Chapter 9:

CELLULAR RESPIRATION

& FERMENTATION

3. The Citric Acid Cycle

2. Glycolysis

4. Oxidative Phosphorylation

1. Overview of Respiration

5. Fermentation

1. Overview of Respiration

Lightenergy

ECOSYSTEM

Photosynthesisin chloroplasts

Cellular respirationin mitochondria

CO2 H2O O2

Organicmolecules

ATP powersmost cellular work

ATP

Heatenergy

Cellular Respiration

Cellular

Respiration is

essentially the

reverse of

photosynthesis

Catabolism of

energy rich

organic

molecules is

coupled to

the endergonic

synthesis of ATP

Important Concepts

Before examining the process of Cellular

Respiration it is important to review some

important concepts that are central to the process:

• mitochondrial structure

• oxidation/reduction reactions

• the central role of HYDROGEN and its

electrons (e–), and electron carriers

• substrate-level phosphorylation vs

oxidative phosphorylation

• gradual or incremental release of energy

Mitochondrial StructureMitochondria have 2 membranes (inner

and outer) and 2 distinct compartments:• intermembrane space • matrix

inner membrane is folded into cristae to increase surface area

Oxidation/Reduction There are 2 ways in which are said to be

oxidized or reduced:

OXIDATION = the loss of electrons

REDUCTION = the gain of electrons1

becomes oxidized

(loses electron)

becomes reduced

(gains electron)

becomes oxidized

becomes reduced

Reactants Products

Energy

WaterCarbon dioxideMethane(reducing

agent)

Oxygen(oxidizing

agent)

becomes oxidized

becomes reduced

OXIDATION = the partial loss of electrons

REDUCTION = the partial gain of electrons2

becomes oxidized

becomes reduced

Oxidation/Reduction & Hydrogen

The electrons associated with hydrogen atoms are

key to the oxidation states of organic molecules in

biochemical processes:

• electrons (e–) are transferred as part of a hydrogen atom

in most biochemical reactions (i.e., with a proton)

• when a molecule gains hydrogen it is “reduced” and

when a molecule loses hydrogen it is “oxidized”

When e– of H are transferred from a less electronegative

atom to a more electronegative atom, energy is released!!

Nicotinamide(oxidized form)

NAD

(from food)

Dehydrogenase

Reduction of NAD

Oxidation of NADH

Nicotinamide(reduced form)

NADH

Dehydrogenase

Electron CarriersHydrogens and their e– will be captured by electron

carriers such as NADH and delivered elsewhere:

Substrate

Product

ADP

P

ATP

Enzyme Enzyme

Substrate-level Phosphorylation

A phosphate group from an organic molecule is

transferred directly to ADP to make ATP

• an enzyme called a “kinase” catalyzes the reaction

involving the transfer of a phosphate from one substrate

to another, ADP

Oxidative Phosphorylation

“Oxidative Phosphorylation” refers to the

phosphorylation of ADP to make ATP by a

process that depends on oxidation-reduction

reactions (“oxidative”).

As we shall see this is more complex than it

sounds and involves two key processes:

1) A series of oxidation/reduction reactions involving

the “Electron Transport Chain” which will produce an

electrochemical gradient of H+ ions

2) Coupling the energy stored in this electrochemical

gradient of H+ to the endergonic synthesis of ATP

(a) Uncontrolled reaction (b) Cellular respiration

Explosiverelease of

heat and lightenergy

Controlledrelease ofenergy for

synthesis ofATP

Fre

e e

nerg

y, G

Fre

e e

nerg

y, G

H2 1/2 O2 2 H 1/2 O2

1/2 O2

H2O H2O

2 H+ 2 e

2 e

2 H+

ATP

ATP

ATP

(from food via NADH)

Respiration Liberates Energy Gradually

Electrons

carried

via NADH

Electrons carried

via NADH and

FADH2

Citric

acid

cycle

Pyruvate

oxidation

Acetyl CoA

Glycolysis

Glucose Pyruvate

Oxidative

phosphorylation:

electron transport

and

chemiosmosis

CYTOSOL MITOCHONDRION

ATP ATP ATP

Substrate-level

phosphorylationSubstrate-level

phosphorylation

Oxidative

phosphorylation

Cellular Respiration at a Glance

2. Glycolysis

What is Glycolysis?

Glycolysis is a metabolic pathway occurring

in the cytoplasm that is essentially “phase 1”

of the catabolism of glucose and other

monosaccharides.

• technically glycolysis is NOT part of cellular

respiration, though two of the products of this

pathway are used in cellular respiration:

2 ATP 2 NADH* 2 pyruvate*

*used in cellular respiration

Energy Investment Phase

Glucose

2 ADP 2 P

4 ADP 4 P

Energy Payoff Phase

2 NAD+ 4 e 4 H+

2 Pyruvate 2 H2O

2 ATP used

4 ATP formed

2 NADH 2 H+

NetGlucose 2 Pyruvate 2 H2O

2 ATP

2 NADH 2 H+2 NAD+ 4 e 4 H+

4 ATP formed 2 ATP used

Glycolysis uses ATP to make ATP

• 2 ATP must be

“consumed”

during the

catabolism of

glucose in

order to

produce 4 ATP

net yield of

2 ATP

Glycolysis: Energy Investment Phase

ATP ATPGlucose Glucose 6-phosphate Fructose 6-phosphate Fructose 1,6-bisphosphate

Dihydroxyacetonephosphate

Glyceraldehyde3-phosphate

Tostep 6

ADP ADP

Hexokinase Phosphogluco-

isomerasePhospho-

fructokinase

Aldolase

Isomerase

12 3

4

5

The Energy Investment Reactions

The initial reactions of glycolysis require the hydrolysis

(i.e., “consumption” or “loss”) of 2 ATP in order to add

2 phosphates to intermediates of the pathway thus

increasing their potential energy (PE).

Glycolysis: Energy Payoff Phase

2 ATP 2 ATP2 NADH

2 NAD + 2 H

2 P i

2 ADP

1,3-Bisphospho-glycerate

3-Phospho-glycerate

2-Phospho-glycerate

Phosphoenol-pyruvate (PEP)

Pyruvate

2 ADP

2 2 2

2 H2O

Phospho-

glycerokinase

Phospho-

glyceromutaseEnolase Pyruvate

kinase

67 8

9

10

Triose

phosphate

dehydrogenase

The Energy Payoff Reactions

In the subsequent steps of glycolysis, the PE invested is

used to accomplish:

• the addition of 2 more phosphates

• the capture of 2 pairs of energy-rich e– associated with

hydrogen (reduction of 2 NAD+ to NADH)

• the synthesis of 4 ATP (substrate-level phosphorylation)

3. The Citric Acid Cycle

What is the Citric Acid Cycle?

The Citric Acid Cycle (CAC) is a metabolic

pathway occurring in the mitochondrial matrix

that is essentially “phase 2” of the catabolism

of glucose:

• pyruvate from glycolysis is first catabolized to

acetyl-Coenzyme A before entering the CAC

• all carbons from the original glucose will be

completely oxidized to waste CO2

• more energy-rich e– in hydrogens will be

captured by electron carriers

• 2 more ATP by substrate-level phosphorylation

Pyruvate

Transport protein

CYTOSOL

MITOCHONDRION

CO2 Coenzyme A

NAD + HNADH Acetyl CoA

1

2

3

Oxidation of Pyruvate to Acetyl-CoA

Pyruvate

NAD

NADH

+ HAcetyl CoA

CO2

CoA

CoA

CoA

2 CO2

ADP + P i

FADH2

FAD

ATP

3 NADH

3 NAD

Citric

acid

cycle

+ 3 H

Acetyl CoA

Acetyl CoA derived

from pyruvate (or the

catabolism of fatty

acids and other

organic molecules)

will directly enter the

Citric Acid Cycle

NADH

1

Acetyl CoA

CitrateIsocitrate

-Ketoglutarate

Succinyl

CoA

Succinate

Fumarate

Malate

Citric Acid

Cycle

NAD

NADH

NADH

FADH2

ATP

+ H

+ H

+ H

NAD

NAD

H2O

H2O

ADP

GTP GDP

P i

FAD

3

2

4

5

6

7

8

CoA-SH

CO2

CoA-SH

CoA-SH

CO2

Oxaloacetate

Acetyl CoA is

combined with

the end product

of the pathway

(oxaloacetate)

to produce citric

acid to begin

the cycle again

Per acetyl group,

the cycle yields:

1 ATP

3 NADH

1 FADH2

2 CO2

Keeping Score

Up to this point, the original molecule of

glucose has yielded the following:

GLYCOLYSIS – 2 ATP 2 NADH

OXIDATION of PYRUVATE – 2 NADH 2 CO2

CITRIC ACID CYCLE – 2 ATP 6 NADH

2 FADH2 4 CO2

TOTAL – 4 ATP 10 NADH 2 FADH2 6 CO2

• NADH & FADH2 provide energy-rich e– for the synthesis

of many more ATP by oxidative phosphorylation…

4. Oxidative Phosphorylation

Oxidative Phosphorylation involves 2 distinct

processes:

1) Electron Transport

• e- from NADH and FADH2 are passed along the electron

transport chain (ETC) via oxidation-reduction reactions

• convert energy from e- into energy stored in H+ gradient

2) Chemiosmosis

• energy from H+ gradient harnessed to make ATP

Overview of

Oxidative Phosphorylation

both processes occur across the inner mitochondrial membrane

Proteincomplexof electroncarriers

(carrying electronsfrom food)

Electron transport chain

Oxidative phosphorylation

Chemiosmosis

ATPsynth-ase

I

II

III

IVQ

Cyt c

FADFADH2

NADH ADP P iNAD

H

2 H + 1/2O2

H

HH

21

H

H2O

ATP

Electron Transport & Chemiosmosis

Basics of Electron Transport

Occurs within the inner mitochondrial membrane

High energy electrons supplied by:

• NADH & FADH2

Electron Transport generates H+ gradient between

intermembrane space & matrix

Requires O2 as the final electron acceptor

• provides proton motive force for ATP synthesis

• anaerobic respiration involves other final

electron acceptors

Protein complex

of electron

carriers

(carrying electrons from food)

INTERMEMBRANE

SPACE

MITOCHONDRIAL MATRIX

H

H

H

2 H + 1/2 O2 H2O

NAD

FADH2 FAD

Q

NADH

I

II

III

IV

Cyt c

Electron Transport Chain

Basics of Chemiosmosis

Energy derived from the flow of H+ is used to

synthesize ATP (from ADP & Pi)

• H+ flows “down” concentration gradient through

ATP synthase in the inner mitochondrial membrane

Yields up to 28 ATP per glucose molecule!

“The flow of a H+ from high to low concentration”

• in addition to 2 ATP in glycolysis, 2 ATP in Krebs cycle

• ATP synthase = enzyme complex that catalyzes:

ADP + Pi ATPH+ flow

INTERMEMBRANE SPACE

Rotor

StatorH

Internal

rod

Catalytic

knob

ADP

+

P i ATP

MITOCHONDRIAL MATRIX

ATP Synthase

The enzyme complex

known as ATP synthase

couples the energy released

by chemiosmosis to the

synthesis of ATP

• as H+ ions flow through ATP

synthase, the rotor portion of

the enzyme complex and the

internal rod actually rotate

• rotation of the internal rod

provides the force needed to

to get ADP & Pi bound by the

catalytic knob close enough to

react and form ATP

Electron shuttlesspan membrane MITOCHONDRION

2 NADH

2 NADH 2 NADH 6 NADH

2 FADH2

2 FADH2

or

2 ATP 2 ATP about 26 or 28 ATP

Glycolysis

Glucose 2 Pyruvate

Pyruvate oxidation

2 Acetyl CoA

Citricacidcycle

Oxidativephosphorylation:electron transport

andchemiosmosis

CYTOSOL

Maximum per glucose:About

30 or 32 ATP

Summary of Cellular Respiration

CarbohydratesProteins

Fatty

acids

Amino

acids

Sugars

Fats

Glycerol

Glycolysis

Glucose

Glyceraldehyde 3- P

NH3 Pyruvate

Acetyl CoA

Citric

acid

cycle

Oxidative

phosphorylation

Respiration

& other

Organic

Molecules

In addition to

glucose, other

organic molecules

“feed” into the

process of

respiration at

various stages

Phosphofructokinase

Glucose

GlycolysisAMP

Stimulates

Fructose 6-phosphate

Fructose 1,6-bisphosphate

Pyruvate

Inhibits Inhibits

ATP Citrate

Citric

acid

cycle

Oxidative

phosphorylation

Acetyl CoA

Regulation of

Respiration

Allosteric regulation of

key enzymes such as

phosphofructokinase

helps keep the entire

process in balance.

5. Fermentation

2 ADP 2 ATP

Glucose Glycolysis

2 Pyruvate

2 CO22

2 NADH

2 Ethanol 2 Acetaldehyde

(a) Alcohol fermentation (b) Lactic acid fermentation

2 Lactate

2 Pyruvate

2 NADH

Glucose Glycolysis

2 ATP2 ADP 2 Pi

NAD

2 H

2 Pi

2 NAD

2 H

AnimalsYeast

The Purpose of FermentationNAD+

NADH

glycolysis fermentation

In the absence of O2,

glycolysis is the only

source of ATP

• fermentation ensures sufficient NAD+ is available for glycolysis

Glucose

CYTOSOLGlycolysis

Pyruvate

No O2 present:

Fermentation

O2 present:

Aerobic cellular

respiration

Ethanol,

lactate, or

other products

Acetyl CoA

MITOCHONDRION

Citric

acid

cycle

Fermentation or Respiration?

If O2 is

present,

respiration is

preferred

(more ATP!),

however

without O2

fermentation

is the only

option to

produce ATP

Glucose

Pyruvate

NAD+

NADH

Saccharomyces

AspergillusLactobacillus

StreptococcusClostridium

EscherichiaAcetobacterPropionibacterium

Fermentation

Fermentationproducts

CO2, propionic acid Lactic acid CO2, ethanol

Swiss cheese

Acetone, isopropanol Acetic acid

NADH

NAD+

Cheddar cheese,yogurt, soy sauce Wine, beer

Nail polish remover,rubbing alcohol

Vinegar

Variety in Fermentation Products

• different organisms produce different fermentation products

Key Terms for Chapter 9

• ATP synthase

• electron carriers (NADH, FADH2)

• electron transport chain (ETC), chemiosmosis

• glycolysis, fermentation

• substrate-level vs oxidative phosphorylation,

• Citric Acid Cycle

• oxidation vs reduction

• mitochondria: inner & outer membranes, matrix,

cristae