Cellular Respiration How Cells Harvest Energy Chapter 6

Cellular Respiration How Cells Harvest Energy Chapter 6

ATP Is Universal Energy Source n n 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 Aerobic pathways n n Evolved first Don’t require oxygen Start with glycolysis in cytoplasm Completed in cytoplasm n n 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 n NADH delivers electrons to a series of protein complexes in an electron transport chain n As electrons move from carrier to carrier, their energy is released in small quantities

In cellular respiration, electrons “fall” down an energy staircase and finally reduce O 2 NADH NAD+ ATP + 2 e Controlled release of energy for synthesis of ATP H+ El ec tro n ch tra ain ns po rt + 2 H 2 e H 2 O 1 2 O 2

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

Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Glucose 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 CO 2 and acetyl co-A, which enters the Krebs cycle Acetyl Co. A (acetyl coenzyme A) Pyruvic acid CO 2

The Krebs cycle completes the oxidation of glucose, generating many NADH and FADH 2 molecules • The Krebs cycle is a series of redox reactions in which enzymes strip away electrons and H+ Acetyl Co. A KREBS CYCLE 2 2 CO 2

Oxidative phosphorylation powers most ATP production n The electrons from NADH and FADH 2 travel down the electron transport chain to oxygen

Chemiosmosis powers most ATP production n n 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 n n Occurs in the inner mitochondrial membrane Coenzymes (NADH, FADH 2) 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 n n Electron transport systems are embedded in inner mitochondrial compartment NADH and FADH 2 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 n Glycolysis n n Krebs cycle n n 2 ATP formed by substrate-level phosphorylation ETC & Chemiosmosis (oxidative phosphorylation) n n 2 ATP formed by substrate-level phosphorylation 32 - 34 ATP formed Total ATP molecules formed: 36 - 38

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

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

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