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Chapter 19 Bioenergetics. How the Body Converts Food to Energy. Metabolism. Metabolism: the sum of all chemical reactions involved in maintaining the dynamic state of a cell or organism pathway: a series of consecutive biochemical reactions

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Chapter 19 Bioenergetics

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Chapter 19 bioenergetics

Chapter 19 Bioenergetics

How the Body Converts Food to Energy



  • Metabolism: the sum of all chemical reactions involved in maintaining the dynamic state of a cell or organism

    • pathway: a series of consecutive biochemical reactions

    • catabolism: the biochemical pathways that are involved in generating energy by breaking down large nutrient molecules into smaller molecules with the concurrent production of energy

    • anabolism: the pathways by which biomolecules are synthesized (use ATP energy to build larger molecules from smaller building blocks)



  • metabolism is the sum of catabolism and anabolism

Stages of catabolism

Stages of Catabolism

Catabolic reactions are organized into three stages:

  • In Stage 1, digestion breaks down large molecules into smaller ones that enter the bloodstream

  • In Stage 2, molecules enter the cells and are broken down into two- and three-carbon compounds

  • In Stage 3, compounds are oxidized in the citric acid cycle to provide energy (ATP) for anabolic processes

Stages of catabolism1

Stages of Catabolism

Cells and mitochondria

Cells and Mitochondria

  • Animal cells have many components, each with specific functions; some components along with one or more of their functions are:

    • nucleus: where replication of DNA takes place

    • lysosomes: remove damaged cellular components and some unwanted foreign materials

    • Golgi bodies: package and process proteins for secretion and delivery to other cellular components

    • mitochondria: responsible for generation of most of the energy for cells

Components of eukaryotic cells

Components of Eukaryotic Cells

A rat liver cell

A Rat Liver Cell

A mitochondrion

A Mitochondrion

  • Schematic of a mitochondrion cut to reveal its inner organization

Common catabolic pthwy

Common Catabolic Pthwy

  • The two parts to the common catabolic pathway

    • citric acid cycle, also called the tricarboxylic acid or Krebs cycle

    • oxidative phosphorylation, also called the electron transport chain, or the respiratory chain

  • The four principal compounds participating in the common catabolic pathway are:

    • AMP, ADP, and ATP

    • NAD+/NADH

    • FAD/FADH2

    • coenzyme A; abbreviated CoA or CoA-SH

Adenosine triphosphate

Adenosine Triphosphate

  • ATP is the most important compound involved in the transfer of phosphate groups

    • ATP contains two phosphoric anhydride bonds and one phosphoric ester bond

Adenosine triphosphate1

Adenosine Triphosphate

  • hydrolysis of the terminal phosphate of ATP gives ADP, phosphate ion, and energy

  • hydrolysis of a phosphoric anhydride liberates more energy than hydrolysis of a phosphoric ester

  • we say that ATP and ADP contain high-energy phosphoric anhydride bonds

  • ATP is a universal carrier of phosphate groups

  • it is also a common currency for the storage and transfer of energy

Hydrolysis of atp

Hydrolysis of ATP

  • The hydrolysis of ATP to ADP releases 7.3 kcal (31 kJ/mole)

    ATP  ADP + Pi + 7.3 kcal (31 kJ/mole)

  • The hydrolysis of ADP to AMP releases 7.3 kcal (31 kJ/mole)

    ADP  AMP + Pi + 7.3 kcal (31 kJ/mole)

Coenzymes nad nadh 2

Coenzymes NAD+/NADH2

  • Nicotinamide adenine dinucleotide (NAD+) is a biological oxidizing agent

Coenzymes nad nadh

Coenzymes NAD+/NADH

  • NAD+ is a two-electron oxidizing agent, and is reduced to NADH

  • NADH is a two-electron reducing agent, and is oxidized to NAD+

  • NAD+ and NADH are also hydrogen ion transporting molecules

Coenzymes nad nadh1

Coenzymes NAD+/NADH

  • When a compound is oxidized by an enzyme, 2H+ and 2e- are removed by a coenzyme, which is reduced

  • NAD+ (nicotinamide adenine dinucleotide) participates in reactions that produce a carbon-oxygen double bond (C=O)

  • For example, NAD+ participates in the oxidation of ethanol:

Coenzymes fad fadh 2

Coenzymes FAD/FADH2

  • Flavin adenine dinucleotide (FAD) is also a biological oxidizing agent

Coenzymes fad fadh 21

Coenzymes FAD/FADH2

  • FAD is a two-electron oxidizing agent, and is reduced to FADH2

  • FADH2 is a two-electron reducing agent, and is oxidized to FAD

Coenzymes fad fadh 22

Coenzymes FAD/FADH2

  • FAD participates in reactions that produce a carbon-carbon double bond (C=C)


    —CH2—CH2—  —CH=CH— + 2H+ + 2e-


    FAD + 2H+ + 2e- FADH2

Coenzyme a

Coenzyme A

  • Coenzyme A (CoA) is an acetyl-carrying group

    • like NAD+ and FAD, coenzyme A contains a unit of ADP

    • CoA is often written CoA-SH to emphasize the fact that it contains a sulfhydryl group

    • the vitamin part of coenzyme A is pantothenic acid

    • the acetyl group of acetyl CoA is bound as a high-energy thioester

Coenzyme a1

Coenzyme A

Citric acid cycle

Citric Acid Cycle

  • overview: the two carbon acetyl group of acetyl CoA is fed into the cycle and oxidized to 2 CO2

  • there are four oxidation steps in the cycle

Citric acid cycle1

Citric Acid Cycle

  • Step 1: condensation of acetyl CoA with oxaloacetate

    • the high-energy thioester of acetyl CoA is hydrolyzed

    • this hydrolysis provides the energy to drive Step 1

    • citrate synthase is an allosteric enzyme; it is inhibited by NADH, ATP, and succinyl-CoA

Citric acid cycle2

Citric Acid Cycle

  • Step 2: dehydration and rehydration, catalyzed by aconitase, gives isocitrate

    • citrate is achiral; it has no stereocenter

    • aconitate is also achiral

    • isocitrate is chiral; it has 2 stereocenters and 4 stereoisomers are possible

    • only one of the 4 possible stereoisomers is formed in the cycle

Citric acid cycle3

Citric Acid Cycle

  • Step 2 (cont’d): Citrate isomerizes to isocitrate

  • The tertiary –OH group in citrate is converted to a secondary –OH that can be oxidized

Citric acid cycle4

Citric Acid Cycle

  • Step 3: oxidation of isocitrate followed by decarboxylation gives a-ketoglutarate

    • isocitrate dehydrogenase is an allosteric enzyme; it is inhibited by ATP and NADH, and activated by ADP and NAD+

Citric acid cycle5

Citric Acid Cycle

  • Step 4: oxidative decarboxylation of -ketoglutarate to succinyl-CoA

    • the two carbons of the acetyl group of acetyl CoA are still present in succinyl CoA and in succinate

    • this multienzyme complex is inhibited by ATP, NADH, and succinyl CoA; it is activated by ADP and NAD+

Citric acid cycle6

Citric Acid Cycle

  • Step 5: formation of succinate

    • the two CH2-COO- groups of succinate are now equivalent

    • this is the first energy-yielding step of the cycle; a molecule of GTP is produced

Citric acid cycle7

Citric Acid Cycle

  • Step 6: oxidation of succinate to fumarate

  • Step 7: hydration of fumarate to L-malate

    • L-malate is chiral and can exist as a pair of enantiomers; it is produced in the citric acid cycle as a single stereoisomer

Citric acid cycle8

Citric Acid Cycle

  • Step 8: oxidation of malate

    • oxaloacetate now can react with acetyl CoA to start another round of the cycle by repeating Step 1

Citric acid cycle9

Citric Acid Cycle

In one turn of the citric acid cycle:

  • Two decarboxylations remove two carbons as 2CO2

  • Four oxidations provide hydrogen for 3NADH and one FADH2

  • A direct phosphorylation forms GTP which is used to form ATP

    Overall reaction of citric acid cycle:

Citric acid cycle10

Citric Acid Cycle

  • Control of the cycle

    • controlled by three feedback mechanisms

    • citrate synthase: inhibited by ATP, NADH, and succinyl CoA; also product inhibition by citrate

    • isocitrate dehydrogenase: activated by ADP and NAD+, inhibited by ATP and NADH

    • -ketoglutarate dehydrogenase complex: inhibited by ATP, NADH, and succinyl CoA; activated by ADP and NAD+

Ca cycle in catabolism

CA Cycle in Catabolism

  • The catabolism of proteins, carbohydrates, and fatty acids all feed into the citric acid cycle at one or more points

Electron carriers

Electron Carriers

  • The electron transport chain consists of electron carriers that accept H+ ions and electrons from the reduced coenzymes NADH and FADH2

  • The H+ ions and electrons are passed down a chain of carriers until in the last step they combine with oxygen to form H2O

  • Oxidative phosphorylation is the process by which the energy from transport is used to synthesize ATP

Oxidative phosphorylation

Oxidative Phosphorylation

  • Carried out by four closely related multisubunit membrane-bound complexes and two electron carriers, coenzyme Q and cytochrome c

    • in a series of oxidation-reduction reactions, electrons from FADH2 and NADH are transferred from one complex to the next until they reach O2

    • O2 is reduced to H2O

    • as a result of electron transport, protons are pumped across the inner membrane to the intermembrane space

Electron transport system

Electron Transport System

  • The electron carriers in the electron transport system are attached to the inner membrane of the mitochondrion

  • They are organized into four protein complexes:

    Complex I NADH dehydrogenase

    Complex IISuccinate dehydrogenase

    Complex IIICoQ-Cytochrome c reductase

    Complex IVCytochrome c Oxidase

Electron transport chain

Electron Transport Chain

Complex i

Complex I

  • The sequence starts with complex I

    • this large complex contains some 40 subunits, among them are a flavoprotein, several iron-sulfur (FeS) clusters, and coenzyme Q (CoQ, ubiquinone)

    • complex I oxidizes NADH to NAD+

    • the oxidizing agent is CoQ, which is reduced to CoQH2

    • some of the energy released in this reaction is used to move 2H+ from the matrix into the intermembrane space

Complex ii

Complex II

  • complex II oxidizes FADH2 to FAD

  • the oxidizing agent is CoQ, which is reduced to CoQH2

  • the energy released in this reaction is not sufficient to pump protons across the membrane

Complex iii

Complex III

  • complex III delivers electrons from CoQH2 to cytochrome c (Cyt c)

  • this integral membrane complex contains 11 subunits, including cytochrome b, cytochrome c1, and FeS clusters

  • complex III has two channels through which the two H+ from CoQH2 are pumped from the matrix into the intermembrane space

Complex iv

Complex IV

  • complex IV is also known as cytochrome oxidase

  • it contains 13 subunits, one of which is cytochrome a3

  • electrons flow from Cyt c (oxidized) in complex III to Cyt a3 in complex IV

  • from Cyt a3 electrons are transferred to O2

  • during this redox reaction, H+ are pumped from the matrix into the intermembrane space

  • Summing the reactions of complexes I - IV, six H+ are pumped out per NADH and four H+ per FADH2

  • Coupling of ox and phos

    Coupling of Ox and Phos

    • To explain how electron and H+ transport produce the chemical energy of ATP, Peter Mitchell proposed the chemiosmotic theory

      • the energy-releasing oxidations give rise to proton pumping and a pH gradient across the inner mitochondrial membrane

      • there is a higher concentration of H+ in the intermembrane space than inside the mitochondrion

      • this proton gradient provides the driving force to propel protons back into the mitochondrion through the enzyme complex called proton translocating ATPase

    Coupling of ox and phos1

    Coupling of Ox and Phos

    • protons flow back into the matrix through channels in the F0 unit of ATP synthase

    • the flow of protons is accompanied by formation of ATP in the F1 unit of ATP synthase

  • The functions of oxygen are:

    • to oxidize NADH to NAD+ and FADH2 to FAD so that these molecules can return to participate in the citric acid cycle

    • provide energy for the conversion of ADP to ATP

  • Coupling of ox and phos2

    Coupling of Ox and Phos

    • The overall reactions of oxidative phosphorylation are:

    The energy yield

    The Energy Yield

    • A portion of the energy released during electron transport is now built into ATP

      • for each two-carbon acetyl unit entering the citric acid cycle, we get three NADH and one FADH2

      • for each NADH oxidized to NAD+, we get three ATP

      • for each FADH2 oxidized to FAD, we get two ATP

      • thus, the yield of ATP per two-carbon acetyl group oxidized to CO2 is

    Other energy forms

    Other Energy Forms

    • The chemical energy of ATP is converted by the body to several other forms of energy

    • Electrical energy

      • the body maintains a K+ concentration gradient across cell membranes; higher inside and lower outside

      • it also maintains a Na+ concentration gradient across cell membranes; lower inside, higher outside

      • Special transport proteins in cell membranes constantly pump K+ into and Na+ out of the cells

      • this pumping requires energy, which is supplied by the hydrolysis of ATP to ADP

      • thus, the chemical energy of ATP is transformed into electrical energy, which operates in neurotransmission

    Other forms of energy

    Other Forms of Energy

    • Mechanical energy

      • ATP drives the alternating association and dissociation of actin and myosin and, consequently, the contraction and relaxation of muscle tissue

    • Heat energy

      • hydrolysis of ATP to ADP yields 7.3 kcal/mol

      • some of this energy is released as heat to maintain body temperature




    Chapter 19

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