Chapter 9 Cellular Respiration: Harvesting Chemical Energy

Energy flows into an ecosystem as sunlight and leaves as heat Photosynthesis generates O 2 and organic molecules, which are used in cellular respiration 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

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

Catabolic Pathways and Production of ATP Fermentation is a partial degradation of sugars that occurs without O 2 (no oxygen) Aerobic respiration consumes organic molecules and O 2 and yields ATP (requires O2) Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O 2 (does not require oxygen) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration C 6 H 12 O O 2  6 CO H 2 O + Energy (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

The Principle of Redox In oxidation, a substance loses electrons, or is oxidized - The electron donor is called the reducing agent In reduction, a substance gains electrons, or is reduced - The electron receptor is called the oxidizing agent Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

becomes oxidized (loses electron) becomes reduced (gains electron) becomes oxidized becomes reduced

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

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

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

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 (input 2 ATP) Energy payoff phase (produces 4 ATP) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 9-8 Energy investment phase Glucose 2 ADP + 2 P 2 ATPused formed 4 ATP Energy payoff phase 4 ADP + 4 P 2 NAD e – + 4 H + 2 NADH + 2 H + 2 Pyruvate + 2 H 2 O Glucose Net 4 ATP formed – 2 ATP used2 ATP 2 NAD e – + 4 H + 2 NADH + 2 H +

The citric acid cycle (Kreb Cycle) In the presence of O 2, pyruvate enters the mitochondrion (matrix) Pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis Generates 1 ATP, 3 NADH, and 1 FADH 2 per turn The NADH and FADH 2 produced by the cycle relay electrons extracted from food to the electron transport chain Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Animation: Mitochondria Animation: Mitochondria

Fig CYTOSOLMITOCHONDRION NAD + NADH+ H Pyruvate Transport protein CO 2 Coenzyme A Acetyl CoA

Fig Pyruvate NAD + NADH + H + Acetyl CoA CO 2 CoA Citric acid cycle FADH 2 FAD CO NAD H + ADP +P i ATP NADH

The Pathway of Electron Transport The electron transport chain is in the cristae of the mitochondrion NADH and FADH 2 donate electrons to the electron transport chain Electrons drop in free energy as they go down the chain and are finally passed to O 2, forming H 2 O Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Electrons are passed through a number of proteins The electron transport chain generates no ATP The chain’s function is to break the large free- energy drop from food to O 2 into smaller steps that release energy in manageable amounts Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings VCAC: Cellular Processes: Electron Transport Chain: First Look VCAC: Cellular Processes: Electron Transport Chain: First Look VCAC: Cellular Processes: ATP Synthase: The MovieTransport VCAC: Cellular Processes: ATP Synthase: The MovieTransport

Fig INTERMEMBRANE SPACE Rotor H+H+ Stator Internal rod Cata- lytic knob ADP + P ATP i MITOCHONDRIAL MATRIX

ATP Production by Cellular Respiration Glycolysis- 2 ATP Citric Acid cycle- 2 ATP ATP Synthase- 32 or 34 Total- 38 ATP Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fermentation and anaerobic respiration Most 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 or anaerobic respiration to produce ATP Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Types of Fermentation Two common types are alcohol fermentation and lactic acid fermentation In alcohol fermentation pyruvate is converted to ethanol Yeast - in brewing, winemaking, and baking Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

In lactic acid fermentation, lactate is the end product Fungi and bacteria - make cheese and yogurt Human muscle cells use lactic acid fermentation to generate ATP when O 2 is scarce Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig Glucose Glycolysis Pyruvate CYTOSOL No O 2 present: Fermentation O 2 present: Aerobic cellular respiration MITOCHONDRION Acetyl CoA Ethanol or lactate Citric acid cycle