CHEMISTRY-VIII NOTES PREPARED BY Dr.

DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

Biological oxidation 4 hours Max. Marks: 10

Bioenergetics- Introduction, stages of energy transformation. Exergonic and
endergonic reactions. Relationship between ΔG0 and Keq. High energy phosphates
– definition, examples, structural features of ATP that makes it a high energy
phosphate (electrostatic repulsion, opposing resonance, solvation). Examples of
high energy phosphates other than ATP (PEP, 1-3-diphosphoglycerate). Energy
coupling in biological reactions (explain the concept with suitable examples).
Biological oxidation- comparison of oxidation with combustion using glucose as
an example. Redox potentials of some biological important half reactions (NAD+
and FAD+). Calculation of energy yield from biological redox reaction (oxidation
of NADH by oxygen, reduction of acetaldehyde by NADH). Mitochondrial
electron transport chain (schematic representation of electron carriers),
oxidative phosphorylation, Substrate level phosphorylation.

Bioenergetics: The field of biochemistry concerned with the use and

transformation of energy by living organisms is called bioenergetics. It predicts
whether a biological process is possible or not.

Stages of biological energy transformation: The flow of energy through living

systems can be thought of as occurring in three stages.

1) Photosynthesis (conversion of solar energy to chemical bond energy of

fuels): In this process, green plants trap solar energy and use it to drive a
sequence of reactions which results in the formation of carbohydrates.
Carbon dioxide and water are converted in the presence of chlorophyll to
sugars. Excess sugar is stored as starch. In effect, solar energy is captured
and stored as chemical energy.
Page 1 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

Ultimately, all organisms depend on photosynthesis, as it is the method by

which basic food is created. Photosynthesis also helps the plant convert
ADP and Pi to ATP. This process is called photosynthetic phosphorylation.
This ATP is then used for the energy requiring functions of the plant.
2) Cellular respiration (oxidation of fuels to generate ATP): The food created
by plants is used by all other organisms. Starch is digested to glucose which
is then oxidized using oxygen. This process of oxidation of glucose in living
cells is called cellular respiration. Part of the energy liberated is converted
in the form of ATP while the rest is lost as heat. Stepwise oxidation of
glucose enables the synthesis of 38 ATP per molecule of glucose in aerobic
organism i.e. those that use oxygen. Cellular respiration takes place via the
metabolic pathways of glycolysis, Kreb’s cycle and the electron transport
chain, which mostly occur in mitochondria.

3) Utilization of ATP to perform biological work: ATP is a high energy

molecule. On hydrolysis, it releases a large amount of energy. This energy is
used to drive reactions which require an input of energy (endergonic
reaction). Since ATP itself is obtained from reactions which release energy (
exergonic reactions), it can be considered to be the link between energy –
releasing and energy –requiring reactions.

Page 2 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

The energy of ATP is used to perform biochemical work. For example, synthesis of
cell material, muscular contraction(movement), active transport across
membranes(this is osmotic work as substances are transported against a
concentration gradient i.e. from a region of low concentration to a region of high
concentration), light production(as in fireflies), electricity generation(transmission
of nervous impulses),etc.

Exergonic reactions: The reaction in which change in free energy is released

(i.e. G = -ve) and therefore proceeds spontaneously is called exergonic reaction.

For example:

ATP + H2O ADP + Pi G0 = -7.3 kcal mol-1

ADP + H2O AMP + Pi G0 = -7.3 kcal mol-1

PPi
2Pi G0 = -8.0 kcal mol-1
Pyrophosphate

Endergonic reactions: The reaction in which change in free energy is absorbed

(required) (i.e. G = +ve) and therefore proceeds non -spontaneously is called
endergonic reaction.

Page 3 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

ADP + Pi ATP + H2O G0 = +7.3 kcal mol-1

AMP + Pi ADP + H2O G0 = +7.3 kcal mol-1

2Pi PPi G0 = +8.0 kcal mol-1

Pyrophosphate

Relationship between standard free energy change ( 𝚫𝑮𝟎 ) and

equilibrium constant of a reaction: Free energy is the maximum amount of
useful work done from a process or system at constant temperature,
pressure and volume. Free energy is a state function which depends on
initial and final stages.
Consider a reaction,

aA + bB cC + dD

The free energy (G) is related to standard free energy change (G0) by the following
equation
[C]c [D]d
G = G0 + 2.303 RT log
[A]a [B]b

Where, R is the gas constant and K is the temperature in kelvin

At equilibrium,

[C]c [D]d
G = 0 and = Keq
[A]a [B]b

Therefore the above equation reduces to

G0 = - 2.303 RT log Keq

This equation gives the relationship between standard change in free energy and
equilibrium constant of a reaction.

Page 4 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

Significance of standard free energy:

If ∆𝐺 0 = 0, when (Keq = 1), the reaction is said be at equilibrium, no

free energy is available and there is no net conversion of reactants to the
products.

If ∆𝐺 0 < 0(-ve), when (Keq = 1), the reaction is said be spontaneous,

the free energy is available without input of external energy and the conversion
of reactants to the products takes place without input of energy.

If ∆𝐺 0 > 0(+ve), when Keq = -1), the reaction is said be non-

spontaneous, the free energy is available with external energy output and the
conversion of reactants to the products takes place with input of external
energy.

High energy phosphates: Those phosphate compounds which are present in the
biological system which, on hydrolysis, liberates at least 7.3 KCal/mol (30.54
KJ/mol) free energy at pH 7.0 are called high energy phosphate compounds.
Examples: Adenosine triphosphate (ATP)
Role of ATP as a high energy compound: Since exergonic reactions drive
endergonic reactions in cells by energy coupling, a single common intermediates
formed in all exergonic reactions which can be used for all endergonic reactions
would be extremely advantageous. In cells, ATP is the common coupling
intermediate molecule which enables free energy to be transferred from energy-
yielding (catabolic) to energy required (anabolic) processes.

ATP is produced from ADP and Pi using energy released during

photosynthesis and by the breakdown (catabolism) of food molecules. It is then
used to derive almost all anabolic biochemical processes, during which it gets

Page 5 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

converted back to ADP and Pi. This ATP-ADP cycle is the fundamental method by
which energy is exchanged in biological systems and is shown in the figure.

ATP

Catabolism Energy utilization

Photosynthesis(green plants)
Biosynthesis of cell macromolecules
Cellular respiration muscle contraction and movement active
(in all organism) transport across membranes nerve
impulses, thermogenesis
Energy yielding reaction with
Energy requiring process with
G < 0 ( -ve)
G > 0 ( +ve)

ADP + Pi

ATP - ADP cycle

ATP serves as the immediate donor of free energy in most cellular endergonic
processes and is formed and used up rapidly. Thus is called the energy currency of
the cell. When it is hydrolyzed to ADP and Pi it yields a large amount of free
energy. Hence it is called a high energy or energy rich compound.

Mg2+
ATP ADP + Pi G0 = -7.3 kcal mol-1

Structure of ATP

ATP has a structure ideally suited to make it an energy rich, universal energy
currency.

Page 6 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

NH2

N
N
- - -
O O O N
N
-
O P O P O P O CH2
O O
O H H
H H

OH OH

Adenosine -5' -triphosphate (ATP)

The factors which cause the release o he large amount of energy released during
hydrolysis of ATP are:

1) Electrostatic repulsion: Electrostatic repulsion between the four negative

charges on its three phosphate groups
2) Opposing resonance energy: The products of hydrolysis (ADP and Pi) are
more stabilized by resonance than ATP itself.
3) Solvation: The products of hydrolysis (ADP and Pi) are more stabilized by
solvation (interaction with water) than ATP itself (solvation leads to the
negative charges being shielded from each other, reducing mutual
repulsion)
[within cells, Mg2+ serves to stabilize ATP by partially neutralizing the
negative charges on its oxygen atoms, diminishing repulsion. In
physiological conditions, therefore, ATP is stable until acted upon by an
enzyme].

Page 7 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

Other high energy compounds: Those phosphate compounds other than ATP
having change in standard free energy (G0) values even more negative than ATP (-
7.3 kcal/mol) due to donation of phosphoryl group to ADP, forming ATP are called other
high energy compound. These compounds also called the super-high energy
compounds.

Examples: Phosphoenolpyruvate (PEP) (-14.8 kcal/mol), 1,3-diphosphoglycerate(-11.8

kcal/mol) and phosphocreatine (-10.3 kcal/mol)

Energy of coupling reaction and Coupling reaction: The energy required or

liberated by combining (coupling) of exergonic reaction with endergonic
reactions is called energy of coupling reaction and net reaction is called
coupling reaction.

Page 8 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

For example:

(1)

Glucose + Pi Glucose -6-phoshate + H2O G0 = + 3.3 kcalmol-1

ATP + H2O ADP + Pi G0 = - 7.3 kcalmol-1

Net reaction Glucose + ATP Glucose -6-phoshate + ADP G0 = - 4.0 kcalmol-1
Thus, overall, coupled reactions are exergonic

The formation of glucose 6-phosphate from glucose and nitrogen

phosphate is thermodynamically unfavourable because it is an endergonic
reaction with a G0 of +3.3kcalmol-1. In cells, this reaction is coupled to the
hydrolysis of ATP to ADP and Pi, which is thermodynamically favourable. Being
an exergonic reaction with a G0 of -7.3 kcalmol-1. Thus the phosphorylation of
glucose occurs with a next G0 of -4.0 kcal mol-1

(2)

Phosphoenolpyruvate Pyruvate + Pi G0 = - 14.8 kcalmol-1

ADP + Pi ATP + H2O G0 = 7.3 kcalmol-1

Net reaction Phosphoenolpyruvate + ADP Pyruvate + ATP G0 = - 7.5 kcalmol-1

Thus, overall, coupled reactions are exergonic

In energy coupling the free energy released in a thermodynamically

favourable exergonic reaction is used to derive a thermodynamically
unfavourable endergonic reaction. An exergonic reaction and an endrgonic
reaction are coupled by the sharing of a common intermediate, so that the
overall free energy change for the coupled reactions is negative. Frequently,
ATP acts as the common intermediate between coupled reactions.

Page 9 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

Biological oxidation: Oxidation is defined as an increase in the oxygen content of

a molecule and or a decrease in its hydrogen content in a metabolism.

Oxidation reaction achieved by using following methods

1) By removal of electrons alone from a substrate.

For example:

Fe2+ Fe3+ + e-
Reduced state Oxidized state

2) By removal of electrons along with protons (dehydrogenation in effect).

AH2 A + 2e- + 2H+

Reduced state Oxidized state

In such reactions, the electron(s) are transferred from the electron donor to an
electron acceptor. The electron acceptor thus gets reduced. Reduction is a
reaction is which a substance gains electrons (or hydrogen).

For example:

Fe2+ Electron acceptor

Fe3+ Reduced electron acceptor

A comparison of biological oxidation and combustion: When one mole (180.2g)

of glucose undergoes complete combustion, the free energy change is
approximately -686 kcal (-2870kj). This energy is released in one step, largely in

Page 10 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

form of heat (enthalpy energy). This cannot be used by biological systems, which
are isothermal (heat must flow from a region of higher temperature to a region of
lower temperature to do work). Also, since energy is released in a single
irreversible step or explosion, it cannot be conserved efficiently, as only one ATP
can be synthesized per step.

In addition to no measurable work being performed, the excessive heat

produced would also be fatal to be organism.

C6H12O6 + 6O2 6CO2 + 6H2O G = -686 kcalmol-1

Biological systems oxidize glucose in many steps. This enables the control of
energy input and output at each step, which in turn energy to be conserved much
more efficiently than in combustion. Here again G is -686 kcalmol-1. But because
energy is released in small fractions a large percentage can be conserved in a
form much more manageable. In those steps where energy released is sufficient
enough, it is conserved in the form of ATP. Expect those steps where ∆G is
extremely large, most steps are reversible. Therefore, metabolic pathways can be
reversed when necessary.

Totally, 36 to 38 ATPs are obtained from each glucose molecule oxidized. The rest
of the energy is dissipated as heat.

Page 11 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

Calculating the free energy stored in 38 ATP (38 x -7.3 kcal =277.4 kcal), the
thermodynamic efficiency of biological oxidation is 277.4 x 100/686 = 46%

Electron-transfer or redox reaction: When a molecule donates electrons, it is

called a reducing agent or reductant. On donating electrons the molecule is
oxidized. The electrons have to be accepted by another molecule. This is called the
oxidizing agent or oxidant. On accepting electrons, this gets reduced.

Thus an oxidation – reduction or redox reaction is one in which electrons are

transferred from one molecule to another. Reducing and oxidizing agents which
react with one another are called as conjugate reductant –oxidant pairs (conjugate
redox pairs).

For redox reactions the following general equation applies.

Here, Fe2+ and Fe3+ are a conjugate redox pair

A redox reaction is composed of two coupled half reactions

For example:

Page 12 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

Standard oxidation reduction potential (standard redox potential)

The electrical potential or electromotive force measured during the reaction

under standard conditions (250C, both electron donor and its conjugate electron
acceptor at 1M concentration, 1 atm. Pressure and pH 7). It is measured against a
reference standard, the hydrogen (H2: H+) electrode, at pH 7. Under these
conditions, the hydrogen electrode has an emf of -0.42.

Page 13 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

Conjugate redox pairs with more negative redox potentials have a

greater tendency to donate electrons(and become oxidized), systems with more
positive redox potentials tend to accept electrons(and become reduced). Thus,
electrons will flow spontaneously from the substance with lower redox potential
(higher reducing power) to the substance with the higher redox potential (lower
reducing power). As they do so, there is a release of free energy which depends
on E0. This is defined as the difference between the redox potential of the electron-
accepting system and that of the electron-donating system.

Standard redox potentials of some conjugate redox pairs involved in biological

electron transport

Conjugate redox pair SRP (E0

volts)
Some substrate pairs
-0.48

-0.38

-0.18
-0.17
0.03
Components of electron –transport chain
-0.42
-0.32
-0.22

Page 14 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

-0.22
0.00

+0.04
+0.08
+0.22
+0.25
+0.29
+0.385
+0.82

Redox pairs with highly negative E0 values are strong reducing
agents i.e. they have a strong tendency to donate electrons. These electrons have a
strong potential to combine with protons and O2 to form water. Redox pairs with
higher (more positive) ∆E0 values are strong oxidizing agents. i.e. they have a
tendency to accept electrons. Therefore, electrons tend to flow from more
negative system to more positive system. Oxygen is the ultimate electron
acceptor (oxidizing agent.

The relationship between G0 and E0: Electron flow from electronegative to
electropositive systems because free energy is lost in the process, reducing the free
energy of the reacting system (leading to stability). G0, all standard free energy change
of redox reaction at pH 7, can be calculated from the change in redox potential E0 of
the substrates and products using the equation.

Page 15 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

Where n is the number of electrons transferred, F is a constant called Faraday’s

constant 23.06 kcal V-1 mol-1 or 96500 JV-1 mol-1)

Problems:

1) Calculate the free energy change of biological redox reaction between the
oxidation of NADH by oxygen

Solution: We know that

Thus the change in potential energy (E0) of the above half cell reactions is given
by

∆E0 = E0 Electron acceptor - E Electron donor

= 0.82 volts – (-0.32 volts)

∆E0 = + 1.14 V n = 2 (No. of electrons involved in the reaction)

Now, ∆G0 = - nF∆E0 ∆G0 = - nF∆E0

= - 2(23.06 kcal ) (1.14v) =- 2(96.5kj/vmol) (1.14v)

Page 16 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

∴ ∆G0 = -52.6 kcal/volt ∴ ∆G0 = -220.02 kJ/mol

2) Calculate the free energy change of biological redox reaction between the
reduction of acetaldehyde by NADH

Solution: We know that

The two half reactions are

Thus the change in potential energy ( E0) of the above half cell reactions is given
by

∆E0 = E0 Electron acceptor - E Electron donor

= -0.197 volts – (-0.32 volts)

∆E0 = 0.123 V n = 2 (No. of electrons involved in the reaction)

Page 17 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

Now, ∆G0 = - nF∆E0 ∆G0 = - nF∆E0

= - 2(23.06 k cal) (0.123v) =- 2(96.5kj/vmol) (0.123v)

∴ ∆G0 = -5.67 kcal/volt ∴ ∆G0 = -23.739 kJ

Mitochondrial electron transport chain: The ETC is the final common

pathway by which electrons derived from various fuel molecules metabolism are
transferred to oxygen

In aerobic organisms, fuel molecules such as glucose or fatty acids

(obtained from food) are broken down by stepwise oxidation reactions, forming
CO2 and H2O ultimately. These oxidation reactions release electrons with high
potential energy which are enzymatically transferred to certain coenzymes,
usually NAD+ (nicotinamide adenine dinucleotide) or FAD (flavin adenin
dinucleotide).

NAD+ accepts two electrons (from various substances) in the form of

a hydride ion (OH- or H- + 2e) and gets reduced to NADH. FAD accepts two
electrons (From some substrates) in the form of two hydrogen atoms (2H + +2e)
and gets reduced to FADH2.

Page 18 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

The energy from the oxidation of the fuel (food) molecules is

conserved by the transfer of electrons to NAD+ and FAD. The reduced coenzymes
(NADH and FADH2) are thus energy rich. If they donate a pair of electrons directly
to oxygen (in one step), a large amount of energy would be released. However,
most of this energy would be water fully liberated as heat. Therefore, aerobic
eukaryotes use a special pathway to ensure that energy is released in steps,
thereby enabling much more efficient conservation of energy.

Electrons from NADH to FADH2 are transferred to oxygen via a specialized set of
electron carriers called the electron transport chain (ETC) or respiratory chain.

Page 19 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

The electron transport chain is a series of electrons carrying enzymes that

accepts electrons from NADH and FADH2 and transfers then to oxygen in a
regulated, stepwise manner. The ETC is associated with the inner membrane of
mitochondria. As shown in figure.

It is the final oxidative pathway in aerobic eukaryotes, channelizing the electrons

derived from different fuels (carbohydrates, lipids, amino acids to oxygen, the final
electron acceptor.

Hydrogen atom from NADH donate their electrons to the ETC and escape as H+
ions into the aqueous medium. In the last step, when oxygen accepts two electrons,
these two protons are again taken up from the medium to form H 2O.

The series of electron carrying proteins constituting the ETC contain tightly
bound prosthetic groups. Each electron carrier can accept electrons from the preceding
carrier and transfer then to the following one, in a specific sequence.

As the electrons flow stepwise along the ETC, redox potentials(E 0) of the
electron carriers successively become progressively higher(more positive), until they
react oxygen, the terminal electron acceptor or oxidizing agent. Energy is released in
discrete steps. Whenever the energy release is sufficient enough, it is used for the
synthesis of ATP. It must be emphasized, however, that electron transport through the
ETC does not directly lead to ATP synthesis.

The main route of the ETC can be represented as

Page 20 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

In effect, electrons from NADH and FADH2 fall down a staircase, each step
representing an electron carrier. Each fall releases free energy. Wherever possible, this
is used (indirectly) to convert ADP and Pi to ATP, instead of wasting, it as heat9 as
occurs in the one step combustion of glucose).Since ATP synthesis in this manner
requires oxygen as the ultimate oxidizing agent, this method of ATP generation is called
oxidative phosphorylation.

Principle of the electron transport chain

Page 21 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

Note: (i) FADH2 donates electrons to the ETC staircase at a point below NADH, and so
leads to the synthesis of fewer ATP

(ii) Free energy not utilized for ATP synthesis is liberated as heat.

Arrangement and sequence of electron carriers: In the electron transport chain, the
electron carriers are arranged such that the electrons accepted from NADH and FADH 2
flow down a free energy gradient (i.e. in the direction of increasing redox potentials) to
oxygen.

Most of the electron carriers are precisely organized into four complexes [each
complex consists of numerous proteins and prosthetic groups(tightly held cofactors),
many of which for clarity are not mentioned here].These complexes are embedded in
the structure of the inner mitochondrial membrane. Two mobile electron carriers interlink
the above complexes. (a) coenzyme Q or ubiquinone and (b) Cytochrome C. Each
electron carrier undergoes cyclic reduction and oxidation by accepting electrons from
the previous carrier and then donating them to the next carrier.

Coenzyme Q is a relatively small lipid soluble molecule, freely mobile in the inner
mitochondrial membrane. It is also called ubiquinone because it can exist in quinine
form and is found ubquitiously in most organisms. CoQ has a long polyisopropenoid tail
and can accept two electrons and two protons to form CoOH 2 (ubiquinol). However it
passes on electrons one at a time.

Cytochrome is the only protein electron carrier not built into the structure of the inner
mitochondrial membrane. It is a small mobile water soluble protein loosely attached to
the outer surface of membrane.

The cytochrome is electron transporting enzyme which contain a haeme

prosthetic group. They transfer electrons through the ETC one at a time. On accepting
an electron, Fe3+ of the prosthetic group is reduced to Fe2+. On donating it, Fe2+ is
reconverted to Fe3+

Page 22 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

Electron flow through the ETC does not directly lead to ATP synthesis. Instead,
at certain points, proton translocation takes place across the membrane creating a
proton gradient. This is used for ATP synthesis by a fifth complex (not part of the ETC)
called synthase.

Note; Protein containing the cofactors FMN and FAD are called flavoproteins.

Sequence of electron carriers:

Complex I (NADH dehydrogenase complex), containing flavin

mononucleotide(FMN) as cofactor, accepts two electrons from NADH in the form of a
hydride ion(H+). FMN is reduced to FMNH2. FMNH2 then passes on the electrons (one
at a time) to coenzyme Q, which is converted to CoQH2

Complex II is (succinate dehydrogenase complex) contains the prosthetic group

Flavin adenine dinucleotide(FAD). This accepts electrons from some substrates as
shown in figure beleow) FAD gets converted to FADH 2. The electrons are then
transferred to CoQ , reducing it to CoQH2. Thus CoQ accepts electrons from both
complex I and compex II. It then delivers them to complex III

Page 23 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

Complex III is sometimes called the cytochrome bc 1 complex because it contains

cytochromes b and C1. It transfers electrons from CoQH2 to cytochrome c

Cytochrome c transfers electrons from complex III to complex IV

Complex IV (cytochrome oxidase) also contains two cytochromes (a and a3). It

catalyzes a complex reaction, transferring four electrons from four cytochrome c
molecules, along with four H+ ions from the surrounding medium to molecular oxygen.
Two molecules of water are formed.

Cytochromes a and a3 contain one copper atom each. While their atoms oscillate
between the Fe3+ AND Fe2+ states, their copper atoms oscillate between Cu 2+ and Cu+
states. Only cytochrome oxidase is directly oxidizable by oxygen. This reaction
accounts for more than 90% of the oxygen utilized by cells.

Only the reactions catalyzed by complexes I, III and IV release sufficient energy
to synthesise ATP from ADP and Pi. As electrons pass through them, these complexes
translocate protons across the inner mitochondrial membrane. These three complexes
are sometimes indicated as sites of ATP synthesis. This is incorrect, because actual
ATP synthesis occurs in ATP synthetase (complex V).

A highly simplified scheme showing the sequence of electron carriers in the ETC is-

Page 24 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

Note: The substrates that donate electrons to NAD include pyruvate, α-ketoglutarate,
isocitrate, malate, glutamate and β-hydroxyacyl CoA.

Those that donate electrons to succinate dehydrogenase include succinate and

fatty acyl CoA.

A somewhat more detailed scheme of electron carriers of the ETC, but still not
showing the iron –sulphur proteins is given in below figure

Oxidative phosphorylation: Electron transport through the ETC from NADH and
NADH2 (powerful electron donor) to oxygen (avid electron acceptor) proceeds with the
liberation of a large amount of free energy. Oxidative phosphorylation is the process by
which the energy released by electron transport through the ETC is (indirectly)
conserved by the phosphorylation of ADP to ATP. It is the main method of energy
conservation in aerobic cells.

Page 25 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

The most widely accepted hypothesis to explain how the free energy released by
electron transfer through the ETC is used to produce ATP is called the chemi-osmotic
hypothesis. This was proposed by Peter Mitchell in 1961

In brief, the hypothesis proposes that as electrons pass the ETC, protons are
transported (trans-located) across the inner mitochondrial membrane from the matrix to
the inter membrane space. This creates a proton gradient across the membrane. The
protons then flow back into the mitochondrial matrix through ATP synthase. This drives
ATP synthesis as shown below.

Differences between Oxidative phosphorylation and substrate level

phosphorylation

Oxidative phosphorylation substrate level phosphorylation

ADP and Pi are converted to ATP using ADP and Pi are converted to ATP using

Page 26 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

the energy released in the electron the energy released by the hydrolysis of a
transport chain by transfer of electrons high energy substrate.
from high energy reduced coenzymes
(NADH and FADH2)
Requires oxygen as final electron acceptor Does not require oxygen
Takes place in the inner mitochondrial Takes place in the cytoplasm or
membrane mitochondrial matrix
Carried out by ATP synthase Carried out by various kinase enzymes

Sites of energy conservation in the ETC: In the ETC, only three reactions (steps)
release enough energy to pump H+ ions from the mitochondrial matrix, across the inner
mitochondrial membrane, into the inter membrane space. These are the reactions
catalyzed by (1) NADH dehydrogenase complex (complex I), (2) Cytochrome bc1
complex (complex III) and (3) cytochrome oxidae complex(complex IV)

In other words, only at these three points is sufficient energy liberated to

account for the conversion of ADP and Pi to ATP. In traditional representations of the
ETC, these three sites were referred to as sites of ATP synthesis. This interpretation
has now been discarded, as it is known that ATP synthase is the site where ATP
synthesis occurs. Instead these sites are called sites of energy conservation.

P: O ratio: P:O ratio is the ratio of the number of inorganic phosphate (Pi) molecules
consumed for each oxygen atom reduced to H 2O. Measurements over the years have
shown that every NADH molecule oxidized leads to the synthesis of three ATP
molecules i.e., P:O ratio of NADH is 3. Similarly, every FADH 2 molecule oxidized
through the ETC was found to lead the synthesis of two ATP molecules i.e. P: O ratio of
FADH2 is 2

FADH2 leads to less ATP synthesis (2moleules) because donates electrons to

the ETC at a point which bypasses the first site of proton translocation or energy
conservation (complex I).

Page 27 of 28
CHEMISTRY-VIII NOTES PREPARED BY Dr. DHONDIBA VISHWANATH SURYAWANSHI, GFGC KR PURAM BENGALURU-36

NADH generates 3 molecules of ATP by oxidative phosphorylation. The

synthesis of an ATP molecule requires ~7.3 kcal of energy. (7.3 x3 = 21.9) kcal is
conserved when three ATP are formed. Since NADH contains 52.6 kcal of energy, the
percentage efficiency of energy conservation is 21.9 x 100 / 52.6 = 41.6%.

Substrate level phosphorylation: Energy coupling is used to regenerate ATP from

ADP by substrate level phosphorylation. In substrate level phosphorylation, the
exergonic hydrolysis of super high energy compounds is coupled to the
endergonic phosphorylation of ADP to form ATP. Often, a phosphoryl group may
be directly transferred from a high energy compound to ADP. Substrate level
phosphorylation is independent of the electron transport chain and no oxygen is
required.

Substrate –level phosphorylation is the only method of conservation of energy

(i.e. ATP synthesis) in anaerobic organisms. It also takes place during early phases
of carbohydrate breakdown in both animals and plants.

Page 28 of 28