Cellular Respiration: Harvesting Chemical Energy Chapter 9 Cellular Respiration: Harvesting Chemical Energy

powers most cellular work Light energy ECOSYSTEM CO2 + H2O Photosynthesis in chloroplasts Cellular respiration in mitochondria Organic molecules + O2 ATP powers most cellular work Heat

Ask a Cell Biologist: What is Cellular Respiration? Cellular respiration is the aerobic harvesting of energy from organic molecules It is a catabolic pathway It contains mostly exergonic reactions that release energy

Summary Equation for Cellular Respiration C6H12O6 + 6O2 6CO2 + 6H2O + ATP + Heat glucose oxygen Carbon dioxide

Greater number of bonds = Greater potential energy O = C = O carbon dioxide glucose

Oxidation-Reduction Reactions Oxidation-Reduction (Redox) reactions Transfer electrons from one reactant to another by oxidation and reduction

Oxidation-Reduction Reactions In oxidation A substance LOSES electrons, or is oxidized In reduction A substance GAINS electrons, or is reduced

becomes oxidized (loses electron) becomes reduced (gains electron) Example of Redox Reaction becomes oxidized (loses electron) becomes reduced (gains electron) Figure 9.UN01

Oxidation of Organic Fuel During cellular respiration Glucose is oxidized and oxygen is reduced It is the movement of hydrogen atoms and their electrons from glucose that are important C6H12O6 + 6O2 6CO2 + 6H2O + Energy BECOMES OXIDIZED BECOMES REDUCED

Mitochondrion: Site of Cellular Respiration Intermembrane space Outer membrane Free ribosomes in the mitochondrial matrix Mitochondrial DNA Inner Cristae Matrix 100 µm

Stages of Cellular Respiration Glycolysis Breaks down glucose into two molecules of pyruvate (pyruvic acid) Citric acid cycle (Krebs Cycle) Completes the breakdown of energy originally in glucose Electron transport chain Generates lots of ATP

Oxidative phosphorylation: electron transport and chemiosmosis Pyruvate oxidation Glycolysis Citric acid cycle Glucose Pyruvate Acetyl CoA CYTOSOL MITOCHONDRION Figure 9.6 An overview of cellular respiration. ATP ATP ATP

Glycolysis Glycolysis Means “splitting of sugar” Breaks down glucose into pyruvate Occurs in the cytoplasm of the cell Ancient pathway No oxygen required!

Energy investment phase Glycolysis: 2 Phases Glycolysis Citric acid cycle Oxidative phosphorylation ATP 2 ATP 4 ATP used formed Glucose 2 ADP + 2 P 4 ADP + 4 2 NAD+ + 4 e- + 4 H + 2 NADH + 2 H+ 2 Pyruvate + 2 H2O Energy investment phase Energy payoff phase 4 ATP formed – 2 ATP used 2 NAD+ + 4 e– + 4 H +

Gradual Harvesting of Energy Electrons from organic compounds Are usually first transferred to NAD+, an electron shuttle NAD+ becomes reduced to NADH as it accepts electrons and H’s from glucose Dehydrogenase is an enzyme which helps move the electrons by removing 2 H atoms (and their electrons) from glucose and giving them to NAD+ NAD+ + 2H NADH + H+ Dehydrogenase

Hydrogen Ions = H+ = Proton HYDROGEN ATOM

Substrate Level Phosphorylation Both glycolysis and the citric acid cycle Can generate ATP by substrate-level phosphorylation ENZYME ATP ADP PRODUCT SUBSTRATE P +

Citric Acid Cycle The citric acid cycle completes the energy-yielding oxidation of organic molecules The citric acid cycle Takes place in the matrix of the mitochondrion

Pre-citric Acid Cycle Before the citric acid cycle can begin Pyruvate must first be converted to acetyl CoA, which links the cycle to glycolysis CYTOSOL MITOCHONDRION NADH + H+ NAD+ 2 3 1 CO2 Coenzyme A Pyruvate Acetyl CoA S CoA C CH3 O Transport protein O–

Oxidative phosphorylation Citric Acid Cycle ATP 2 CO2 3 NAD+ 3 NADH + 3 H+ ADP + P i FAD+ FADH2 Citric acid cycle CoA Acetyl CoA NADH CO2 Pyruvate (from glycolysis, 2 molecules per glucose) Glycolysis Oxidative phosphorylation Oxaloacetate (4 C) Citrate (6 C)

Electron Transport Chain Mitochondrial inner membrane proteins (carrier molecules) pass electrons in a series of steps instead of in one explosive reaction Uses the energy from the electron transfer to form ATP

Electron Transport Chain H2O O2 NADH FADH2 FMN FE•S O FAD Cyt b Cyt c1 Cyt c Cyt a Cyt a3 2 H + + 1⁄2 I II III IV CARRIER MOLECULES 10 20 30 40 50 Free energy (G) relative to O2 (kcl/mol) At the end of the chain Electrons are passed to oxygen, forming water This makes oxygen the final electron acceptor oxygen

Electron transport chain Oxidative phosphorylation Chemiosmosis Chemiosmosis and the electron transport chain Oxidative phosphorylation. electron transport and chemiosmosis Glycolysis ATP Inner Mitochondrial membrane H+ P i Protein complex of electron carners Cyt c I II III IV (Carrying electrons from, food) NADH FADH2 NAD+ FAD+ 2 H+ + 1/2 O2 H2O ADP + Electron transport chain Electron transport and pumping of protons (H+), which create an H+ gradient across the membrane Chemiosmosis ATP synthesis powered by the flow Of H+ back across the membrane synthase Q Oxidative phosphorylation Intermembrane space mitochondrial matrix

Chemiosmosis At certain steps along the electron transport chain Electron transfer causes protein complexes to pump H+ from the mitochondrial matrix to the intermembrane space The resulting H+ gradient Stores energy Drives chemiosmosis in ATP synthase

But what’s Chemiosmosis really? Is an energy-coupling mechanism that uses energy in the form of a H+ gradient across a membrane to drive cellular work Is referred to as a proton-motive force But what’s Chemiosmosis really?

Chemiosmosis ATP synthase Is the enzyme that actually makes ATP INTERMEMBRANE SPACE H+ P i + ADP ATP MITOCHONDRIAL MATRIX

Following the Electrons (Energy) During respiration, most energy flows in this sequence Glucose to NADH to electron transport chain to chemiosmosis to ATP About 40% of the energy in a glucose molecule Is transferred to ATP during cellular respiration, making approximately 32 ATP

Oxidative phosphorylation: electron transport and chemiosmosis Review of Stages of Cellular Respiration Electron shuttles span membrane MITOCHONDRION 2 NADH or 2 FADH2 2 NADH 2 NADH 6 NADH 2 FADH2 Oxidative phosphorylation: electron transport and chemiosmosis Glycolysis Pyruvate oxidation Citric acid cycle Glucose 2 Pyruvate 2 Acetyl CoA Figure 9.16 ATP yield per molecule of glucose at each stage of cellular respiration.  2 ATP  2 ATP  about 26 or 28 ATP About 30 or 32 ATP Maximum per glucose: CYTOSOL

So what happens when there is little or no O2? Without O2, the ETC will cease to operate BUT glycolysis couples with fermentation to still produce 2 ATP’s Without the ETC however, fermentation needs an alternate way to generate NAD+

Fermentation Two common types of fermentation are: Alcohol fermentation Lactic acid fermentation

Alcohol Fermentation Pyruvate is converted to ethanol in two steps Releases CO2 from pyruvate making acetaldehyde Acetaldehyde reduced by NADH to ethanol which regenerates NAD+ Many bacteria undergo alcohol fermentation in anaerobic conditions Alcohol fermentation by yeast is used in brewing, winemaking, and baking

Lactic Acid Fermentation Pyruvate directly reduced by NADH to form lactate (ionized form of lactic acid) No CO2 released Fungi and bacteria undergo lactic acid fermentation Used to make cheese and yogurt Human muscle cells will undergo lactic acid fermentation when O2 scarce during heavy exercise

Summary of Types of Fermentation 2 ADP  2 P i 2 ATP 2 ADP  2 P i 2 ATP Glucose Glycolysis Glucose Glycolysis 2 Pyruvate 2 NAD  2 NADH 2 CO2 2 NAD  2 NADH  2 H  2 H 2 Pyruvate Figure 9.17 Fermentation. 2 Ethanol 2 Acetaldehyde 2 Lactate (a) Alcohol fermentation (b) Lactic acid fermentation Fermentation Overview

Pyruvate as a key juncture in catabolism Glucose Glycolysis CYTOSOL Pyruvate No O2 present: Fermentation O2 present: Aerobic cellular respiration MITOCHONDRION Ethanol, lactate, or other products Acetyl CoA Figure 9.18 Pyruvate as a key juncture in catabolism. Citric acid cycle Pyruvate as a key juncture in catabolism