How Cells Harvest Energy Chapter 6 Cellular Respiration How Cells Harvest Energy Chapter 6

ATP Is Universal Energy Source Photosynthetic organisms get energy from the sun The light energy is converted to the chemical bond energy of ATP

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

Redox reactions release energy when electrons “fall” from a hydrogen carrier to oxygen NADH delivers electrons to a series of protein complexes in an electron transport chain As electrons move from carrier to carrier, their energy is released in small quantities

Electron transport chain In cellular respiration, electrons “fall” down an energy staircase and finally reduce O2 H2O NAD+ NADH ATP H+ Controlled release of energy for synthesis of ATP Electron transport chain 2 O2 1 2e +

Main Pathways Start with Glycolysis Glycolysis occurs in cytoplasm Reactions are catalyzed by enzymes Glucose 2 Pyruvate (six carbons) (three carbons)

Glycolysis harvests chemical energy by oxidizing glucose to pyruvate

Net Energy Yield from Glycolysis Energy investment phase: 2 ATP invested Energy releasing phase: 2 NADH formed 4 ATP formed Net yield: 2 ATP and 2 NADH

Pyruvate is chemically groomed for the Krebs cycle Each molecule of pyruvate is broken down to form CO2 and acetyl co-A, which enters the Krebs cycle Acetyl CoA (acetyl coenzyme A) Pyruvic acid CO2

The Krebs cycle completes the oxidation of glucose, generating many NADH and FADH2 molecules Acetyl CoA The Krebs cycle is a series of redox reactions in which enzymes strip away electrons and H+ 2 KREBS CYCLE 2 CO2

Oxidative phosphorylation powers most ATP production The electrons from NADH and FADH2 travel down the electron transport chain to oxygen

Chemiosmosis powers most ATP production Energy released by the electrons is used to pump H+ into the space between the mitochondrial membranes (intermembrane space) by active transport In chemiosmosis, the H+ ions diffuse back through the inner membrane through ATP synthase , which capture the energy to synthesize ATP

Overview: Oxidative Phosphorylation Occurs in the inner mitochondrial membrane Coenzymes (NADH, FADH2) deliver electrons to electron transport systems Electron transport sets up H+ ion gradients Flow of H+ down concentration gradients powers ATP formation

Overview: Electron Transport System Electron transport systems are embedded in inner mitochondrial compartment NADH and FADH2 give up electrons that they picked up in earlier stages to electron transport system Electrons are transported through the system The final electron acceptor is oxygen

Summary of Aerobic Cellular Respiration Glycolysis 2 ATP formed by substrate-level phosphorylation Krebs cycle ETC & Chemiosmosis (oxidative phosphorylation) 32 - 34 ATP formed Total ATP molecules formed: 36 - 38

Anaerobic Pathways Do not use oxygen Produce less ATP than aerobic pathways Two types Lactic acid fermentation Alcoholic fermentation

Lactic Acid Fermentation 2 Lactate NAD+ NADH 2 ATP 2 ADP + 2 2 Pyruvate GLYCOLYSIS P Glucose

Alcoholic Fermentation NAD+ NADH 2 GLYCOLYSIS 2 ADP + 2 P ATP Glucose 2 Pyruvate released CO2 2 Ethanol