Cellular Respiration: Harvesting Chemical Energy Chapter 9 Cellular Respiration: Harvesting Chemical Energy

Living cells require energy from outside sources Overview: Life Is Work Living cells require energy from outside sources Some animals, such as the giant panda, obtain energy by eating plants, and some animals feed on other organisms that eat plants For the Discovery Video Space Plants, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 9-1 Figure 9.1 How do these leaves power the work of life for the giant panda?

Energy flows into an ecosystem as sunlight and leaves as heat Photosynthesis generates O2 and organic molecules, which are used in cellular respiration Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Organic molecules Cellular respiration in mitochondria Fig. 9-2 Light energy ECOSYSTEM Photosynthesis in chloroplasts Organic molecules CO2 + H2O + O2 Cellular respiration in mitochondria Figure 9.2 Energy flow and chemical recycling in ecosystems ATP ATP powers most cellular work Heat energy

Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels Several processes are central to cellular respiration and related pathways Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

C6H12O6 + 6O2  6CO2 + 6H2O + (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 + 6O2  6CO2 + 6H2O + (ATP+heat) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

In oxidation, a substance loses electrons, or is oxidized 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) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

becomes oxidized (loses electron) becomes reduced (gains electron) Fig. 9-UN1 becomes oxidized (loses electron) becomes reduced (gains electron)

becomes oxidized becomes reduced Fig. 9-UN2 becomes oxidized becomes reduced

An example is the reaction between methane and O2 The electron donor (the one that is oxidized) is called the reducing agent The electron receptor (the one that is reduced) 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Methane (reducing agent) Oxygen (oxidizing agent) Fig. 9-3 Reactants Products becomes oxidized becomes reduced Figure 9.3 Methane combustion as an energy-yielding redox reaction Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water

Oxidation of Organic Fuel Molecules During Cellular Respiration During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced: Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 9-UN3 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 9-UN4 Dehydrogenase The electron is tracked through the reaction as a hydrogen atom: H (one proton, one electron).

NADH H+ NAD+ + 2[H] + H+ 2 e– + 2 H+ 2 e– + H+ Dehydrogenase Fig. 9-4 2 e– + 2 H+ 2 e– + H+ NADH H+ Dehydrogenase Reduction of NAD+ NAD+ + 2[H] + H+ Oxidation of NADH Nicotinamide (reduced form) Nicotinamide (oxidized form) Figure 9.4 NAD+ as an electron shuttle

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

(a) Uncontrolled reaction (b) Cellular respiration Fig. 9-5 H2 + 1/2 O2 2 H + 1/2 O2 (from food via NADH) Controlled release of energy for synthesis of ATP 2 H+ + 2 e– ATP Explosive release of heat and light energy ATP Electron transport chain Free energy, G Free energy, G ATP 2 e– Figure 9.5 An introduction to electron transport chains 1/2 O2 2 H+ H2O H2O (a) Uncontrolled reaction (b) Cellular respiration

The Stages of Cellular Respiration: A Preview Cellular respiration has three stages: Glycolysis (breaks down glucose into two molecules of pyruvate) The citric acid cycle (completes the breakdown of glucose) Oxidative phosphorylation (accounts for most of the ATP synthesis) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Electrons carried via NADH Electrons carried via NADH and FADH2 Fig. 9-6-3 Electrons carried via NADH Electrons carried via NADH and FADH2 Oxidative phosphorylation: electron transport and chemiosmosis Glycolysis Citric acid cycle Glucose Pyruvate Mitochondrion Cytosol Figure 9.6 An overview of cellular respiration ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Enzyme Enzyme ADP P Substrate + ATP Product Fig. 9-7 Figure 9.7 Substrate-level phosphorylation Product

Glycolysis occurs in the cytoplasm and has two major phases: Concept 9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate Glycolysis occurs in the cytoplasm and has two major phases: Energy investment phase Energy payoff phase Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Energy investment phase Fig. 9-8 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 9.8 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+

Glucose ATP 1 Hexokinase ADP Glucose-6-phosphate Fig. 9-9-1 Figure 9.9 A closer look at glycolysis Glucose-6-phosphate

Glucose-6-phosphate 2 Phosphogluco- isomerase Fructose-6-phosphate Fig. 9-9-2 Glucose ATP 1 Hexokinase ADP Glucose-6-phosphate Glucose-6-phosphate 2 Phosphoglucoisomerase 2 Phosphogluco- isomerase Fructose-6-phosphate Figure 9.9 A closer look at glycolysis Fructose-6-phosphate

Fructose- 1, 6-bisphosphate Fig. 9-9-3 Glucose ATP 1 1 Hexokinase ADP Fructose-6-phosphate Glucose-6-phosphate 2 2 Phosphoglucoisomerase ATP 3 Phosphofructo- kinase Fructose-6-phosphate ATP 3 3 ADP Phosphofructokinase ADP Figure 9.9 A closer look at glycolysis Fructose- 1, 6-bisphosphate Fructose- 1, 6-bisphosphate

Aldolase Isomerase Fructose- 1, 6-bisphosphate 4 5 Dihydroxyacetone Fig. 9-9-4 Glucose ATP 1 Hexokinase ADP Glucose-6-phosphate 2 Phosphoglucoisomerase Fructose- 1, 6-bisphosphate 4 Fructose-6-phosphate Aldolase ATP 3 Phosphofructokinase ADP Figure 9.9 A closer look at glycolysis 5 Isomerase Fructose- 1, 6-bisphosphate 4 Aldolase 5 Isomerase Dihydroxyacetone phosphate Glyceraldehyde- 3-phosphate Dihydroxyacetone phosphate Glyceraldehyde- 3-phosphate

Glyceraldehyde- 3-phosphate Fig. 9-9-5 2 NAD+ 6 Triose phosphate dehydrogenase 2 NADH 2 P i + 2 H+ 2 2 1, 3-Bisphosphoglycerate Glyceraldehyde- 3-phosphate 2 NAD+ 6 Triose phosphate dehydrogenase 2 P 2 NADH i + 2 H+ Figure 9.9 A closer look at glycolysis 2 1, 3-Bisphosphoglycerate

2 2 ADP 2 ATP 2 3-Phosphoglycerate 1, 3-Bisphosphoglycerate 7 Fig. 9-9-6 2 NAD+ 6 Triose phosphate dehydrogenase 2 NADH 2 P i + 2 H+ 2 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP 2 1, 3-Bisphosphoglycerate 2 ADP 2 3-Phosphoglycerate 7 Phosphoglycero- kinase 2 ATP Figure 9.9 A closer look at glycolysis 2 3-Phosphoglycerate

2 3-Phosphoglycerate 8 Phosphoglycero- mutase 2 2-Phosphoglycerate Fig. 9-9-7 2 NAD+ 6 Triose phosphate dehydrogenase 2 NADH 2 P i + 2 H+ 2 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP 2 3-Phosphoglycerate 2 3-Phosphoglycerate 8 Phosphoglyceromutase 8 Phosphoglycero- mutase 2 2-Phosphoglycerate Figure 9.9 A closer look at glycolysis 2 2-Phosphoglycerate

2 2-Phosphoglycerate Enolase 2 H2O 2 Phosphoenolpyruvate 9 Fig. 9-9-8 2 NAD+ 6 Triose phosphate dehydrogenase 2 NADH 2 P i + 2 H+ 2 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP 2 2-Phosphoglycerate 2 3-Phosphoglycerate 8 Phosphoglyceromutase 9 Enolase 2 H2O 2 2-Phosphoglycerate 9 Enolase Figure 9.9 A closer look at glycolysis 2 H2O 2 Phosphoenolpyruvate 2 Phosphoenolpyruvate

2 Phosphoenolpyruvate 2 ADP 10 Pyruvate kinase 2 ATP 2 Pyruvate Fig. 9-9-9 2 NAD+ 6 Triose phosphate dehydrogenase 2 NADH 2 P i + 2 H+ 2 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP 2 Phosphoenolpyruvate 2 ADP 2 3-Phosphoglycerate 8 10 Phosphoglyceromutase Pyruvate kinase 2 ATP 2 2-Phosphoglycerate 9 Enolase 2 H2O Figure 9.9 A closer look at glycolysis 2 Phosphoenolpyruvate 2 ADP 10 Pyruvate kinase 2 ATP 2 Pyruvate 2 Pyruvate