Harvesting Chemical Energy: Cellular Respiration Chapter 8

Cellular Respiration: THE BIG PICTURE Cellular respiration is the process by which organisms can get energy (ATP) from their food (glucose) Cellular respiration is CRITICAL for life Occurs in BOTH plants and animals Two main mechanisms  Aerobic cellular Respiration – Requires Oxygen  Anaerobic cellular Respiration – Does not require Oxygen

Main Types of Cellular Respiration Pathways Aerobic Respiration Evolved later Require oxygen Start with glycolysis in cytoplasm Completed in mitochondria Anaerobic Respiration Evolved first Don’t require oxygen Start with glycolysis in cytoplasm Completed in cytoplasm

Aerobic Respiration Overall Equation: C 6 H O 2 6CO 2 + 6H 2 0 glucose oxygen carbon water dioxide Several steps occur in the middle (intermediates) Each step (rxn) catalyzed by enzymes

Aerobic respiration Overview Stage One: Glycolysis (cytoplasm) Stage Two: Preparation for Krebs (mitochondrial matrix) Krebs Cycle (matrix) Stage Three: Electron Transfer Chain (across inner membrane of mitochondria)

The Role of Coenzymes Several oxidation-reduction rxns take place in aerobic respiration (Glucose gets oxidized to carbon dioxide) In order to aid in the redox rxns, enzymes use coenzymes NAD+ and FAD to carry electrons from glucose derivatives to the electron transfer chain NAD + and FAD accept electrons and hydrogen to become NADH and FADH 2 during the first two stages of aerobic respiration (Glycolysis, Krebs) and deliver electrons and hydrogen to the Electron Transfer Chain to make ATP

Coenzymes: NAD + & FAD Nicotinamide adenine dinucleotide (NAD + )

Stage One: Glycolysis Glucose (6-carbon) is broken down into 2 molecules of pyruvate (3-carbon) Yields 2 ATP by substrate level phosphorylation

1 NADH Glycolysis: Overall Reaction Glucose (6C) 2 ATP 2 ADP Fructose Bisphosphate (6C) G3P (3C) G3P (3C) Pyruvate (3C) Pyruvate (3C) 2 ADP 2 ATP 2 ADP 2 ATP 1 NAD +

Aerobic Respiration: 1. Glycolysis

Glycolysis: Net Yield Energy requiring steps: 2 ATP invested Energy releasing steps: 2 NADH formed 4 ATP formed Net yield: 2 ATP + 2 NADH + 2 molecules of Pyruvate

What happens next? Depends on the organism and the presence of oxygen If oxygen is around: Aerobic respiration, proceed to Krebs cycle If no oxygen: Anaerobic respiration, Proceed to Fermentation

Second Stage: Krebs cycle Preparatory reactions: Oxidation of pyruvate  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

Oxidation of Pyruvate

Results of the Second Stage All of the carbon atoms 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

Coenzyme Reductions during First Two Stages Glycolysis2 NADH Preparatory reactions 2 NADH Krebs cycle 2 FADH NADH Total 2 FADH NADH

Occurs in the mitochondria Coenzymes deliver electrons to electron transfer chains Electron transfer sets up H + ion gradients Flow of H + down gradients powers ATP formation Electron Transfer Chain

Importance of Oxygen Why does aerobic respiration require oxygen? Oxygen acts as the final electron (and hydrogen ion) acceptor in ETC  Binds to leftover e - and H + to form water  “Clean up crew”

ATP Actual Yield

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 +

686 kcal of energy are released 7.5 kcal are conserved in each ATP When 32 ATP form, 240 kcal (32 X 7.5) are captured in ATP Efficiency is 240 / 686 X 100 = 35% Most energy is lost as heat Efficiency of Aerobic Respiration

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

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 +

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 Evolution of Metabolic Pathways