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© 2017 Pearson Education, Inc.

Life Is Work Living cells require energy from outside sources to do work This work includes assembling polymers, membrane transport, moving, and reproducing Animals obtain energy to do this work by feeding on other animals or photosynthetic organisms Energy enters ecosystem as sunlight and leaves as heat The chemical elements essential to life are recycled © 2017 Pearson Education, Inc.

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 powers most cellular work ATP Heat energy © 2017 Pearson Education, Inc.

Catabolic Pathways and Production of ATP The breakdown of organic molecules via cellular respiration is exergonic Fermentation is a partial degradation of sugars that occurs without O2 Aerobic respiration consumes organic molecules and O2 and yields ATP © 2017 Pearson Education, Inc.

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy (ATP + heat) Catabolic Pathways and Production of ATP 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) The transfer of electrons during chemical reactions releases energy stored in organic molecules This released energy is ultimately used to synthesize ATP © 2017 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) © 2017 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 are excellent sources of high-energy electrons Energy is released as the electrons associated with hydrogen ions are transferred to oxygen, a lower energy state Figure 9.UN03 In-text figure, cellular respiration redox reaction, p. 166 becomes oxidized becomes reduced © 2017 Pearson Education, Inc.

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 © 2017 Pearson Education, Inc.

Uncontrolled reaction Cellular respiration H2 + ½ O2 2 H + ½ O2 Controlled release of energy 2 H+ + 2 e– Free energy, G Free energy, G Electron transport chain ATP Explosive release of energy ATP ATP 2 e– Figure 9.5 An introduction to electron transport chains ½ O2 2 H+ H2O H2O Uncontrolled reaction Cellular respiration © 2017 Pearson Education, Inc.

The Stages of Cellular Respiration: Harvesting of energy from glucose 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) © 2017 Pearson Education, Inc.

1. Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Glycolysis (“sugar splitting”) breaks down glucose into two molecules of pyruvate Glycolysis occurs whether or not O2 is present © 2017 Pearson Education, Inc.

CITRIC ACID CYCLE OXIDATIVE PHOSPHORYL- ATION PYRUVATE OXIDATION GLYCOLYSIS Figure 9.UN06 In-text figure, mini-map, glycolysis, p. 170 ATP © 2017 Pearson Education, Inc.

Electron shuttles span membrane 2 NADH or 2 FADH2 2 NADH GLYCOLYSIS Glucose 2 Figure 9.16a ATP yield per molecule of glucose at each stage of cellular respiration (part 1: glycolysis) Pyruvate + 2 ATP © 2017 Pearson Education, Inc.

2. After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules In the presence of O2, pyruvate enters a mitochondrion (in eukaryotic cells), where the oxidation of glucose is completed © 2017 Pearson Education, Inc.

CITRIC ACID CYCLE OXIDATIVE PHOSPHORYL- ATION PYRUVATE OXIDATION GLYCOLYSIS Figure 9.UN07 In-text figure, mini-map, pyruvate oxidation, p. 171 © 2017 Pearson Education, Inc.

PYRUVATE OXIDATION 2 Acetyl CoA CITRIC ACID CYCLE 2 NADH 6 NADH 2 FADH2 PYRUVATE OXIDATION 2 Acetyl CoA CITRIC ACID CYCLE Figure 9.16b ATP yield per molecule of glucose at each stage of cellular respiration (part 2: citric acid cycle) + 2 ATP © 2017 Pearson Education, Inc.

2. The Citric Acid Cycle The citric acid cycle, also called the Krebs cycle, completes the breakdown of pyruvate to CO2 The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain © 2017 Pearson Education, Inc.

CITRIC ACID CYCLE OXIDATIVE PHOSPHORYL- ATION PYRUVATE OXIDATION GLYCOLYSIS Figure 9.UN08 In-text figure, mini-map, citric acid cycle, p. 173 ATP © 2017 Pearson Education, Inc.

3. 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 © 2017 Pearson Education, Inc.

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 multiprotein complexes Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O Electrons are transferred from NADH or FADH2 to the electron transport chain © 2017 Pearson Education, Inc.

CITRIC ACID CYCLE OXIDATIVE PHOSPHORYL- ATION PYRUVATE OXIDATION GLYCOLYSIS Figure 9.UN09 In-text figure, mini-map, oxidative phosphorylation, p. 174 ATP © 2017 Pearson Education, Inc.

Electron transport chain NADH (least electronegative) 50 2 e– NAD+ Complexes I-IV each consist of multiple proteins with electron carriers. FADH2 2 e– FAD Free energy (G) relative to O2 (kcal/mol) 40 I FMN II Fe•S Fe•S Q III Cyt b 30 Fe•S Cyt c1 IV Cyt c Cyt a Electron transport chain Cyt a3 20 Figure 9.13 Free-energy change during electron transport 10 2 e– 2 H+ + ½ O2 (most electronegative) H2O © 2017 Pearson Education, Inc.

3. Chemiosmosis: The Energy-Coupling Mechanism The energy released as electrons are passed down the electron transport chain is used to pump H+ from the mitochondrial matrix to the intermembrane space H+ then moves down its concentration gradient back across the membrane, passing through the protein complex ATP synthase H+ moves into binding sites on the rotor of ATP synthase, causing it to spin in a way that catalyzes phosphorylation of ADP to ATP This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work © 2017 Pearson Education, Inc.

Stator H+ INTERMEMBRANE SPACE Rotor Internal rod Catalytic knob ADP + Figure 9.14 ATP synthase, a molecular mill ADP + P i ATP MITOCHONDRIAL MATRIX © 2017 Pearson Education, Inc.

Electron transport chain 2 Chemiosmosis ATP synthase Protein complex of electron carriers H+ H+ H+ Cyt c IV Q III I II 2 H+ + ½ O2 H2O FADH2 FAD NADH NAD+ Figure 9.15 Chemiosmosis couples the electron transport chain to ATP synthesis ADP + P i ATP (carrying electrons from food) H+ 1 Electron transport chain 2 Chemiosmosis Oxidative phosphorylation © 2017 Pearson Education, Inc.

An Accounting of ATP Production by Cellular Respiration During cellular respiration, most energy flows in this 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 rest of the energy is lost as heat © 2017 Pearson Education, Inc.

Electron shuttles span membrane MITOCHONDRION CYTOSOL GLYCOLYSIS 2 NADH or CYTOSOL 2 FADH2 2 NADH 2 NADH 6 NADH 2 FADH2 GLYCOLYSIS PYRUVATE OXIDATION OXIDATIVE PHOSPHORYLATION CITRIC ACID CYCLE Glucose 2 Pyruvate 2 Acetyl CoA (Electron transport and chemiosmosis) Figure 9.16 ATP yield per molecule of glucose at each stage of cellular respiration + 2 ATP + 2 ATP + about 26 or 28 ATP About 30 or 32 ATP Maximum per glucose: © 2017 Pearson Education, Inc.

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 © 2017 Pearson Education, Inc.

In alcohol fermentation, pyruvate is converted to ethanol in two steps The first step releases CO2 from pyruvate The second step produces NAD+ and ethanol Alcohol fermentation by yeast is used in brewing, winemaking, and baking © 2017 Pearson Education, Inc.

In lactic acid fermentation, pyruvate is reduced by NADH, forming NAD+ and lactate as end products, 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 during strenuous exercise when O2 is scarce © 2017 Pearson Education, Inc.

Comparing Fermentation with Anaerobic and Aerobic Respiration All use glycolysis (net ATP = 2) to oxidize glucose and harvest the chemical energy of food In all three, NAD+ is the oxidizing agent that accepts electrons during glycolysis © 2017 Pearson Education, Inc.

Glucose Glycolysis CYTOSOL Pyruvate No O2 present: O2 present: Fermentation Aerobic cellular respiration MITOCHONDRION Ethanol, lactate, or other products Acetyl CoA Figure 9.18 Pyruvate as a key juncture in catabolism CITRIC ACID CYCLE © 2017 Pearson Education, Inc.

Amino acids Fatty acids CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION Proteins Carbohydrates Fats Amino acids Sugars Glycerol Fatty acids GLYCOLYSIS Glucose Glyceraldehyde 3- P NH3 Pyruvate Figure 9.19_5 The catabolism of various molecules from food (step 5) Acetyl CoA CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION © 2017 Pearson Education, Inc.