How Cells Release Stored Energy Chapter 8

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

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

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

Summary Equation for Aerobic Respiration C 6 H O 2 6CO 2 + 6H 2 0 glucose oxygen carbon water glucose oxygen carbon water dioxide dioxide

Overview of Aerobic Respiration CYTOPLASM Glycolysis Electron Transfer Phosphorylation Krebs Cycle ATP 2 CO 2 4 CO water 2 NADH 8 NADH 2 FADH 2 2 NADH 2 pyruvate e - + H + e - + oxygen (2 ATP net) glucose Typical Energy Yield: 36 ATP e-e- e - + H + ATP H+H+ e - + H + ATP 2 4 Figure 8.3 Page 135

The Role of Coenzymes NAD + and FAD accept electrons and hydrogen NAD + and FAD accept electrons and hydrogen Become NADH and FADH 2 Become NADH and FADH 2 Deliver electrons and hydrogen to the electron transfer chain Deliver electrons and hydrogen to the electron transfer chain

A simple sugar A simple sugar (C 6 H 12 O 6 ) Atoms held together by covalent bonds Atoms held together by covalent bonds Glucose In-text figure Page 136

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

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

The Krebs Cycle Overall Products Coenzyme A Coenzyme A 2 CO 2 2 CO 2 3 NADH 3 NADH FADH 2 FADH 2 ATP ATP Overall Reactants Acetyl-CoA Acetyl-CoA 3 NAD + 3 NAD + FAD FAD ADP and P i ADP and P i

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

Creating an H + Gradient NADH OUTER COMPARTMENT INNER COMPARTMENT

Making ATP: Chemiosmotic Model ATP ADP + P i INNER COMPARTMENT

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

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

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

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

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

Alcoholic Fermentation C 6 H 12 O 6 ATP NADH 2 acetaldehyde electrons, hydrogen from NADH 2 NAD ADP 2 pyruvate 2 4 energy output energy input GLYCOLYSIS ETHANOL FORMATION 2 ATP net 2 ethanol 2 H 2 O 2 CO 2

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

Processes Are Linked sunlight energy water + carbon dioxide PHOTOSYNTHESIS AEROBIC RESPIRATION sugar molecules oxygen In-text figure Page 146