Cellular Respiration Chapter 8 BIOL 1000 Dr Mohamad

Cellular Respiration Chapter 8 BIOL 1000 Dr. Mohamad H. Termos

Cellular Respiration • Breaks down nutrient molecules for the production of ATP • Consumes oxygen and produces carbon dioxide (CO 2) • Aerobic • Usually involves the breakdown of glucose to CO 2 and H 2 O • Energy is extracted from the glucose molecule: • Released step-wise • Allows ATP to be produced efficiently 2

Glucose Catabolism • Three major pathways of glucose catabolism • Glycolysis • Glucose (6 C) two pyruvic acid molecules • Anaerobic fermentation • Occurs in the absence of oxygen • pyruvic acid lactic acid • Aerobic respiration • Occurs in the presence of oxygen • pyruvic acid CO 2 and H 2 O 26 -3 (3 C)

Cellular Respiration • Cellular respiration includes four phases: –Glycolysis –Preparatory (prep) reaction –Citric acid cycle –Electron transport chain (ETC) • Many steps in each phase • NAD+ and FAD accept electrons to be transferred to other phases for more energy 4

Glucose Catabolism • Two coenzymes of special importance to glucose catabolism • NAD+ (nicotinamide adenine dinucleotide) • Derived from niacin (B vitamin) • NAD+ + 2 H NADH + H+ • FAD (flavin adenine dinucleotide) • Derived from riboflavin • FAD + 2 H FADH 2 26 -5

Outside the Mitochondria: Glycolysis • Glycolysis occurs outside mitochondria • • • Two ATP are used Glucose Pyruvate (3 carbon molecule) Gain two NADH Four ATP are produced; 2 are consumed Net gain of two ATP (4 ATP produced - 2 ATP consumed) 6

Outside the Mitochondria: Fermentation • Pyruvate is a “biological crossroads” • If O 2 is available to the cell, pyruvate enters the mitochondria for aerobic respiration. • If O 2 is not available to the cell, fermentation (anaerobic process) occurs in the cytoplasm. 7

Anaerobic Fermentation • In an exercising muscle, demand for ATP may exceed the oxygen supply • The only ATP cells can make under these circumstances are the 2 ATP produced by glycolysis • Glycolysis cannot continue without supply of NAD+ • NADH donates a pair of electrons to pyruvic acid reducing it to lactic acid • Lactic acid leaves the cells • Enter bloodstream and transported to the liver • When oxygen becomes available the liver oxidizes it back to pyruvic acid • Oxygen is created by exercising muscle 26 -8

Anaerobic Fermentation • Drawbacks of anaerobic fermentation • Wasteful, because most of the energy of glucose is still in the lactic acid and has contributed no useful work • Lactic acid is toxic and contributes to muscle fatigue • Skeletal muscle is relatively tolerant of anaerobic fermentation, cardiac muscle less so • The brain employs no anaerobic fermentation 26 -9

Aerobic Respiration • Most ATP generated in mitochondria, which requires oxygen. In the presence of oxygen, pyruvic acid enters the mitochondria 26 -10

Inside the Mitochondria • The Preparatory (prep) Reaction: Connects glycolysis to the citric acid cycle. Pyruvate, enters the mitochondria. Pyruvate (3 carbons) 2 -carbon group (Carbon dioxide released and electrons are picked up as hydrogen atom by NAD+ ) • Citric Acid Cycle (Krebs cycle): Occurs in mitochondria. NADH and FADH 2 capture energy rich electrons and ATP is formed. Produces: 4 CO 2, 2 ATP, 6 NADH and 2 FADH 2 per glucose molecule 11

Inside the Mitochondria • Electron Transport Chain (ETC): Takes place in the mitochondria. Receives electrons from NADH & FADH 2 and produces ATP. • Oxygen serves as the final electron acceptor and combines with hydrogen ions to form water. • ATP moves out of mitochondria and is used for cellular work. It can be broken down to ADP and inorganic phosphate. These molecules are returned to the mitochondria for more ATP production 12

Inside the Mitochondria • Energy yield from glucose metabolism: • Net yield per glucose: 1 - From glycolysis – 2 ATP, 2 -From citric acid cycle – 2 ATP, 3 - From electron transport chain – 32 or 34 ATP • Energy content: 1 - Reactant (glucose) 686 kcal, 2 - Energy yield (36 ATP) 263 kcal • The rest of the energy from glucose is lost as heat 13

The Metabolic Pool Concept 14