PowerLecture: Chapter 8 How Cells Release Stored Energy 1

Impacts, Issues: When Mitochondria Spin Their Wheels More than 100 mitochondrial disorders are known Friedreich’s ataxia, caused by a mutant gene, results in loss of cordination, weak muscles, and visual problems Animal, plants, fungus, and most protists depend on structurally sound mitochondria Defective mitochondria can result in life threatening disorders 2

When Mitochondria Spin Their Wheels Fig. 8-1, p.122 3

“Killer” Bees Descendents of African honeybees that were imported to Brazil in the 1950s More aggressive, wider-ranging than other honeybees Africanized bee’s muscle cells have large mitochondria 4

ATP Is Universal Energy Source Photosynthesizers get energy from the sun Animals get energy second- or third-hand from plants or other organisms Regardless, the energy is converted to the chemical bond energy of ATP 5

Making ATP Plants make ATP during photosynthesis Cells of all organisms make ATP by breaking down carbohydrates, fats, and protein 6

Main Types of Energy-Releasing Pathways Anaerobic pathways Evolved first Don’t require oxygen Start with glycolysis in cytoplasm Completed in cytoplasm Aerobic pathways Evolved later Require oxygen Start with glycolysis in cytoplasm Completed in mitochondria 7

Main Types of Energy-Releasing Pathways start (glycolysis) in cytoplasm start (glycolysis) in cytoplasm completed in cytoplasm completed in mitochondrion Anaerobic Energy-Releasing Pathways Aerobic Respiration Fig. 8-2, p.124 8

Summary Equation for Aerobic Respiration C6H1206 + 6O2 6CO2 + 6H20 glucose oxygen carbon water dioxide 9

Transfer Phosphorylation Typical Energy Yield: 36 ATP CYTOPLASM glucose ATP 2 ATP 4 Glycolysis e- + H+ (2 ATP net) 2 NADH 2 pyruvate Overview of Aerobic Respiration e- + H+ 2 CO2 2 NADH e- + H+ 8 NADH 4 CO2 KrebsCycle e- + H+ 2 ATP 2 FADH2 e- Electron Transfer Phosphorylation 32 ATP H+ water e- + oxygen Typical Energy Yield: 36 ATP Fig. 8-3, p. 135 10

The Role of Coenzymes NAD+ and FAD accept electrons and hydrogen Become NADH and FADH2 Deliver electrons and hydrogen to the electron transfer chain 11

Glucose A simple sugar (C6H12O6) Atoms held together by covalent bonds In-text figure Page 126 12

Glycolysis Occurs in Two Stages Energy-requiring steps ATP energy activates glucose and its six-carbon derivatives Energy-releasing steps The products of the first part are split into three- carbon pyruvate molecules ATP and NADH form 13

Glycolysis glucose to second stage of aerobic GYCOLYSIS pyruvate to second stage of aerobic respiration or to a different energy-releasing pathway GLUCOSE Fig. 8-4a, p.126 14

ENERGY-REQUIRING STEPS fructose–1,6–bisphosphate Glycolysis ENERGY-REQUIRING STEPS OF GLYCOLYSIS glucose ATP 2 ATP invested ADP P glucose–6–phosphate P fructose–6–phosphate ATP ADP P P fructose–1,6–bisphosphate DHAP Fig. 8-4b, p.127 15

ENERGY-RELEASING STEPS 1,3–bisphosphoglycerate 1,3–bisphosphoglycerate Glycolysis ENERGY-RELEASING STEPS OF GLYCOLYSIS P P PGAL PGAL NAD+ NAD+ NADH NADH Pi Pi P P P P 1,3–bisphosphoglycerate 1,3–bisphosphoglycerate substrate-level phsphorylation ADP ADP ATP ATP 2 ATP invested P P 3–phosphoglycerate 3–phosphoglycerate Fig. 8-4c, p.127 16

Glycolysis P P 2–phosphoglycerate 2–phosphoglycerate H2O H2O P P PEP substrate-level phsphorylation ADP ADP ATP ATP 2 ATP produced pyruvate pyruvate Fig. 8-4d, p.127 17

Glycolysis: Net Energy Yield Energy requiring steps: 2 ATP invested Energy releasing steps: 2 NADH formed 4 ATP formed Net yield is 2 ATP and 2 NADH 20

Second Stage Reactions Preparatory reactions Pyruvate is oxidized into two-carbon acetyl units and carbon dioxide NAD+ is reduced Krebs cycle The acetyl units are oxidized to carbon dioxide NAD+ and FAD are reduced 21

Second Stage Reactions inner mitochondrial membrane outer mitochondrial membrane inner compartment outer compartment Fig. 8-6a, p.128 22

Preparatory Reactions pyruvate coenzyme A (CoA) NAD+ NADH O O carbon dioxide CoA acetyl-CoA 23

The Krebs Cycle Overall Reactants Overall Products Acetyl-CoA 3 NAD+ FAD ADP and Pi Overall Products Coenzyme A 2 CO2 3 NADH FADH2 ATP 24

Preparatory Reactions Acetyl-CoA Formation pyruvate Preparatory Reactions coenzyme A NAD+ (CO2) NADH CoA acetyl-CoA Krebs Cycle CoA oxaloacetate citrate NAD+ NADH NADH NAD+ FADH2 NAD+ FAD NADH ADP + phosphate group ATP Fig. 8-7a, p.129 25

Results of the Second Stage All of the carbon molecules in pyruvate end up in carbon dioxide Coenzymes are reduced (they pick up electrons and hydrogen) One molecule of ATP forms Four-carbon oxaloacetate regenerates 27

Two pyruvates cross the inner mitochondrial membrane. outer mitochondrial compartment 2 NADH inner mitochondrial compartment 6 NADH Krebs Cycle Eight NADH, two FADH 2, and two ATP are the payoff from the complete break-down of two pyruvates in the second-stage reactions. 2 FADH2 ATP 2 The six carbon atoms from two pyruvates diffuse out of the mitochondrion, then out of the cell, in six CO 6 CO2 Fig. 8-6b, p.128 28

Coenzyme Reductions during First Two Stages Glycolysis 2 NADH Preparatory reactions 2 NADH Krebs cycle 2 FADH2 + 6 NADH Total 2 FADH2 + 10 NADH 29

Phosphorylation glycolysis e– KREBS CYCLE electron transfer glucose Phosphorylation ATP 2 PGAL ATP 2 NADH 2 pyruvate glycolysis 2 FADH2 2 CO2 e– 2 acetyl-CoA 2 NADH H+ H+ 2 6 NADH KREBS CYCLE ATP Krebs Cycle ATP H+ 2 FADH2 ATP H+ 4 CO2 36 ATP H+ H+ ADP + Pi electron transfer phosphorylation H+ H+ H+ Fig. 8-9, p.131 30

Electron Transfer Phosphorylation Occurs in the mitochondria Coenzymes deliver electrons to electron transfer chains 31

Creating an H+ Gradient Electron transfer sets up H+ ion gradients OUTER COMPARTMENT NADH INNER COMPARTMENT 32

Making ATP: Chemiosmotic Model Flow of H+ down gradients powers ATP formation ATP INNER COMPARTMENT ADP + Pi 33

Importance of Oxygen Electron transport phosphorylation requires the presence of oxygen Oxygen withdraws spent electrons from the electron transfer chain, then combines with H+ to form water 34

Anaerobic Pathways Do not use oxygen Produce less ATP than aerobic pathways Two types Fermentation pathways Anaerobic electron transport 35

Fermentation Pathways Begin with glycolysis Do not break glucose down completely to carbon dioxide and water Yield only the 2 ATP from glycolysis Steps that follow glycolysis serve only to regenerate NAD+ 36

Alcoholic Fermentation glycolysis C6H12O6 Alcoholic Fermentation 2 ATP energy input 2 ADP 2 NAD+ 2 NADH 4 ATP 2 pyruvate energy output 2 ATP net ethanol formation 2 H2O 2 CO2 2 acetaldehyde electrons, hydrogen from NADH 2 ethanol Fig. 8-10d, p.132 37

Lactate Fermentation glycolysis C6H12O6 ATP 2 energy input 2 ADP 2 NAD+ 2 NADH 4 ATP 2 pyruvate energy output 2 ATP net lactate fermentation electrons, hydrogen from NADH 2 lactate Fig. 8-11, p.133 38

Anaerobic Electron Transport Carried out by certain bacteria Electron transfer chain is in bacterial plasma membrane Final electron acceptor is compound from environment (such as nitrate), not oxygen ATP yield is low 39

Summary of Energy Harvest (per molecule of glucose) Glycolysis 2 ATP formed by substrate-level phosphorylation Krebs cycle and preparatory reactions Electron transport phosphorylation 32 ATP formed 40

Energy Harvest Varies NADH formed in cytoplasm cannot enter mitochondrion It delivers electrons to mitochondrial membrane Membrane proteins shuttle electrons to NAD+ or FAD inside mitochondrion Electrons given to FAD yield less ATP than those given to NAD+ 41

Efficiency of Aerobic Respiration 686 kcal of energy are released 7.5 kcal are conserved in each ATP When 36 ATP form, 270 kcal (36 X 7.5) are captured in ATP Efficiency is 270 / 686 X 100 = 39 percent Most energy is lost as heat 42

complex carbohydrates electron transfer phosphorylation Alternative Energy Sources FOOD fats glycogen complex carbohydrates proteins fatty acids glycerol simple sugars (e.g., glucose) amino acids NH3 carbon backbones glucose-6-phosphate urea PGAL 2 ATP glycolysis 4 ATP (2 ATP net) NADH pyruvate acetyl-CoA NADH CO2 NADH, FADH2 Krebs Cycle 2 ATP CO2 e– ATP ATP electron transfer phosphorylation ATP many ATP H+ water e– + oxygen Fig. 8-13b, p.135 43

Evolution of Metabolic Pathways When life originated, atmosphere had little oxygen Earliest organisms used anaerobic pathways Later, noncyclic pathway of photosynthesis increased atmospheric oxygen Cells arose that used oxygen as final acceptor in electron transport 44

Processes Are Linked p.136b 45