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

Life Is Work Living cells The giant panda Require transfusions of energy from outside sources to perform many tasks The giant panda Obtains energy for its cells by eating plants

powers most cellular work The energy stored in the organic molecules of food comes from the sun 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

Photosynthesis generates oxygen and organic molecules Used by the mitochondria of eukaryotes as fuel for cellular respiration Respiration breaks this fuel down generating ATP The waste products (CO2 and water) are the raw material for photosynthesis

Catabolic pathways yield energy by oxidizing organic fuels Organic componds store energy in their arrangment of atoms With the help of enzymes a cell degrades complex organic molecules to more simple ones metabolic pathway that release stored energy by breaking down complex molecules are called catabolic pathways Example: fermentation (partial degradation of sugars that occurs without the use of oxygen)

Catabolic Pathways and Production of ATP The breakdown of organic molecules is exergonic Releases energy to the environment

Cellular respiration Is the most prevalent and efficient catabolic pathway Consumes oxygen and organic molecules such as glucose Yields ATP Mitochondria Organic compounds + Oxygen → CO2 + Water + Energy

To keep working Cells must regenerate ATP From ADP and phosphate To understand this we need to examine the fundamental chemical processes: oxidation and reduction

Redox Reactions: Oxidation and Reduction Catabolic pathways yield energy Due to the transfer of electrons The relocation of electrons releases energy stored in organic molecules This energy is used to synthesize ATP

The Principle of Redox Redox reactions Transfer electrons from one reactant to another by oxidation and reduction This electron transfers are oxidation-reduction reactions or redox reactions the loss of electrons from one substance is called oxidation Addition of electrons to enother substance is called reduction Notice: adding is called reduction. Negatively charged electrons added to an atom reduce the amount of positive charge of that atom.

Example of redox reaction: Na is electron donor or reducing agent Cl is electron acceptor or oxidizing agent Na + Cl Na+ + Cl– becomes oxidized (gives electron) becomes reduced (accepts electron)

Do not completely exchange electrons Some redox reactions Do not completely exchange electrons Some of them change the degree of electron sharing in covalent bonds Oxygen is one of the most potent of all oxidizing agents 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

Oxidation of Organic Fuel Molecules During Cellular Respiration Glucose is oxidized and oxygen is reduced The electrons lose potential energy along the way and energy is released Hydrogen is transferred from glucose to oxygen By oxidizing glucose, respiration liberates stored energy from glucose and makes it available for ATP synthesis C6H12O6 + 6O2 6CO2 + 6H2O + Energy becomes oxidized becomes reduced

Stepwise Energy Harvest via NAD+ and the Electron Transport Chain Cellular respiration Oxidizes glucose in a series of steps Each step catalyzed by an enzyme At key steps electrons are stripped from the glucose The hydrogen atoms are not directly trasferred to oxygen but are usually transferred to a coenzyme NAD+ (nicotinamide adenine dinucleotide) Derivative of the vitamin niacin Functions as oxidizing agent during respiration (electron acceptor)

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

NADH, the reduced form of NAD+ Passes the electrons to the electron transport chain NADH recieved hydrogen in the reaction Each NADH molecule formed during respiration represents stored energy that can be tapped to make ATP

(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

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 number of molecules built into the inner membrane of a mitochondrion

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

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

Glycolysis The citric acid cycle Breaks down glucose into two molecules of pyruvate Occurs in cytosol The citric acid cycle Completes the breakdown of glucose by oxidizing a derivative of pyruvate to CO2 Takes place in mitochondrial matrix

Some steps in glycolysis and citric acid cycle are redox reactions Dehydrogenase enzymes trasfer electrons from substrates to NAD+ forming NADH

Oxidative phosphorylation as third step in respiration Is driven by the electron transport chain Generates ATP Powered by the redox reactions of the electron transport chain

Both glycolysis and the citric acid cycle Can generate ATP by substrate-level phosphorylation This mode of ATP synthesis occurs when an enzyme transfers a phosphate group from a substrate to ADP as in oxidative phosphorylation Substrate molecule refers to an organic molecule generated during the catabolism of glucose Figure 9.7 Enzyme ATP ADP Product Substrate P +

Glycolysis harvests energy by oxidizing glucose to pyruvate Means “splitting of sugar” Breaks down glucose into pyruvate Occurs in the cytoplasm of the cell

Energy investment phase Glycolysis consists of two major phases Energy investment phase Energy payoff phase 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

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

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 Final product of glycolysis

completes the energy-yielding oxidation of organic molecules The citric acid cycle completes the energy-yielding oxidation of organic molecules The citric acid cycle 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 Figure 9.12

Summary! Cellular respiration Redox reactions Glycolysis Citric acid cycle

Oxidative phosphorylation During it, 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 Most components of the chain are proteins which exist in multiprotein complexes I to IV Includes also prosthetic groups Non-protein components essential for the catalytic functions of certain enzymes

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

Generally, the electron transport chain makes no ATP directly Makes fall of electrons from food to oxygen easier, breaking a large free-energy drop into a series of smaller steps that release energy in manageable amounts How? → Chemiosmosis

Chemiosmosis: The Energy-Coupling Mechanism ATP synthase Is the enzyme that actually makes ATP from ADP and inorganic phosphate Located at the inner membrane of the mitochondrion Works like an ion pump running in reverse Uses the energy of an existing ion gradient to power ATP synthesis It is proton gradient (hydrogen) The process in which the energy is stored and used to drive cellular work is called chemiosmosis

In general terms: Chemiosmosis is an energy-coupling mechanism that uses energy stored in the form of an H+ gradient across a membrane to drive cellular work

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 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 → NADH → electron transport chain → proton-motive force → ATP But how many ATPs can be generated?

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 molecule Is transferred to ATP during cellular respiration, making approximately 38 ATP This number however cannot be fixed Mostly because some of the reactions are not directly coupled

enables some cells to produce ATP without the use of oxygen Fermentation enables some cells to produce ATP without the use of oxygen Why? Oxidation is the loss of electrons to any electron acceptor not only 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+ can be reused to oxidize sugar by glyocolysis Two types of fermentation: 1. alcohol fermentation 2. lactic acid fermentation

(a) Alcohol fermentation In alcohol fermentation Pyruvate is converted to ethanol (ethyl alcohol) in two steps The first step releases CO2 from the pyruvate which is converted to the two-carbon compound acetaldehyde In the second step acetaldehyde is reduced by NADH to ethanol This regenerates the supply of NAD+ needed for the continuation of glycolysis 2 ADP + 2 P1 2 ATP Glycolysis Glucose 2 NAD+ 2 NADH 2 Pyruvate 2 Acetaldehyde 2 Ethanol (a) Alcohol fermentation H OH CH3 C O – O CO2 2

(b) Lactic acid fermentation During lactic acid fermentation Pyruvate is reduced directly to NADH to form lactate as a waste product No release of CO2 This fermentation by certain fungi and bacteria is used in the dairy industry to make cheese and other milk products 2 ADP + 2 P1 2 ATP Glycolysis Glucose 2 NAD+ 2 NADH 2 Lactate (b) Lactic acid fermentation O– C O CH3 OH H

Fermentation and Cellular Respiration Compared Both fermentation and cellular respiration Use glycolysis to oxidize glucose and other organic fuels to pyruvate Net production of 2 molecules of ATP by substrate-level phosphorylation NAD+ is the oxidizing agent that accepts electrons from food during glycolysis

The key difference is the contrasting mechanism for oxidizing NADH back to NAD+ In fermentation the final electron acceptor is an organic molecule such as pyruvate (lactic acid fermentation) or acetaldehyde (alcohol fermentation) In respiration the final electron acceptor is oxygen which also 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 Early organisms produced ATP exclusively from glycolysis which does not require oxygen

Glycolysis and the citric acid cycle connect to many other metabolic pathways Can be catabolic or anabolic pathways

The Versatility of Catabolism Catabolic pathways Funnel electrons from many kinds of organic molecules into cellular respiration To make ATP Glycolysis can accept a wide range of carbohydrates for catabolism Starch, glycogen (polysaccharides), proteins and fats

Most of the energy of a fat is stored in the fatty acids Beta oxidation breaks fatty acids down to two-carbon fragments Those enter the citric acid cycle as acetyl CoA Fats are great fuels 1 gram of fat oxidized by respiration produces more than twice as much ATP as a gram of carbohydrates

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 Can be diverted into anabolic pathways as precursors for other molecules

Regulation of Cellular Respiration via Feedback Mechanisms Is controlled by allosteric enzymes at key points in glycolysis and the citric acid cycle Feedback inhibition: the end product of the anabolic pathway inhibits the enzyme that catalyzes an early step of the pathway Example: phosphofructokinase which catalyzes 3rd step in glycolysis

Allosteric enzyme Inhibited by ATP and stimulated by AMP As ATP accumulates, inhibition of the enzyme slows down glycolysis Active again upon conversion of ATP to ADP (and AMP)

Summary! Oxidative phosphorylation Electron transport chain Chemiosmosis Fermentations Allosteric enzymes