Overview: Life Is Work Living cells Require energy from outside sources to perform their many tasks Photosynthesis takes light energy and converts it into chemical e Cell Resp. Takes chemical e and makes ATP

The giant panda Obtains energy for its cells by eating plants Figure 9.1 Mmmmm makes ATP

powers most cellular work Energy Flows into an ecosystem as sunlight and leaves as heat Light energy ECOSYSTEM CO2 + H2O Photosynthesis in chloroplasts Cellular respiration in mitochondria Organic molecules + O2 ATP powers most cellular work Heat energy Figure 9.2 Go through cycle

Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels

Catabolic Pathways and Production of ATP The breakdown of organic molecules is exergonic -G

One catabolic process, fermentation Is a partial degradation of sugars that occurs without oxygen Bacteria, yeast, and animals can do this process.

Cellular respiration Is the most prevalent and efficient catabolic pathway Consumes oxygen and organic molecules such as glucose Yields ATP This process is similar to the combustion of Gas in a car

To keep working Cells must regenerate ATP

Redox Reactions: Oxidation and Reduction Catabolic pathways yield energy Due to the transfer of electrons

The Principle of Redox Redox reactions Transfer electrons from one reactant to another by oxidation and reduction

Examples of redox reactions Na + Cl Na+ + Cl– becomes oxidized (loses electron) becomes reduced (gains electron) Here Cl takes the electron from Na Therefore Na is oxidized and Cl- is reduced Na is the reducing agent. It will become oxidized and it give away electrons Cl is the oxidizing agent. It will be reduced bc is will gain electrons

Oxidation Reaction Or the gain of oxygen. The loss of electrons from a substance. Or the gain of oxygen. C6H12O6 + 6O2 6CO2 + 6H2O + energy Oxidation glucose ATP Oxygen is very electronegative, and is on of the most potent oxidizing agents because it will accept electrons Glucose is the reducing agent b/c it will give away electrons by the way of H.

C6H12O6 + 6O2  6CO2 + 6H2O + energy Reduction Reaction The gain of electrons to a substance. Or the loss of oxygen. Reduction glucose ATP C6H12O6 + 6O2  6CO2 + 6H2O + energy

Oxidation of Organic Fuel Molecules During Cellular Respiration Glucose is oxidized and oxygen is reduced C6H12O6 + 6O2 6CO2 + 6H2O + Energy becomes oxidized becomes reduced Glucose Gained O and lost Electrons Oxygen gained hydrogens and gained e

Some redox reactions Do not completely exchange electrons Change the degree of electron sharing in covalent bonds CH4 H C O Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water + 2O2 CO2 Energy 2 H2O becomes oxidized becomes reduced Reactants Products Figure 9.3

Stepwise Energy Harvest via NAD+ and the Electron Transport Chain Cellular respiration Oxidizes glucose in a series of steps Each step catalyzed by a specific enzyme At key steps electrons are stripped from glucose

Electrons from organic compounds Are usually first transferred to NAD+, a coenzyme NAD+ H O O– P CH2 HO OH N+ C NH2 N Nicotinamide (oxidized form) + 2[H] (from food) Dehydrogenase Reduction of NAD+ Oxidation of NADH 2 e– + 2 H+ 2 e– + H+ NADH Nicotinamide (reduced form) Figure 9.4 The electrons usually will go to NAD to make NADH Where 2 electrons and 1 proton is donated to NAD+ The other H+ is released into the surrounding solution

NADH, the reduced form of NAD+ Passes the electrons to the electron transport chain In this pass very little energy is lost Each NADH formed during CR represents stored e. This stored E is tapped to synthesize ATP as electrons are passed from NADH to oxygen

(a) Uncontrolled reaction If electron transfer is not stepwise A large release of energy occurs As in the reaction of hydrogen and oxygen to form water (a) Uncontrolled reaction Free energy, G H2O Explosive release of heat and light energy Figure 9.5 A H2 + 1/2 O2 Burning like a hot fire. We could not eat enough to keep up with our needs.

The electron transport chain Passes electrons in a series of steps instead of in one explosive reaction Uses the energy from the electron transfer to form ATP Consists of a chain of molecules built into the inner membrane of the mitochondria.

Electron transport chain (b) Cellular respiration (from food via NADH) 2 H+ + 2 e– 2 H+ 2 e– H2O Controlled release of energy for synthesis of ATP ATP Electron transport chain Free energy, G (b) Cellular respiration + Figure 9.5 B The electron transfer from NADH to oxygen in an exergonic rxn –53kcal/mol

The Stages of Cellular Respiration: A Preview Respiration is a cumulative function of three metabolic stages Glycolysis The citric acid cycle Oxidative phosphorylation

Glycolysis The citric acid cycle Breaks down glucose into two molecules of pyruvate The citric acid cycle Completes the breakdown of glucose

Oxidative phosphorylation Is driven by the electron transport chain Generates ATP

Oxidative phosphorylation: electron transport and chemiosmosis An overview of cellular respiration Figure 9.6 Electrons carried via NADH Glycolsis Glucose Pyruvate ATP Substrate-level phosphorylation Electrons carried via NADH and FADH2 Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis Oxidative Mitochondrion Cytosol

Both glycolysis and the citric acid cycle Can generate ATP by substrate-level phosphorylation Figure 9.7 Enzyme ATP ADP Product Substrate P +

Concept 9.2: Glycolysis harvests energy by oxidizing glucose to pyruvate Means “splitting of sugar” Breaks down glucose into pyruvate Occurs in the cytoplasm of the cell Key in on the starting and end products of the reactions rather than the steps of the reaction

Energy investment phase Glycolysis consists of two major phases Energy investment phase Energy payoff phase Takes place in the cytoplasm No oxygen needed Glycolysis Citric acid cycle Oxidative phosphorylation ATP 2 ATP 4 ATP used formed Glucose 2 ATP + 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 + Figure 9.8

Phosphoglucoisomerase A closer look at the energy investment phase Dihydroxyacetone phosphate Glyceraldehyde- 3-phosphate H OH HO CH2OH O P CH2O CH2 C CHOH ATP ADP Hexokinase Glucose Glucose-6-phosphate Fructose-6-phosphate Phosphoglucoisomerase Phosphofructokinase Fructose- 1, 6-bisphosphate Aldolase Isomerase Glycolysis 1 2 3 4 5 Oxidative phosphorylation Citric acid cycle Figure 9.9 A Invests 2 ATP and glucose

1, 3-Bisphosphoglycerate A closer look at the energy payoff phase 2 NAD+ NADH 2 + 2 H+ Triose phosphate dehydrogenase P i P C CHOH O CH2 O– 1, 3-Bisphosphoglycerate 2 ADP 2 ATP Phosphoglycerokinase 3-Phosphoglycerate Phosphoglyceromutase CH2OH H 2-Phosphoglycerate 2 H2O Enolase Phosphoenolpyruvate Pyruvate kinase CH3 6 8 7 9 10 Pyruvate Figure 9.8 B 4 ATP 2 NADH 2 pyruvate

Concept 9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules Takes place in the matrix of the mitochondrion

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 Acetyle CoA S CoA C CH3 O Transport protein O– Figure 9.10

Oxidative phosphorylation An overview of the citric acid cycle ATP 2 CO2 3 NAD+ 3 NADH + 3 H+ ADP + P i FAD FADH2 Citric acid cycle CoA Acetyle CoA NADH CO2 Pyruvate (from glycolysis, 2 molecules per glucose) Glycolysis Oxidative phosphorylation Figure 9.11

A closer look at the citric acid cycle Acetyl CoA NADH Oxaloacetate Citrate Malate Fumarate Succinate Succinyl CoA a-Ketoglutarate Isocitrate Citric acid cycle S SH FADH2 FAD GTP GDP NAD+ ADP P i CO2 H2O + H+ C CH3 O COO– CH2 HO HC CH 1 2 3 4 5 6 7 8 Glycolysis Oxidative phosphorylation ATP Figure 9.12 A closer look at the citric acid cycle Figure 9.12 1 glucose makes Makes 2 ATP 6 NADH 2 FADH2

Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis NADH and FADH2 Donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation

The Pathway of Electron Transport In the electron transport chain Electrons from NADH and FADH2 lose energy in several steps

At the end of the chain Electrons are passed to oxygen, forming water 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 Multiprotein complexes 10 20 30 40 50 Free energy (G) relative to O2 (kcl/mol) Figure 9.13 Chemiosmosis: Proteins 1-4 pump the protons across the membrane. The Conc. Gradient of H+ is made from the chemiosmosis process is used to make ATP

Chemiosmosis: The Energy-Coupling Mechanism ATP synthase Is the enzyme that actually makes ATP INTERMEMBRANE SPACE H+ P i + ADP ATP A rotor within the membrane spins clockwise when H+ flows past it down the H+ gradient. A stator anchored in the membrane holds the knob stationary. A rod (for “stalk”) extending into the knob also spins, activating catalytic sites in the knob. Three catalytic sites in the stationary knob join inorganic Phosphate to ADP to make ATP. MITOCHONDRIAL MATRIX Figure 9.14

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 Is referred to as a proton-motive force

Chemiosmosis Is an energy-coupling mechanism that uses energy in the form of a H+ gradient across a membrane to drive cellular work

Electron transport chain Oxidative phosphorylation 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 Figure 9.15

An Accounting of ATP Production by Cellular Respiration During respiration, most energy flows in this sequence Glucose to NADH to electron transport chain to proton-motive force to ATP

There are three main processes in this metabolic enterprise Electron shuttles span membrane CYTOSOL 2 NADH 2 FADH2 6 NADH Glycolysis Glucose 2 Pyruvate Acetyl CoA Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis MITOCHONDRION by substrate-level phosphorylation by oxidative phosphorylation, depending on which shuttle transports electrons from NADH in cytosol Maximum per glucose: About 36 or 38 ATP + 2 ATP + about 32 or 34 ATP or Figure 9.16

About 40% of the energy in a glucose molecules transferred to ATP during cellular respiration, making approximately 38 ATP

In the absence of oxygen Concept 9.5: Fermentation enables some cells to produce ATP without the use of oxygen Cellular respiration Relies on oxygen to produce ATP In the absence of oxygen Cells can still produce ATP through fermentation

Glycolysis Can produce ATP with or without oxygen, in aerobic or anaerobic conditions Couples with fermentation to produce ATP

Fermentation consists of Types of Fermentation Fermentation consists of Glycolysis plus reactions that regenerate NAD+, which can be reused by glyocolysis

In alcohol fermentation Pyruvate is converted to ethanol in two steps, one of which releases CO2

During lactic acid fermentation Pyruvate is reduced directly to NADH to form lactate as a waste product

(a) Alcohol fermentation (b) Lactic acid fermentation 2 ADP + 2 P1 2 ATP Glycolysis Glucose 2 NAD+ 2 NADH 2 Pyruvate 2 Acetaldehyde 2 Ethanol (a) Alcohol fermentation 2 Lactate (b) Lactic acid fermentation H OH CH3 C O – O O– CO2 2 Figure 9.17

Fermentation and Cellular Respiration Compared Both fermentation and cellular respiration Use glycolysis to oxidize glucose and other organic fuels to pyruvate

Fermentation and cellular respiration Differ in their final electron acceptor Cellular respiration Produces more ATP

Pyruvate is a key juncture in catabolism Glucose CYTOSOL Pyruvate No O2 present Fermentation O2 present Cellular respiration Ethanol or lactate Acetyl CoA MITOCHONDRION Citric acid cycle Figure 9.18

The Evolutionary Significance of Glycolysis Occurs in nearly all organisms Probably evolved in ancient prokaryotes before there was oxygen in the atmosphere

Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways

The Versatility of Catabolism Catabolic pathways Funnel electrons from many kinds of organic molecules into cellular respiration

The catabolism of various molecules from food Amino acids Sugars Glycerol Fatty Glycolysis Glucose Glyceraldehyde-3- P Pyruvate Acetyl CoA NH3 Citric acid cycle Oxidative phosphorylation Fats Proteins Carbohydrates Figure 9.19

Biosynthesis (Anabolic Pathways) The body Uses small molecules to build other substances These small molecules May come directly from food or through glycolysis or the citric acid cycle

Regulation of Cellular Respiration via Feedback Mechanisms Is controlled by allosteric enzymes at key points in glycolysis and the citric acid cycle

Fructose-1,6-bisphosphate The control of cellular respiration Glucose Glycolysis Fructose-6-phosphate Phosphofructokinase Fructose-1,6-bisphosphate Inhibits Pyruvate ATP Acetyl CoA Citric acid cycle Citrate Oxidative phosphorylation Stimulates AMP + – Figure 9.20