LE 8-8

Phosphate groups

Ribose

Adenine

Using Hydrolysis to

break the phosphate bond

LE 8-9

Adenosine triphosphate (ATP)

Energy

P P P

PPP i

Adenosine diphosphate (ADP)Inorganic phosphate

H2O

+ +

How ATP Performs Work• ATP drives endergonic reactions by

phosphorylation, transferring a phosphate group to some other molecule, such as a reactant

• The recipient molecule is now phosphorylated• The three types of cellular work (mechanical,

transport, and chemical) are powered by the hydrolysis of ATP

LE 8-11

NH2

Glu

P i

P i

P i

P i

Glu NH3

P

P

P

ATPADP

Motor protein

Mechanical work: ATP phosphorylates motor proteins

Protein moved

Membraneprotein

Solute

Transport work: ATP phosphorylates transport proteins

Solute transported

Chemical work: ATP phosphorylates key reactants

Reactants: Glutamic acidand ammonia

Product (glutamine)made

+ +

+

LE 9-2

ECOSYSTEM

Lightenergy

Photosynthesisin chloroplasts

Cellular respirationin mitochondria

Organicmolecules+ O2

CO2 + H2O

ATP

powers most cellular work

Heatenergy

Several processes are central to cellular respiration and related pathways

Production of ATP• The breakdown of organic molecules is

exergonic• Fermentation is a partial degradation of sugars

that occurs without oxygen• Cellular respiration consumes oxygen and

organic molecules and yields ATP• Although carbohydrates, fats, and proteins are all

consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose:

C6H12O6 + 6O2 6CO2 + 6H2O + Energy (ATP + heat)

• The transfer of electrons during chemical reactions releases energy stored in organic molecules

• This released energy is ultimately used to synthesize ATP

• During cellular respiration, the fuel (such as glucose) is oxidized and oxygen is reduced:

C6H12O6 + 6O2 6CO2 + 6H2O + Energy

becomes oxidized

becomes reduced

the Electron Transport Chain• In cellular respiration, glucose and other

organic molecules are broken down in a series of steps

• Electrons from organic compounds are usually first transferred to NAD+, a coenzyme

• As an electron acceptor, NAD+ functions as an oxidizing agent during cellular respiration

• Each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP

• NADH passes the electrons to the electron transport chain

• Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction

• Oxygen pulls electrons down the chain in an energy-yielding tumble

• The energy yielded is used to regenerate ATP

LE 9-5

2 H+ + 2 e–

2 H

(from food via NADH)

Controlledrelease ofenergy for

synthesis ofATP ATP

ATP

ATP

2 H+

2 e–

H2O

+ 1/2 O21/2 O2H2 +

1/2 O2

H2O

Explosiverelease of

heat and lightenergy

Cellular respirationUncontrolled reaction

Fre

e en

erg

y, G

Fre

e en

erg

y, G

Electro

n tran

spo

rt chain

The Stages of Cellular Respiration: A Preview

• Cellular respiration has three stages:– Glycolysis (breaks down glucose into two

molecules of pyruvate)

– Kreb’s cycle

– Electron Transport System

• The process that generates most of the ATP is called oxidative phosphorylation because it is powered by redox reactions

LE 9-6_1

Mitochondrion

Glycolysis

PyruvateGlucose

Cytosol

ATP

Substrate-levelphosphorylation

LE 9-6_2

Mitochondrion

Glycolysis

PyruvateGlucose

Cytosol

ATP

Substrate-levelphosphorylation

ATP

Substrate-levelphosphorylation

Kreb’sCycle

LE 9-6_3

Mitochondrion

Glycolysis

PyruvateGlucose

Cytosol

ATP

Substrate-levelphosphorylation

ATP

Substrate-levelphosphorylation

Krebscycle

ATP

Oxidativephosphorylation

Oxidativephosphorylation:electron transport

andchemiosmosis

Electronscarried

via NADH

Electrons carriedvia NADH and

FADH2

• Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration (electron transport system)

• A small amount of ATP is formed in glycolysis and the Krebs cycle

Glycolysis harvests energy by oxidizing glucose to pyruvate

• Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate

• Glycolysis occurs in the cytoplasm and has two major phases:– Energy investment phase– Energy payoff phase

LE 9-8

Energy investment phase

Glucose

2 ATP used2 ADP + 2 P

4 ADP + 4 P 4 ATP formed

2 NAD+ + 4 e– + 4 H+

Energy payoff phase

+ 2 H+2 NADH

2 Pyruvate + 2 H2O

2 Pyruvate + 2 H2O

2 ATP

2 NADH + 2 H+

Glucose

4 ATP formed – 2 ATP used

2 NAD+ + 4 e– + 4 H+

Net

Glycolysis Citricacidcycle

Oxidativephosphorylation

ATPATPATP

LE 9-9a_1

Glucose

ATP

ADP

Hexokinase

ATP ATP ATP

Glycolysis Oxidationphosphorylation

Citricacidcycle

Glucose-6-phosphate

LE 9-9a_2

Glucose

ATP

ADP

Hexokinase

ATP ATP ATP

Glycolysis Oxidationphosphorylation

Citricacidcycle

Glucose-6-phosphate

Phosphoglucoisomerase

Phosphofructokinase

Fructose-6-phosphate

ATP

ADP

Fructose-1, 6-bisphosphate

Aldolase

Isomerase

Dihydroxyacetonephosphate

Glyceraldehyde-3-phosphate

LE 9-9b_1

2 NAD+

Triose phosphatedehydrogenase

+ 2 H+

NADH2

1, 3-Bisphosphoglycerate

2 ADP

2 ATPPhosphoglycerokinase

Phosphoglyceromutase

2-Phosphoglycerate

3-Phosphoglycerate

LE 9-9b_2

2 NAD+

Triose phosphatedehydrogenase

+ 2 H+

NADH2

1, 3-Bisphosphoglycerate

2 ADP

2 ATPPhosphoglycerokinase

Phosphoglyceromutase

2-Phosphoglycerate

3-Phosphoglycerate

2 ADP

2 ATPPyruvate kinase

2 H2OEnolase

Phosphoenolpyruvate

Pyruvate

The Krebs cycle completes the energy-yielding oxidation of

organic molecules

• Before the Krebs cycle can begin, pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis

LE 9-10

CYTOSOL

Pyruvate

NAD+

MITOCHONDRION

Transport protein

NADH + H+

Coenzyme ACO2

Acetyl Co A

• The Krebs cycle, takes place within the mitochondrial matrix

• The cycle oxidizes organic fuel derived from pyruvate, generating one ATP, 3 NADH, and 1 FADH2 per turn

LE 9-11Pyruvate(from glycolysis,2 molecules per glucose)

ATP ATP ATP

Glycolysis Oxidationphosphorylation

CitricacidcycleNAD+

NADH

+ H+

CO2

CoA

Acetyl CoACoA

CoA

Krebscycle CO2

2

3 NAD+

+ 3 H+

NADH3

ATP

ADP + P i

FADH2

FAD

• The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain

• Following glycolysis and the Krebs cycle, NADH and FADH2 account for most of the energy extracted from food

• These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation

The Pathway of Electron Transport

• The electron transport chain is in the cristae of the mitochondrion

• Most of the chain’s components are proteins, which exist in multiprotein complexes

• The carriers alternate reduced and oxidized states as they accept and donate electrons

• Electrons drop in free energy as they go down the chain and are finally passed to O2, forming water

• The electron transport chain generates no ATP• The chain’s function is to break the large free-

energy drop from food to O2 into smaller steps that release energy in manageable amounts

Chemiosmosis: The Energy-Coupling Mechanism

• Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space

• H+ then moves back across the membrane, passing through channels in ATP synthase

• ATP synthase uses the flow of H+ to drive phosphorylation of ATP

• This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work

LE 9-14

INTERMEMBRANE SPACE

H+ H+

H+H+

H+

H+

H+

H+

ATP

MITOCHONDRAL MATRIX

ADP+

Pi

A rotor within the membrane spins as shown when H+ flows past it down the H+ gradient.

A stator anchored in the membrane holds the knob stationary.

A rod (or “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.

LE 9-15

Protein complexof electroncarriers

H+

ATP ATP ATP

GlycolysisOxidative

phosphorylation:electron transportand chemiosmosis

Citricacidcycle

H+

Q

IIII

II

FADFADH2

+ H+NADH NAD+

(carrying electronsfrom food)

Innermitochondrialmembrane

Innermitochondrialmembrane

Mitochondrialmatrix

Intermembranespace

H+

H+

Cyt c

IV

2H+ + 1/2 O2 H2O

ADP +

H+

ATP

ATPsynthase

Electron transport chainElectron transport and pumping of protons (H+),

Which create an H+ gradient across the membrane

P i

ChemiosmosisATP synthesis powered by the flow

of H+ back across the membrane

Oxidative phosphorylation

An Accounting of ATP Production by Cellular Respiration

• During cellular respiration, most energy flows in this sequence:

glucose NADH electron transport chain proton-motive force ATP

• About 40% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 38 ATP

LE 9-16

CYTOSOL Electron shuttlesspan membrane 2 NADH

or

2 FADH2

MITOCHONDRION

Oxidativephosphorylation:electron transport

andchemiosmosis

2 FADH22 NADH 6 NADH

Citricacidcycle

2AcetylCoA

2 NADH

Glycolysis

Glucose2

Pyruvate

+ 2 ATP

by substrate-levelphosphorylation

+ 2 ATP

by substrate-levelphosphorylation

+ about 32 or 34 ATP

by oxidation phosphorylation, dependingon which shuttle transports electronsform NADH in cytosol

About36 or 38 ATPMaximum per glucose:

Fermentation enables some cells to produce ATP without the use

of oxygen

• Cellular respiration requires O2 to produce ATP

• Glycolysis can produce ATP with or without O2 (in aerobic or anaerobic conditions)

• In the absence of O2, glycolysis couples with fermentation to produce ATP

Types of Fermentation

• Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis

• Two common types are alcohol fermentation and lactic acid fermentation

• In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO2

• Alcohol fermentation by yeast is used in brewing, winemaking, and baking

LE 9-17a

CO2

+ 2 H+

2 NADH2 NAD+

2 Acetaldehyde

2 ATP2 ADP + 2 P i

2 Pyruvate

2

2 Ethanol

Alcohol fermentation

Glucose Glycolysis

• In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2

• Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt

• Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce

LE 9-17b

CO2

+ 2 H+

2 NADH2 NAD+

2 ATP2 ADP + 2 P i

2 Pyruvate

2

2 Lactate

Lactic acid fermentation

Glucose Glycolysis

Fermentation and Cellular Respiration Compared

• Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate

• The processes have different final electron acceptors: an organic molecule (such as pyruvate) in fermentation and O2 in cellular respiration

• Cellular respiration produces much more ATP

LE 9-18

Pyruvate

Glucose

CYTOSOL

No O2 presentFermentation

Ethanolor

lactate

Acetyl CoA

MITOCHONDRION

O2 present Cellular respiration

Citricacidcycle

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

other metabolic pathways

• Gycolysis and the Krebs cycle are major intersections to various catabolic and anabolic pathways

The Versatility of Catabolism• Catabolic pathways funnel electrons from many

kinds of organic molecules into cellular respiration• Glycolysis accepts a wide range of carbohydrates• Proteins must be digested to amino acids; amino

groups can feed glycolysis or the citric acid cycle• Fats are digested to glycerol (used in glycolysis)

and fatty acids (used in generating acetyl CoA) • An oxidized gram of fat produces more than twice

as much ATP as an oxidized gram of carbohydrate

LE 9-19

Citricacidcycle

Oxidativephosphorylation

Proteins

NH3

Aminoacids

Sugars

Carbohydrates

Glycolysis

Glucose

Glyceraldehyde-3- P

Pyruvate

Acetyl CoA

Fattyacids

Glycerol

Fats