson Education, Inc.

Redox reactions that move electrons closer to electronegative atoms, like oxygen, release chemical energy that can be put to work © 2014 Pearson Education, Inc.

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 sythesis © 2014 Pearson Education, Inc.

becomes oxidized becomes reduced Figure 7.UN03 becomes oxidized becomes reduced

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 As an electron acceptor, NAD+ functions as an oxidizing agent during cellular respiration Each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP © 2014 Pearson Education, Inc.

2 e−+ 2 H+ 2 e− + H+ NAD+ NADH H+ Dehydrogenase Reduction of NAD+ Figure 7.4 2 e−+ 2 H+ 2 e− + H+ NAD+ NADH H+ Dehydrogenase Reduction of NAD+ + 2[H] (from food) + H+ Oxidation of NADH Nicotinamide (reduced form) Nicotinamide (oxidized form)

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.

(a) Uncontrolled reaction (b) Cellular respiration Figure 7.5 ½ ½ H2 + O2 2 H + O2 Controlled release of energy 2 H+ + 2 e− ATP ATP Explosive release Electron transport chain Free energy, G Free energy, G ATP 2 e− ½ O2 2 H+ H2O H2O (a) Uncontrolled reaction (b) Cellular respiration

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

Glycolysis (color-coded teal throughout the chapter) Figure 7.UN05 1. Glycolysis (color-coded teal throughout the chapter) 2. Pyruvate oxidation and the citric acid cycle (color-coded salmon) 3. Oxidative phosphorylation: electron transport and chemiosmosis (color-coded violet)

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 ATP ATP ATP Substrate-level Substrate-level Oxidative

The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions © 2014 Pearson Education, Inc.

The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions 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 by substrate-level phosphorylation 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.

Enzyme Enzyme ADP Substrate + ATP Product Figure 7.7 Enzyme Enzyme ADP P Substrate + ATP Product

Glycolysis occurs in the cytoplasm and has two major phases 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.

Citric acid cycle Oxidative phosphorylation Pyruvate oxidation Figure 7.UN06 Citric acid cycle Oxidative phosphorylation Pyruvate oxidation Glycolysis ATP ATP ATP

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+ 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+

Glycolysis: Energy Investment Phase Figure 7.9a Glycolysis: Energy Investment Phase Glyceraldehyde 3-phosphate (G3P) ATP Glucose 6-phosphate Fructose 6-phosphate ATP Fructose 1,6-bisphosphate Glucose ADP ADP Isomerase 5 Hexokinase Phosphogluco- isomerase Phospho- fructokinase Aldolase Dihydroxyacetone phosphate (DHAP) 1 4 2 3

32 Glycolysis: Energy Payoff Phase Figure 7.9b 2 ATP 2 ATP 2 NADH 2 H2O Glyceraldehyde 3-phosphate (G3P) 2 ADP 2 NAD+ + 2 H+ 2 ADP 2 2 2 2 2 Triose phosphate dehydrogenase Phospho- glycerokinase Phospho- glyceromutase Enolase Pyruvate kinase 2 P i 9 1,3-Bisphospho- glycerate 7 3-Phospho- glycerate 8 2-Phospho- glycerate Phosphoenol- pyruvate (PEP) 10 Pyruvate 6 32

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. 33

34 Citric acid cycle Oxidative phosphorylation Pyruvate oxidation Figure 7.UN07 Citric acid cycle Oxidative phosphorylation Pyruvate oxidation Glycolysis ATP ATP ATP 34

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 2 CO2 FADH2 3 NAD+ FAD 3 NADH + 3 H+ ADP + P i ATP

The citric acid cycle, also called the Krebs cycle, completes the breakdown of pyruvate to CO2 The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn © 2014 Pearson Education, Inc. 36

37 Citric acid cycle Oxidative phosphorylation Pyruvate oxidation Figure 7.UN08 Citric acid cycle Oxidative phosphorylation Pyruvate oxidation Glycolysis ATP ATP ATP 37

Citric acid cycle Figure 7.11-6 Acetyl CoA 1 Oxaloacetate 8 2 Malate CoA-SH NADH 1 + H+ H2O NAD+ Oxaloacetate 8 2 Malate Citrate Isocitrate NAD+ Citric acid cycle 3 NADH 7 + H+ H2O CO2 Fumarate CoA-SH α-Ketoglutarate 6 4 CoA-SH 5 FADH2 CO2 NAD+ FAD Succinate P NADH i GTP GDP Succinyl CoA + H+ ADP ATP formation ATP

39 Start: Acetyl CoA adds its two-carbon group to Figure 7.11a Start: Acetyl CoA adds its two-carbon group to oxaloacetate, producing citrate; this is a highly exergonic reaction. Acetyl CoA CoA-SH 1 H2O Oxaloacetate 2 Citrate Isocitrate 39

Isocitrate is oxidized; NAD+ is reduced. Figure 7.11b Isocitrate Redox reaction: Isocitrate is oxidized; NAD+ is reduced. NAD+ NADH 3 + H+ CO2 CO2 release CoA-SH α-Ketoglutarate 4 CO2 release CO2 NAD+ Redox reaction: After CO2 release, the resulting four-carbon molecule is oxidized (reducing NAD+), then made reactive by addition of CoA. NADH Succinyl CoA + H+

Succinate is oxidized; FAD is reduced. Figure 7.11c Fumarate 6 CoA-SH 5 FADH2 FAD Redox reaction: Succinate is oxidized; FAD is reduced. Succinate P i Succinyl CoA GTP GDP ADP ATP formation ATP

42 Redox reaction: Malate is oxidized; NAD+ is reduced. Oxaloacetate 8 Figure 7.11d Redox reaction: Malate is oxidized; NAD+ is reduced. NADH + H+ NAD+ 8 Oxaloacetate Malate 7 H2O Fumarate 42

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. 43