Overview Life Is Work Living cells require energy

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 • Energy flows into an ecosystem as sunlight and leaves as heat • Photosynthesis generates oxygen and organic molecules, which are used in cellular respiration • Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 9 -2 Light energy ECOSYSTEM Photosynthesis in chloroplasts CO 2 + H 2 O Cellular respiration in mitochondria Organic + O molecules 2 ATP powers most cellular work Heat energy

Catabolic Pathways and Production of ATP • The breakdown of organic molecules is exergonic • Fermentation is a partial degradation of sugars that occurs without oxygen • Cellular respiration consumes oxygen and organic molecules and yields ATP • Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose: C 6 H 12 O 6 + 6 O 2 6 CO 2 + 6 H 2 O + Energy (ATP + heat) Copyright © 2005 Pearson Education, Inc. publishing as 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 © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

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) becomes oxidized (loses electron) Xe- + Y X + becomes reduced (gains electron) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ye-

• The electron donor is called the reducing agent • The electron receptor is called the oxidizing agent Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds • An example is the reaction between methane and oxygen Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 9 -3 Products Reactants becomes oxidized CH 4 2 O 2 + CO 2 C Energy 2 H 2 O + becomes reduced H H + H O O O C O H Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water H

Oxidation of Organic Fuel Molecules During Cellular Respiration • During cellular respiration, the fuel (such as glucose) is oxidized and oxygen is reduced: becomes oxidized C 6 H 12 O 6 + 6 O 2 6 CO 2 + 6 H 2 O + Energy becomes reduced Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

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+ (nicotinamide adenine dinucleotide), 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 © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 9 -4 2 e– + 2 H+ NAD+ 2 e– + H+ H+ NADH Dehydrogenase + 2[H] (from food) Nicotinamide (oxidized form) + Nicotinamide (reduced form) H+

Fig. 9 -UN 4 Dehydrogenase Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• 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 • Oxygen pulls electrons down the chain in an energy-yielding tumble • The energy yielded is used to regenerate ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 9 -5 H 2 + 1 / 2 O 2 + 2 H 1 /2 O 2 (from food via NADH) ATP tran ATP rt spo Free energy, G ron Explosive release of heat and light energy t Elec Free energy, G 2 H + + 2 e– Controlled release of energy for synthesis of ATP in cha 2 e– 2 H+ H 2 O Uncontrolled reaction H 2 O Cellular respiration

The Stages of Cellular Respiration: A Preview • Cellular respiration has four stages: – Glycolysis (breaks down glucose into two molecules of pyruvate) – Pyruvate conversion (pyruvate is changed into acetyl Co. A for entry into citric acid cycle) – The citric acid cycle (completes the breakdown of glucose) – Oxidative phosphorylation (accounts for most of the ATP synthesis) • The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 9 -6_1 Glycolysis Pyruvate Glucose Cytosol Mitochondrion ATP Substrate-level phosphorylation

LE 9 -6_2 Glycolysis Pyruvate Glucose Cytosol Citric acid cycle Mitochondrion ATP Substrate-level phosphorylation

LE 9 -6_3 Electrons carried via NADH and FADH 2 Electrons carried via NADH Glycolysis Pyruvate Glucose Cytosol Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis Mitochondrion ATP ATP Substrate-level phosphorylation Oxidative phosphorylation

• Oxidative phosphorylation (indirect ATP synthesis) accounts for almost 90% of the ATP generated by cellular respiration • A small amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation (direct ATP synthesis) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 9 -7 Substrate level phosphorylation - direct synthesis of ATP Enzyme ADP P Substrate + Product ATP

Step 1: Glycolysis • 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 © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 9 -8 Energy investment phase Glucose Glycolysis Citric acid cycle 2 ADP + 2 P 2 ATP used Oxidative phosphorylation Energy payoff phase ATP ATP 4 ADP + 4 P 2 NAD+ + 4 e– + 4 H+ 4 ATP formed 2 NADH + 2 H+ 2 Pyruvate + 2 H 2 O Net Glucose 4 ATP formed – 2 ATP used 2 NAD+ + 4 e– + 4 H+ 2 Pyruvate + 2 H 2 O 2 ATP 2 NADH + 2 H+

LE 9 -9 a_1 Glucose ATP Hexokinase ADP Glucose-6 -phosphate Glycolysis Citric acid cycle ATP Oxidation phosphorylation ATP

LE 9 -9 a_2 Glucose ATP Hexokinase ADP Glucose-6 -phosphate Phosphoglucoisomerase Fructose-6 -phosphate ATP Phosphofructokinase ADP Fructose 1, 6 -bisphosphate Aldolase Isomerase Dihydroxyacetone phosphate Glyceraldehyde 3 -phosphate Glycolysis Citric acid cycle ATP Oxidation phosphorylation ATP

LE 9 -9 b_1 2 NAD+ Triose phosphate dehydrogenase 2 NADH + 2 H+ 1, 3 -Bisphoglycerate 2 ADP Phosphoglycerokinase 2 ATP 3 -Phosphoglycerate Phosphoglyceromutase 2 -Phosphoglycerate

LE 9 -9 b_2 2 NAD+ Triose phosphate dehydrogenase 2 NADH + 2 H+ 1, 3 -Bisphoglycerate 2 ADP Phosphoglycerokinase 2 ATP 3 -Phosphoglycerate Phosphoglyceromutase 2 -Phosphoglycerate 2 H 2 O Enolase Phosphoenolpyruvate 2 ADP Pyruvate kinase 2 ATP Pyruvate

Step 2: Pyruvate Conversion • Before the citric acid cycle can begin, pyruvate must be converted to acetyl Co. A, which links the cycle to glycolysis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 9 -10 MITOCHONDRION CYTOSOL NAD+ NADH + H+ Acetyl Co A Pyruvate Transport protein CO 2 Coenzyme A

Step 3: The Citrate Cycle (a. k. a. Krebs Cycle) • The citric acid cycle, also called the Krebs cycle, takes place within the mitochondrial matrix • The cycle oxidizes organic fuel derived from pyruvate, generating one ATP, 3 NADH, and 1 FADH 2 per turn Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 9 -11 Pyruvate (from glycolysis, 2 molecules per glucose) CO 2 NAD+ NADH + H+ Glycolysis Citric acid cycle ATP Oxidation phosphorylation Co. A Acetyl Co. A Citric acid cycle FADH 2 2 CO 2 3 NAD+ 3 NADH + 3 H+ FAD ADP + P i ATP

• The citric acid cycle has eight steps, each catalyzed by a specific enzyme • The acetyl group of acetyl Co. A 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 FADH 2 produced by the cycle relay electrons extracted from food to the electron transport chain Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 9 -12_1 Glycolysis Citric acid cycle ATP Oxidation phosphorylation ATP Acetyl Co. A H 2 O Oxaloacetate Citrate Isocitrate Citric acid cycle

LE 9 -12_2 Glycolysis Citric acid cycle ATP Oxidation phosphorylation ATP Acetyl Co. A H 2 O Oxaloacetate Citrate Isocitrate CO 2 Citric acid cycle NAD+ NADH + H+ a-Ketoglutarate NAD+ Succinyl Co. A NADH + H+ CO 2

LE 9 -12_3 Glycolysis Citric acid cycle ATP Oxidation phosphorylation ATP Acetyl Co. A H 2 O Oxaloacetate Citrate Isocitrate CO 2 Citric acid cycle NAD+ NADH + H+ Fumarate a-Ketoglutarate FADH 2 NAD+ FAD Succinate GTP GDP ATP Pi Succinyl Co. A NADH + H+ CO 2

LE 9 -12_4 Glycolysis Citric acid cycle ATP Oxidation phosphorylation ATP Acetyl Co. A NADH + H+ H 2 O NAD+ Oxaloacetate Malate Citrate Isocitrate CO 2 Citric acid cycle H 2 O NAD+ NADH + H+ Fumarate a-Ketoglutarate FADH 2 NAD+ FAD Succinate GTP GDP ATP Pi Succinyl Co. A NADH + H+ CO 2

Step 4: Oxidative Phosphorylation • Following glycolysis and the citric acid cycle, NADH and FADH 2 account for most of the energy (potential energy) extracted from food • These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation (powered by redox reactions) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The Pathway of Electron Transport • The electron transport chain is in the inner mitochondrial membrane of the mitochondrion • Most of the chain’s components are proteins, which exist in multiprotein complexes • The carriers alternate reduced and oxidized states as they accept and donate electrons • Electrons are passed to protein complexes (that increase in electronegativity (increased desire to possess electrons)) as they go down the chain and are finally passed to the ultimate electron acceptor O 2 (most electronegative), forming H 2 O Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 9 -13 NADH 50 Free energy (G) relative to O 2 (kcal/mol) FADH 2 40 FMN I Multiprotein complexes FAD Fe • S II Fe • S Q III Cyt b 30 Fe • S Cyt c 1 Glycolysis Citric acid cycle ATP Oxidative phosphorylation: electron transport and chemiosmosis IV Cyt c Cyt a 20 Cyt a 3 10 0 2 H+ + 1/2 O 2 H 2 O ATP

• The electron transport chain generates no ATP • The chain’s function is to break the large freeenergy drop from food to O 2 into smaller steps that release energy in manageable amounts Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Chemiosmosis: The Energy-Coupling Mechanism • Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space • H+ then moves back across the membrane, passing through channels in 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 9 -14 INTERMEMBRANE SPACE H+ H+ H+ A rotor within the membrane spins as shown when H+ flows past it down the H + gradient. H+ A stator anchored in the membrane holds the knob stationary. A rod (or “stalk”) extending into the knob also spins, activating catalytic sites in the knob. H+ ADP + Pi MITOCHONDRAL MATRIX ATP Three catalytic sites in the stationary knob join inorganic phosphate to ADP to make ATP.

LE 9 -15 Inner mitochondrial membrane Glycolysis Citric acid cycle ATP Oxidative phosphorylation: electron transport and chemiosmosis ATP H+ H+ H+ Intermembrane space H+ Cyt c Protein complex of electron carriers Q III I II FADH 2 Inner mitochondrial membrane NADH + H+ IV ATP synthase FAD 2 H+ + 1/2 O 2 H 2 O NAD+ Mitochondrial matrix ATP ADP + P i (carrying electrons from food) H+ Electron transport chain Electron transport and pumping of protons (H+), Which create an H+ gradient across the membrane Oxidative phosphorylation Chemiosmosis ATP synthesis powered by the flow of H+ back across the membrane

An Accounting of ATP Production by Cellular Respiration • During cellular respiration, most energy flows in this sequence: glucose NADH electron transport chain protonmotive force ATP • About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 32 ATP • The variability in whether 30 or 32 total ATP are generated per 1 molecule of glucose depends on which of two shuttle pathways is used to transfer electrons from NADH in the cytosol, generated during glycolysis, to either NAD+ or FAD (one shuttle passes electrons to NAD+ to generate NADH and the other shuttle passes electrons to FAD to generate FADH 2) - see following Figure 9. 16* Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 9 -16 * Electron shuttles span membrane CYTOSOL 2 NADH Glycolysis Glucose 2 Pyruvate MITOCHONDRION 2 NADH or 2 FADH 2 2 NADH 2 Acetyl Co. A 6 NADH Citric acid cycle + 2 ATP by substrate-level phosphorylation Maximum per glucose: About 30 or 32 ATP 2 FADH 2 Oxidative phosphorylation: electron transport and chemiosmosis + about 26 or 28 ATP by oxidation phosphorylation, depending on which shuttle transports electrons form NADH in cytosol

Concept 9. 5: Fermentation enables some cells to produce ATP without the use of oxygen • Cellular respiration requires O 2 to produce ATP • Glycolysis can produce ATP with or without O 2 (in aerobic or anaerobic conditions) • In the absence of O 2, glycolysis couples with fermentation to produce ATP • Additionally, anaerobic respiration uses an electron transport chain with an electron acceptor other than O 2, for example sulfate • Fermentation uses phosphorylation instead of an electron transport chain to generate ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Types of Fermentation • Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis • Two common types are alcohol fermentation and lactic acid fermentation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO 2 • Alcohol fermentation by yeast is used in brewing, winemaking, and baking Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 9 -17 a 2 ADP + 2 P i Glucose 2 ATP Glycolysis 2 Pyruvate 2 NAD+ 2 Ethanol Alcohol fermentation 2 NADH + 2 H+ 2 CO 2 2 Acetaldehyde

• In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO 2 • 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 O 2 is scarce Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 9 -17 b 2 ADP + 2 P i Glucose 2 ATP Glycolysis 2 NAD+ 2 Lactate Lactic acid fermentation 2 NADH + 2 H+ 2 Pyruvate

Fermentation and Cellular Respiration Compared • Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate • The processes have different final electron acceptors: an organic molecule (such as pyruvate) in fermentation and O 2 in cellular respiration • Cellular respiration produces much more ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 9 -18 Glucose CYTOSOL Pyruvate No O 2 present Fermentation O 2 present Cellular respiration MITOCHONDRION Ethanol or lactate Acetyl Co. A Citric acid cycle

Concept 9. 6: Glycolysis and the citric acid cycle connect to many other metabolic pathways • Gycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The Versatility of Catabolism • Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration • Glycolysis accepts a wide range of carbohydrates • Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle • Fats are digested to glycerol (used in glycolysis) and fatty acids (used in generating acetyl Co. A) • An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 9 -19 Proteins Carbohydrates Amino acids Sugars Glycerol Fatty acids Glycolysis Glucose Glyceraldehyde-3 - P NH 3 Fats Pyruvate Acetyl Co. A Citric acid cycle Oxidative phosphorylation

Biosynthesis (Anabolic Pathways) • The body uses small molecules to build other substances • These small molecules may come directly from food, from glycolysis, or from the citric acid cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings