Bioenergetics - entropy, enthalpy, free energy, standard free energy change, laws of thermodynamics, coupled reactions, energy currency, conventions in biochemical energetics, phosphorylation, uncouplers, muscle contraction, chemiosmotic hypothesis.

1. 8. SOME IMPORTANT QUESTION - ANSWER FOR IGNOU BIOCHEMISTRY [CHE 9]
BIOENERGETICS
Q.1. Define enthalpy, entropy, free energy and standard free energy. How are these
related to spontaneity of chemical reactions? [enthalpy, entropy, free energy और standard
free energy को परिभाषित करें I ये chemical reactions की स्वतःप्रवृत्ति (spontaneity) से किस प्रकार
सम्बंधित हैं I]
Ans.:
Enthalpy: It is the heat content of the reacting system. It is defined as the sum of a system's
internal energy (E) and the product of pressure (P) and volume (V) and is denoted by H. Hence
H = E + PV.
Change in Enthalpy (ΔH)- The amount of heat released or absorbed by a chemical reaction at
constant pressure. Or it is the difference in bond energies of products and reactants.
When a reaction releases heat it is said to be an exothermic reaction and the heat content of the
products is less than that of reactants. Hence the change in enthalpy is negative. Likewise the
reactions absorbing heat are called endothermic and change in enthalpy would be positive. [ Δ
H = H (product) - H (reactant)].
Most of the spontaneous reactions at constant pressure are accompanied by evolution of heat
(exothermic) i.e. ΔH is negative but there are some spontaneous reactions which are
endothermic (ΔH - positive), therefore, enthalpy change can not be used as a criterion for
spontaneity.
[Enthalpy - यह reacting system की क
ु ल ऊष्मा (heat content) होती है I यह system की आंतरिक ऊर्जा
तथा pressure (P) एवं volume (V) क
े गुणनफल का क
ु ल योग होता है I इसे H द्वारा चिन्हित करते हैं I H = E
+ PV
Change in Enthalpy (ΔH) - यह एक constant pressure पर एक chemical reaction द्वारा release या
absorb की गई ऊष्मा होती है I या यह products और reactants की bond energies का अंतर होता है I
जब एक reaction में ऊष्मा (heat) release होती है तो उसे exothermic reaction कहते हैं और इसमें
products का heat content (क
ु ल ऊष्मा) reactants की क
ु ल ऊष्मा से कम होता है I इसलिए change in
enthalpy (Δ H) negative होती है I इसी प्रकार, जो reactions ऊष्मा अवशोषित करते हैं उन्हें endothermic
reactions कहते हैं और इनमें Δ H positive होती है {Δ H = H (product) - H (reactant)}
अधिकतर स्वतःप्रवर्तित (spontaneous) reactions exothermic होते हैं, यानि ΔH negative होता है किन्तु
क
ु छ spontaneous reactions endothermic भी होते हैं यानि, ΔH positive होता है I इसलिए enthalpy को
reaction क
े स्वतःप्रवृत्ति (spontaneity) क
े मानदंड क
े रूप में उपयोग नहीं किया जा सकता है I]
Entropy: It is a measure of randomness or disorder in a system. All chemical and biological
processes tend to progress towards a situation of maximum entropy (randomness). At
equilibrium the entropy is maximum. It is denoted by 'S'. e.g. entropy increases with change of
ice to water and to vapour.
As per 2nd law of thermodynamics, a change in a biochemical process will take place
spontaneously if it leads to an increase in randomness (entropy) in the universe (system +
surrounding). However, it is not always possible to use this criterion to predict the spontaneity of
a chemical reaction because (i) entropy changes are not easily measurable and (ii) it requires
measurement of entropy changes of the system as well as of its surroundings, which is difficult.
1

2. [Entropy - यह एक system में यादृच्छिकता (randomness) या अव्यवस्था (disorder) का माप होता है I सभी
chemical और biological प्रक्रियायें अधिकतम entropy (randomness) की स्थिति की ओर अग्रसित रहती हैं
और equilibrium पर entropy maximum होती है I इसे ‘S’ द्वारा चिन्हित करते हैं I उदाहरण - बर्फ से पानी और
भाप बनने क
े साथ entropy बढ़ती है I
Thermodynamics क
े दूसरे नियम क
े अनुसार एक biochemical प्रक्रिया में परिवर्तन तब होगा जब इसक
े
universe (system + surrounding) में randomness (entropy) बढ़ेगी I हालांकि, इस मानदंड को reaction
की स्वतःप्रवृत्ति (spontaneity) क
े पूर्वानुमान हेतु उपयोग नहीं किया जा सकता है क्योंकि (i) entropy change
को मापना आसान नहीं है, और (ii) इसक
े लिए system और surrounding दोनों क
े entropy changes को
मापने की आवश्यकता होगी जो कि मुश्किल है I]
Free energy: is the energy available in a substance to do work. It is denoted by 'G' ( Gibbs free
energy).
G = H - TS
H is enthalpy, S is entropy and T is temperature.
Change in free energy (∆G): is that form of energy capable of doing work under conditions of
constant temperature and pressure.
∆G = ∆H - T∆S
It can also be defined as that portion of total energy change which is available to do work as the
system proceeds towards equilibrium at constant temperature and pressure. The value of ΔG
depends on the free energy of the system after and before the chemical reaction. ΔG =
G(product) - G(reactant). When a reaction proceeds with release of free energy the free energy
change (ΔG) is negative and the reaction is called exergonic reaction. Reacting system that
takes up heat from the surrounding is endergonic and ΔG is positive.
[Free energy -किसी पदार्थ में काम करने हेतु उपलब्ध ऊर्जा को free energy कहते हैं I इसे ‘G’ द्वारा चिह्नित
करते हैं (Gibbs free energy)
G = H - TS
H enthalpy है, S entropy है और T temperature है I
Change in free energy (∆G) - यह energy का वह रूप है जो constant temperature और pressure पर
कार्य करने में सक्षम हो I
∆G = ∆H - T∆S
इसको इस प्रकार भी परिभाषित कर सकते हैं कि यह क
ु ल energy का वो भाग है जो कार्य करने हेतु उपलब्ध हो,
जब system constant temperature और pressure पर equilibrium की तरफ बढ़ता हो I ΔG की value
system की chemical reaction क
े पहले और बाद की free energy पर निर्भर करती है I
ΔG = G (product) - G (reactant)
2

3. जब reaction free energy क
े release क
े साथ आगे बढ़ता है तो free energy change (ΔG) negative होता
है और reaction exergonic reaction कहलाता है I जब reacting system surrounding से heat लेता है तो
वह endergonic कहलाता है और ΔG positive होता है I]
Standard free energy change (∆G°): is the difference between the sum of the free energies of
the products and the sum of the free energies of the reactants at standard state i.e. at a
concentration of 1M, temperature 25°C or 298K and pressure of 1 atmosphere. It can also be
defined as that amount of free energy absorbed or lost per mole when reactants are
transformed into products under standard conditions of concentration of 1M of both reactants
and products and at standard temperature and pressure. ΔG (not ∆G°) is used as a criterion for
a spontaneous reaction and it requires the change in free energy of the reacting system only. A
chemical reaction will occur spontaneously only if ∆G is negative.
[Standard free energy change (∆G°) - यह standard state अर्थात 1M concentration, 25°C या 298K
temperature और 1 atmosphere pressure पर products की क
ु ल free energy और reactants की क
ु ल
free energy का अंतर होता है I दूसरे शब्दों में, standard conditions यानि reactants और products दोनों क
े
1 M concentration और standard temperature एवं pressure पर जब reactants का products परिवर्तन
होता है तो प्रति mole free energy की अवशोषित या खोई हुई मात्रा standard free energy change (∆G°)
कहलाती है I ΔG (∆G° नहीं ) का ही उपयोग एक स्वतःप्रवर्तित (spontaneous) reaction क
े लिए एक मानदंड
क
े रूप में किया जाता है और इसक
े लिए क
े वल reacting system की free energy change की आवश्यकता
होती है I एक chemical reaction स्वतःप्रवर्तित (spontaneous) तभी होगा जब ∆G negative हो I]
Q. 2. What is the difference between ∆G and ∆G°?[∆G और ∆G° में क्या अंतर है?]
Ans.
● ∆G is free energy change and ∆G° is standard free energy change.
[∆G free energy change है और ∆G° standard free energy change है I]
● ∆G is free energy change at constant temperature and pressure while ∆G° is free energy
change at standard state( at a concentration of 1M, temp. 25°C or 298K and pressure of
1atm.).
[∆G constant temperature और pressure पर free energy change होता है जबकि ∆G°
standard state (1M concentration, temp. 25°C या 298K और 1 atm pressure) पर free
energy change होता है I]
● ∆G varies with concentration of reactants and products while ∆G° is a constant for a
given reaction.
[∆G reactants और products क
े concentration क
े अनुसार बदलता है जबकि ∆G° किसी reaction
क
े लिए constant होता है I]
● The criterion of spontaneity of a reaction is negative ∆G and not ∆G° A chemical
reaction will occur spontaneously only if ∆G is negative. A reaction with a positive ∆G°
can go in forward direction if ∆G is negative because actual ∆G depends on the
concentration of reactants and products and the temperature prevailing during the
reaction whereas the standard ∆G° is a constant value at initial concentration of each
component 1M, temp.25°C and pressure 1 atm.
[एक reaction क
े spontaneity का मानदंड negative ∆G होता है ∆G° नहीं I एक reaction तभी
स्वतःप्रवर्तित (spontaneous) होता है जब ∆G negative हो I एक reaction जिसका ∆G° positive हो,
वो भी forward direction में हो सकता है यदि ∆G negative हो क्योंकि actual ∆G reaction क
े दौरान
reactants और products क
े concentration और temperature पर निर्भर करता है जबकि standard
3

4. ∆G° reaction क
े प्रत्येक अवयव क
े प्रारंभिक 1M concentration, temp. 25°C या 298K और 1 atm
pressure पर एक constant value होती है I]
Q.3. What is bioenergetics? Define 1st and 2nd law of thermodynamics. [Bioenergetics
क्या है? Thermodynamics क
े 1st और 2nd law को परिभाषित करें ]
Ans. Bioenergetics is the quantitative study of energy relationships and energy conversions in
biological systems i.e. the study of generation of energy by the breakdown of nutrients, its
storage and utilization in performing various functions of the cells.
[Biological systems में ऊर्जा क
े सम्बंधों और परिवर्तनों क
े मात्रात्मक अध्ययन को bioenergetics कहते हैं
अर्थात कोशिकाओं क
े विभिन्न कार्यों क
े निष्पादन हेतु nutrients क
े breakdown से ऊर्जा क
े उत्पादन, इसक
े
भंडारण और उपयोग का अध्ययन bioenergetics कहलाता है I]
First law of thermodynamics: states that the total energy of a system (any portion of the
universe which we want to study) and its surroundings i.e. the total energy of the universe, is
constant although transformation of energy (heat into work or vice versa) may take place. It may
be transported from one region to another but it can not be created or destroyed. That is why it
is also referred to as the law of conservation of energy.
[ऊष्मागतिकी का प्रथम नियम कहता है कि एक system (universe का कोई हिस्सा जिसका हम अध्ययन करना
चाहते हैं) और इसक
े surroundings की क
ु ल energy constant रहती है अर्थात universe की क
ु ल ऊर्जा स्थिर
रहती है हालांकि इसका स्थानातरण एक क्षेत्र से दूसरे क्षेत्र में हो सकता है I दूसरे शब्दों में ऊर्जा को न तो बनाया जा
सकता है और न ही नष्ट किया जा सकता है, बल्कि इसे एक रूप से दूसरे रूप में बदला जा सकता है (जैसे - ऊष्मा
को कार्य में और कार्य को ऊष्मा में)। इसीलिए इसे ऊर्जा संरक्षण क
े नियम क
े रूप में भी जाना जाता है I]
Second law of thermodynamics: states that in all processes the entropy of the system plus
the surroundings i.e. the entropy of the universe always increases until equilibrium is attained at
which point the entropy is the maximum possible under the prevailing conditions of temperature
and pressure.
[ऊष्मागतिकी का द्वितीय नियम - कहता है कि सभी प्रक्रियाओं में system और surrounding की क
ु ल entropy
अर्थात universe की entropy तब तक बढ़ती है जब तक प्रक्रिया का equilibrium न पहुँच जाए और इस
equilibrium पर temperature और pressure की मौजूदा स्थिति में entropy अधिकतम होती है I
Q.4. What are coupled reactions and what is their advantage?[युग्मित (coupled) reactions
क्या होते हैं और इनक
े क्या लाभ हैं?]
Ans. The endergonic reactions are not spontaneous and are thermodynamically unfavorable but
such reactions can be made thermodynamically favourable(∆G <0) if these are coupled to some
exergonic reactions. Such reactions are called coupled reactions.
The advantage of coupled reactions is based on the additive property of ∆G. The coupling of an
unfavorable reaction makes the unfavorable reaction to proceed. Example: The first step of
glycolysis.
Glucose + Pi ---------> Gl - 6 - P ∆G°' + 13.8 kj per mole
It is an endergonic reaction and would not be spontaneous. When this reaction is coupled to a
highly exergonic reaction :
ATP + H2O -----> ADP + Pi + H+ (∆G°' - 30.5 kj per mole)
The overall reaction becomes:
ATP + Glucose -----> Gl - 6 - P + ADP (∆G°' - 17.2 kj per mole)
Thus ∆G°' of overall reaction is negative and so the reaction would proceed.
4

5. The favorable reaction either precedes (to provide reactants for the unfavorable reaction) or
follows the unfavorable reaction (to remove the product of the unfavorable reaction
continuously) to pull the unfavorable reaction to completion.
[Endergonic reactions spontaneous नहीं होते और thermodynamically unfavorable होते हैं लेकिन ऐसे
reactions को thermodynamically favourable (यानि ∆G <0) बनाया जा सकता है अगर इनको कोई
exergonic reactions क
े साथ युग्मित (coupled) कर दिया जाए I ऐसे reactions को coupled reactions
कहते हैं I
Coupled reactions का लाभ ∆G क
े योगात्मक गुण (additive property) पर आधारित होता है I
Unfavourable reaction की coupling उसे आगे बढ़ने में सहायता करती है I उदाहरण : glycolysis.का पहला
चरण -
Glucose + Pi ---------> Gl - 6 - P (∆G°' + 13.8 kj per mole)
यह एक endergonic reaction है और spontaneous नहीं होता I किन्तु जब यह reaction एक अत्यधिक
exergonic reaction {ATP + H2O -----> ADP + Pi + H+ (∆G°' - 30.5 kj per mole)} क
े साथ युग्मित
(couple) होता है तो reaction का ∆G°’ क
ु ल मिलाकर negative हो जाता है और reaction आगे बढ़ पाता है :
Final reaction -
ATP + Glucose -----> Gl - 6 - P + ADP (∆G°' - 17.2 kj per mole)
Favorable reaction unfavorable reaction क
े पहले हो सकता है ताकि unfavorable reaction क
े लिए
reactant से उपलब्ध हो सक
े या फिर unfavorable reaction क
े बाद में होता है ताकि unfavorable reaction क
े
products को लगातार हटाकर reaction को पूरा होने दे I]
Q.5. Predict the direction of the following reaction at pH 7.0 and 298 K if the
concentration of each reaction partner is 1M.[इस reaction की दिशा (direction) का pH 7.0 और
298 K पर पूर्वानुमान लगाएं यदि प्रत्येक प्रतिक्रिया भागीदार (reaction partner) का concentration 1M है
I]
ATP + Glucose --------> Gl - 6 - P + ADP
Given ∆G°' for Gl - 6 - P formation is + 13.8 and for ATP —> ADP is - 30.5
Ans.
For a spontaneous reaction the ∆G°' should be -ve and in reaction Glucose + Pi -----> Gl - 6 - P
+ H2O the ∆G°' is + 13.8 (Here ∆G = ∆G°' because pH is 7.0 , concentration of reactants is 1M
and temperature 298 K). Therefore, when the above reaction is coupled with reaction ATP +
H2O ------> ADP + Pi; ∆G°' = - 30.5, the ∆G°’ of the overall becomes - 16.7.
ATP + H2O ------> ADP + Pi; ∆G°' = - 30.5
Glucose + Pi -----> Gl - 6 - P + H2O ; ∆G°' = + 13.8
Sum of these two reactions:
Glucose + ATP ----> Gl - 6 - P + ADP; ∆G°' = - 16.7 [- 30.5 - (+13.8) = - 16.7]
Since the overall ∆G°' is negative and there is a large energy drop the reaction will proceed from
left to right and irreversibly.
[एक spontaneous reaction क
े लिए ∆G°' -ve होना आवश्यक है और Glucose + Pi -----> Gl - 6 - P +
H2O reaction में ∆G°' + 13.8 है (यहाँ ∆G = ∆G°' क्योंकि pH 7.0 है, reactants का concentration 1M है
और temperature 298 K है) इसलिए जब यह reaction ATP + H2O ------> ADP + Pi; ∆G°' = - 30.5 से
युग्मित (couple) होता है तो दोनों reaction का क
ु ल ∆G°’ - 16.7 हो जाता है I अब चूंकि ∆G°’ negative है तो
energy में बड़ी कमी आने क
े कारण reaction बाई ओर से दाहिनी ओर बढ़ता है और अपरिवर्तनीय
(irreversibly) ढंग से I
क
ु ल reaction :
5

6. Glucose + ATP ----> Gl - 6 - P + ADP; ∆G°' = - 16.7 [- 30.5 - (+13.8) = - 16.7]
Q.6. Write a short note on conventions in biochemical energetics.[Biochemical
energetics क
े conventions पर एक short note लिखें I]
Ans. Two conventions in biochemical energetics are very important:
1. Biochemical reactions take place at a pH of about 7.0 i.e. [H+] = 10−7
M rather than pH 0.0 i.e.
[H+] = 1.0 M as normally used in physical chemistry. Therefore, a different standard state is
adopted for biochemical reactions in which the [H+] is 10−7
M(pH 7.0) while that of all other
species is 1M(non-biochemical reactions ). The standard free energy change at pH 7.0 is
designated by ∆G°'(delta G° prime) while at pH 0.0 by ∆G°.
For reactions releasing H ions ∆G° is greater than delta G°' by about 40 kJ/mol and those
involving H ions have ∆G° is less than ∆G°' by about 40 kJ/mol. ∆G° = ∆G°' for reactions not
involving H ions.
2. The activity of water molecules in aqueous solutions is arbitrarily set at 1 M although their
concentration is about 55M.
[Biochemical energetics में दो conventions बहुत महत्वपूर्ण हैं :
1. Biochemical reactions होते हैं लगभग pH 7.0 यानि [H+] = 10−7
M पर जबकि physical chemistry में
सामान्य तौर पर pH 0.0 यानि [H+] = 1.0 M का इस्तेमाल होता है I इसलिए biochemical reactions क
े लिए
एक भिन्न standard state को अपनाया गया है जिसमें [H+] 10−7
M (pH 7.0) है जबकि non-biochemical
reactions क
े लिए यह 1M है I pH 7.0 पर standard free energy change को ∆G°'(delta G° prime) द्वारा
निर्दिष्ट करते हैं और pH 0.0 पर इसे ∆G° द्वारा निर्दिष्ट करते हैं I
जिन reactions में H ions release होते हैं उनमें ∆G° ∆G°' से लगभग 40 kJ/mol अधिक होता है और जिन
reactions में H ions शामिल होते हैं उनमें ∆G° ∆G°' से लगभग 40 kJ/mol कम होता है I जिनमें है ions
शामिल ही नहीं होते उनमें ∆G° और ∆G°' बराबर होते हैं (∆G° = ∆G°’) I
2. Aqueous solutions में water की activity स्वेच्छन्दता से (arbitrarily) 1M निर्धारित की गई है जबकि
इसका concentration लगभग 55 M होता है I]
Q.7. Why is ATP considered as energy currency of a cell?[ATP को cell का energy currency
क्यों कहते हैं I]
Ans. ATP is considered as energy currency of a cell because all energy received by the cells, be
it chemical energy of the nutrients or light energy during photosynthesis, is converted into ATP
before it can be utilized for various energy requirements of the cells. In this way ATP acts as an
energy currency just like we have to get our money changed into the local currency of a country
we visit before using it for various purchases in that country. Thus the living cells convert the
energy obtained from external sources into ATP which is then utilized by the cells for different
cell functions as and when required.
[ATP को cells का energy currency इसलिए माना जाता है क्योंकि cells द्वारा प्राप्त सारी ऊर्जा (nutrients
की रासायनिक ऊर्जा या प्रकाश संश्लेषण क
े दौरान प्रकाश ऊर्जा) ATP में बदल जाती है और यही ATP cells की
विभिन्न ऊर्जा अवश्यकताओं की पूर्ति करती है I इसप्रकार ATP cells में एक energy currency क
े रूप में ठीक
उसी प्रकार काम करती है जिस प्रकार किसी दूसरे देश में जाने पर हमें वहाँ क
ु छ भी खरीदने हेतु अपने धन को उस
देश की currency में बदलना पड़ता है I इसप्रकार जीवित कोशिकाएं वाह्य स्रोतों से प्राप्त ऊर्जा को ATP में बदल
देती हैं जो कोशिकाओं द्वारा विभिन्न कार्यों हेतु अवश्यकतानुसार उपयोग होती है I]
6

7. Q.8. What are high energy phosphate compounds? Give some examples.[High energy
phosphate compounds क्या होते हैं? क
ु छ उदाहरण दें I]
Ans. A phosphate compound is referred to as a high energy phosphate compound if its standard
free energy of hydrolysis is negative and numerically larger than 29.2 kJ per mole. e.g. ATP,
ADP, phosphoenolpyruvate, phosphocreatine, acetyl phosphate etc. Such compounds have a
higher tendency to transfer its terminal phosphate group. Higher the ∆G°' value higher will be
their phosphate group transfer potential.
[किसी phosphate compound को एक high energy phosphate compound तब कहा जाता है जब उसक
े
hydrolysis की standard free energy negative हो और 29.2 kJ per mole से अधिक हो I जैसे - ATP, ADP,
phosphoenolpyruvate, phosphocreatine, acetyl phosphate आदि I ऐसे compounds की अपने
phosphate group को transfer करने की प्रवृत्ति अधिक होती है I ∆G°’ की value जितनी अधिक होती है उतनी
ही अधिक compound की phosphate group को transfer करने की सामर्थ्य होती है I]
Compound ∆G°’ (kJ per mole)
ATP - 31.38/- 30.5
ADP - 30.5
Phosphoenolpyruvate - 61.92
Phosphocreatine - 43.1
Acetyl phosphate - 43.1
Q. 9. Why is the ATP/ADP pair as the vehicle of free energy and not ATP/AMP?[Free
energy क
े vehicle क
े रूप में ATP/AMP pair की जगह ATP/ADP का pair का इस्तेमाल क्यों होता है?]
Ans. Hydrolysis of ATP to AMP gives a molecule of PPi which gets further hydrolysed to two
inorganic phosphate ions and thus giving a large amount of free energy making the reaction
almost irreversible. This would make the regeneration of ATP from AMP very difficult(as it would
require a large amount of free energy) as compared to regeneration of ATP from ADP which
would require a moderate amount of free energy.
1. ATP + H2O -----> ADP + Pi (∆G°' - 30.5 kJ/mol)
2. ATP + H2O ------> AMP + PPi (∆G°' - 31.38 kJ/mol)
3. PPi + H2O -------> 2 Pi (∆G°' - 33.0 kJ/mol)
Adding reaction 2 and 3 would give ∆G°' - 64.38 kJ/mol
[ATP की AMP में hydrolysis होने पर PPi का एक molecule बनता है जो फिर पुनः hydrolyzed होकर
inorganic phosphate ions देता है और इस प्रकार अत्याधिक free energy release होती है जिससे reaction
लगभग irreversible हो जाता है I इस कारण AMP से ATP का पुनरुत्पादन मुश्किल होगा क्योंकि इसक
े लिए
अधिक मात्रा में free energy की आवश्यकता होगी I जबकि ADP से ATP क
े पुनरुत्पादन में कम free energy
की आवश्यकता होगी I इसीलिए Free energy क
े vehicle क
े रूप में ATP/AMP pair की जगह ATP/ADP का
pair का इस्तेमाल होता है I]
1. ATP + H2O -----> ADP + Pi (∆G°' = - 30.5 kJ/mol)
2. ATP + H2O ------> AMP + PPi (∆G°' = - 31.38 kJ/mol)
3. PPi + H2O -------> 2 Pi (∆G°' = - 33.0 kJ/mol)
Reaction 2 और 3 का क
ु ल ∆G°' = - 64.38 kJ/mol
7

8. Q.10. Differentiate between substrate level phosphorylation and oxidative
phosphorylation with example.[Substrate level phosphorylation और oxidative
phosphorylation में उदाहरण देकर अंतर बतायें I]
Ans.
● In substrate level phosphorylation the phosphate group is directly removed from a
substrate and transferred to ADP to form ATP whereas in oxidative phosphorylation the
ATP is generated from the oxidation of NADH and FADH2 and transfer of electrons to
molecular oxygen and pumping of protons and the action of ATP synthase.
[Substrate level phosphorylation में एक substrate से phosphate group सीधे हटा कर ADP को
स्थानांतरित (transfer) कर दिया जाता है, जबकि oxidative phosphorylation में ATP उत्पन्न होता
है NADH और FADH2 क
े oxidation तथा molecular oxygen को electrons transfer होने और
protons की pumping एवं ATP synthase क
े action द्वारा I]
● Substrate level phosphorylation occurs in cytosol while oxidative phosphorylation occurs
in mitochondria.
[Substrate level phosphorylation cytosol में होता है जबकि oxidative phosphorylation
mitochondria में ]
● Substrate level phosphorylation is an anaerobic process whereas oxidative
phosphorylation requires oxygen.
[Substrate level phosphorylation एक anaerobic प्रक्रिया है जबकि oxidative phosphorylation
क
े लिए oxygen की आवश्यकता (aerobic प्रक्रिया) होती है I]
● Example:
Substrate level phosphorylation:
Conversion of phosphoenolpyruvate(PEP) to pyruvate. The phosphate group of
PEP is transferred to ADP due to higher phosphate group transfer potential of PEP as
compared to that of ADP.
PEP + ADP ----> Pyruvate + ATP
Oxidative phosphorylation:
Reduced coenzymes NADH and FADH2 are oxidised by a series of reactions called
electron transfer chain with oxygen as the ultimate acceptor of electrons. This electron
transfer is coupled to the synthesis of ATP from ADP and Pi in presence of ATP
synthase and proton gradient across the mitochondrial membrane.
[Examples:
Substrate level phosphorylation : Phosphoenolpyruvate (PEP) का pyruvate में परिवर्तन -
PEP का phosphate group ADP को transfer होता है क्योंकि PEP का phosphate group को
transfer करने का सामर्थ्य (potential) ADP की अपेक्षा अधिक होता है I
PEP + ADP ----> Pyruvate + ATP
Oxidative phosphorylation: Reduced coenzymes NADH और FADH2 का oxidation
reactions की एक श्रृंखला (electron transfer chain) द्वारा होता है जिसमें oxygen electrons का
अंतिम acceptor होता है I Electrons का यह transfer ATP क
े संश्लेषण से युग्मित (coupled) रहता
है जो ADP, Pi, ATP synthase enzyme और mitochondrial membrane क
े आर - पार proton क
े
gradient की मदद से होता है I]
8

9. Q.11. What are uncouplers? How do they act? Give an example.[Uncouplers क्या होते हैं?एक
उदाहरण भी दें I]
Ans. Uncouplers are compounds which inhibit the coupling between the electron transport and
phosphorylation reactions and thus inhibit ATP synthesis without affecting the respiratory chain
and ATP synthase. Such substances, presumably attached to the inner mitochondrial
membrane, provide an alternative and more facile route for the protons to flow back into the
matrix instead of going through the ATP synthase and thereby collapsing the proton motive
force that the cell uses to synthesize most of its ATP. However, electron transport proceeds
unhindered but ATP is not synthesized and thus free energy released on electron transport is
dissipated as heat. In other words these uncouplers short circuit mitochondrial ATP production.
Example: The hibernating animals, some newborn animals and cold adapted mammals have
thermogenin protein (physiological uncoupler) in their brown adipose tissue's mitochondrial
membrane which serves as uncoupler and provides an alternative route for the back flow of
protons into the mitochondrial matrix and thus dissipating the free energy in the form of heat
during electron transport and help in keeping the body warm in cold conditions. Another
example is 2,4-dinitrophenol (chemical uncoupler) which was used for weight loss but now it is
not used due to some serious side effects.
[Uncouplers वो compounds होते हैं जो electron transport और phosphorylation reactions क
े बीच
युग्मन (coupling) को रोकते (inhibit) हैं और इस प्रकार ATP क
े संश्लेषण को रोकते हैं लेकिन respiratory
chain और ATP synthase को प्रभावित किये बिना I दूसरे शब्दों में, ये uncouplers mitochondria द्वारा ATP
क
े उत्पादन की प्रक्रिया को शार्ट सर्कि ट कर देते हैं I उदाहरण : शीतनिंद्रा (hibernating) में रहने वाले जानवर, क
ु छ
नवजात जानवरों और ठंड अनुक
ू लित (cold adapted) mammals में उनक
े brown adipose tissues में
thermogenin protein (physiological uncoupler) होती है जो uncoupler का काम करती हैं और protons क
े
mitochondrial matrix में वापस जाने का एक वैकल्पिक मार्ग प्रदान करती हैं और इस प्रकार electron
transport क
े दौरान free energy ऊष्मा क
े रूप में अपव्यय (dissipate) होती है जो ठंडी परिस्थितियों में शरीर
को गर्म रखने में मदद करती है I एक अन्य उदाहरण है 2,4-dinitrophenol (chemical uncoupler) जिसका
इस्तेमाल weight loss हेतु किया जाता था किन्तु इसक
े क
ु छ गंभीर side effects क
े कारण अब इसका इस्तेमाल
नहीं किया जाता है I].
Q.12. How is ATP hydrolysed in the myosin-ATP complex? Explain with the help of a
suitable diagram.[Myosin-ATP complex में ATP क
ै से hydrolyze होता है? एक चित्र द्वारा समझायें I]
Ans. The binding of ATP to actin-myosin complex dissociates the complex leading to breakdown
of the contact between actin and myosin. This results in formation of myosin-ATP complex
which is acted upon by ATPase of myosin resulting in the formation of myosin-ADP + Pi
complex. This complex binds to actin again as myosin-ADP + Pi complex has high binding
affinity for actin. This accelerates the dissociation of ADP and Pi from myosin and actin-myosin
Complex forms again. All these changes bring about the contraction of muscles.
[ATP की actin-myosin complex से binding होने से इस complex का पृथक्करण हो जाता है जिससे actin
और myosin क
े बीच संपर्क भी टूट जाता है I इस कारण myosin-ATP complex बनता है जिसपर myosin क
े
ATPase की कार्रवाई से myosin-ADP + Pi complex बनता है I यह complex पुनः actin से bind करता है
(actin क
े प्रति अधिक binding affiniy क
े कारण) I इससे ADP और Pi का myosin से पृथक्करण बढ़ जाता है
तथा actin-myosin complex पुनः बन जाता है I इन सभी परिवर्तनों क
े कारण muscles का contraction होता
है I]
9

10. Q.13. Write a note on muscle contraction.[Muscle contraction पर एक note लिखें I
Ans. The contractile elements of muscle cells are called myofibrils which consist of thick and
thin filaments arranged regularly parallel to each other in repeated units called sarcomeres.
These are the contractile units of myofibrils. The thick filaments are made up of bundles of
parallel rod shapped molecules of myosin with projecting "head" structures which are in contact
with thin filaments consisting of two strands of fibrous actin twisted around each other.
Contraction is initiated by release of Ca++ from vesicles of the sarcoplasmic reticulum
(membrane-bound structure in muscle cells) following excitation of the sarcolemma (muscle
plasma membrane). During contraction the thick filaments slide into the spaces between the thin
filaments in each sarcomere, thereby causing a shortening of the entire muscle fibre. This is
followed by binding of ATP to actomyosin complex resulting in breaking of the contact between
actin and myosin and myosin-ATP complex is formed. This myosin-ATP complex is broken
down by ATPase present in myosin head resulting in myosin-ADP-Pi complex. This complex
binds to actin resulting in a rowing type action with the myosin head acting as an oar which is
responsible for the sliding motion of the thick filament (power stroke). Relaxation is produced by
removal of Ca++ from the sarcoplasm and it's segregation in the sarcoplasmic reticulum by ATP
dependent transport process.
[Muscles क
े contractile elements को myofibrils कहते हैं जिसमें thick और thin filaments नियमित रूप से
एक दूसरे क
े समानांतर पुनरावृत्त इकाइयों, जिन्हें sarcomeres कहते हैं, क
े रूप में व्यवस्थित रहते हैं I ये
myofibrils की contractile units होती हैं I Thick filaments rod shape क
े myosin molecules होते हैं जो
समानांतर रूप से bundles क
े रूप में होते हैं I इन thick filaments में बाहर की ओर निकले हुए "head"
structures होते हैं जो thin filaments क
े संपर्क में रहते हैं I Thin filaments आपस में ऐंठे हुए दो fibrous actin
strands से बने होते हैं I Muscle क
े Contraction की शुरूआत होती है sarcolemma (muscle plasma
membrane) क
े excitation क
े फलस्वरुप sarcoplasmic reticulum (membrane-bound structure in
muscle cells) क
े vesicles से calcium ions (Ca++) क
े release होने से I Contraction क
े दौरान प्रत्येक
sarcomere में thick filaments (myosin) thin filaments (actin) क
े बीच क
े spaces में slide करते हैं जिसक
े
कारण पूरे muscle fibre की लम्बाई कम हो जाती है I इसक
े बाद इस actomyosin complex से ATP bind
करता है जिससे actin और myosin क
े बीच का संपर्क टूट जाता है और myosin-ATP complex बन जाता है I
इस myosin-ATP complex को myosin क
े head में उपस्थित ATPase enzyme तोड़ देता है जिससे
myosin-ADP-Pi complex बनता है I यह complex actin से bind करता है जिसक
े कारण एक नौकायन की
10

11. तरह की क्रिया होती है जिसमें myosin head एक चप्पू या पतवार का काम करता है जिसक
े कारण thick
filament का sliding motion होता है (power stroke)I Muscles का relaxation होता है sarcoplasm से
Ca++ क
े हटने और एक ATP पर निर्भर transport प्रक्रिया द्वारा इनक
े sarcoplasmic reticulum में
पृथक्करण क
े साथ I
Q.14. Describe the chemiosmotic hypothesis - the mechanism of oxidative
phosphorylation.[Chemiosmotic hypothesis (mechanism of oxidative phosphorylation)
का वर्णन करें I]
Ans. According to chemiosmotic hypothesis, proposed by Peter Mitchell, as the electrons flow
through through Complexes I, Ill & IV of the electron transport system (during oxidation of NADH
and FADH2) free energy is released which is utilized by electron carriers to transport H+ or
protons from the mitochondrial matrix through inner mitochondrial membrane to outside the
inner membrane to intermembrane space making the matrix alkaline relative to intermembrane
space. This pumping of electrons is unidirectional due to the unique vectorial location of electron
transport proteins across the membrane. The inner mitochondrial membrane is not freely
permeable to protons and this creates a pH gradient due to difference in [H+] and a potential
difference (due to difference in positive charge) across the membrane. These two gradients
result in an energized state of the membrane called 'Proton Motive Force' which provides
energy necessary for the synthesis of ATP from ADP and Pi in presence of enzyme assembly
ATP synthase. The enzyme ATP synthase is located in the inner mitochondrial membrane and
it provides a channel for the flow of excess protons back to the mitochondrial matrix through the
otherwise impermeable inner membrane.
[Peter Mitchell द्वारा प्रस्तावित chemiosmotic hypothesis क
े अनुसार NADH और FADH2 क
े oxidation
क
े दौरान जब electron transport system क
े Complexes l, lll एवं lV क
े माध्यम से electrons flow करते हैं
तो free energy release होती है जो electron carriers द्वारा mitochondrial matrix से H+ या protons क
े
inner mitochondrial membrane क
े रास्ते intermembrane space में transport करने हेतु इस्तेमाल होती है
I Protons (H+) की इस प्रकार pumping mitochondrial matrix को intermembrane space की तुलना में
alkaline बना देती है I Electrons की यह pumping, electron transport proteins की membrane क
े आर -
पार एक अनोखी सदिशीय स्थिति (vectorial location) होने क
े कारण, एकदिशीय (unidirectional) होती है I
Mitochondria की inner membrane protons क
े लिए स्वतंत्र रूप से पारगम्य (freely permeable) नहीं होती
है और इस कारण membrane क
े आर - पार, [H+] में अंतर क
े कारण एक pH gradient और positive charge
में अंतर क
े कारण एक potential difference बन जाता है I ये दो gradients membrane को एक energised
state प्रदान करते हैं जिसे 'Proton Motive Force' कहते हैं और यह force ATP synthase enzyme
assembly की उपस्थिति में ADP और Pi से ATP क
े संश्लेषण हेतु आवश्यक energy प्रदान करता है I Enzyme
11

12. ATP synthase inner mitochondrial membrane में स्थित होता है और यह अतिरिक्त protons क
े अभेद्य
(impermeable) inner mitochondrial membrane क
े रास्ते mitochondrial matrix में वापस बहाव हेतु एक
मार्ग प्रदान करता है I]
Q.15. Standard free energy change alone can not predict the direction of a biochemical
reaction. Comment.[क
े वल Standard free energy change द्वारा ही एक biochemical reaction की
दिशा का पूर्वानुमान नहीं लगाया जा सकता, इस पर टिप्पणी करें I]
Ans. The standard free energy change (ΔG°') is a constant for a given biochemical reaction as it
is the free energy change at standard state i.e. at the initial concentration of each component
1M, pH 7.0, temperature
25°C (298 K) and pressure 1 atm. Whereas the actual free energy change for a given
biochemical reaction (ΔG) is a function of reactant and product concentrations and temperature
prevailing during the reaction and it is related to ΔG°' by the following equation :
ΔG =ΔG°' + RT In ([product]/[reactant])
Or
ΔG =ΔG°'+ 2.303 RT log([product]/[reactant])
where R is gas constant (8.314 J/K/mol) and T is temperature (298K)
The criterion for spontaneity of a biochemical reaction is ΔG and not ΔG°' and the reaction can
go in the forward direction only if ΔG is negative. However, a reaction with a positive ΔG°' can
also go in the forward direction. This is possible if the term RT In ([product]/[reactant]) is
negative and has a larger absolute value than ΔG°'. This could be possible by immediate
removal of the products of a reaction(i.e. by changing the concentration of products and
reactants) which would keep the ratio [product]/[reactant] well below 1, thereby the term RT In
([product]/[reactant]) would be large and negative as compared to ΔG°' and so the value of ΔG
becomes negative using this equation. In this way a nonspontaneous reaction becomes
spontaneous. This happens many times in living cell depending on the requirements of the cell
metabolism. ΔG°' will be able to indicate the direction of a biochemical reaction only under the
conditions when its value is similar to ΔG. Therefore, ΔG°' alone cannot predict the direction of a
biochemical reaction, it is the ΔG which can always predict the direction of a biochemical
reaction.
[किसी biochemical reaction क
े लिए standard free energy change (ΔG°') एक constant होता है क्योंकि
यह standard state में free energy change होता है, यानि जब इसक
े प्रत्येक component का प्रारंभिक
(initial) concentration 1M, pH 7.0, temperature 25°C (298 K) और pressure 1 atm हो I जबकि किसी
biochemical reaction का actual free energy change (ΔG) निर्भर करता है reaction क
े दौरान reactant
और product क
े concentration और temperature पर और यह ΔG°' से निम्न equation द्वारा सम्बद्ध
रहता है :
12

13. ΔG = ΔG°' + RT In ([product]/[reactant]) या ΔG = ΔG°'+ 2.303 RT log ([product]/[reactant])
जिसमें R gas constant (8.314 J/K/mol) और T temperature (298K) है I
एक biochemical reaction की spontaneity का मानदंड ΔG होता है, ΔG°' नहीं और reaction forward
direction में तभी बढ़ता है जब ΔG negative हो I हालांकि, एक reaction जिसका ΔG°' positive हो वह भी
forward direction में बढ़ सकता है I यह तभी संभव होता है term RT In ([product]/[reactant]) negative हो
और इसका निरपेक्ष मान (absolute value) ΔG°' से अधिक हो I यह तभी संभव हो सकता है जब reaction में
बनने वाले products को तुरंत हटा दिया जाए (यानि products और reactants का concentration परिवर्तन
करक
े )जिससे [product]/[reactant] ratio 1 से काफ़ी कम रहे I इसक
े परिणाम स्वरुप term RT In
([product]/[reactant]) का मान ΔG°’ से अधिक और negative होगा, इसलिए ΔG भी इस equation में
negative होगा और इस प्रकार एक nonspontaneous reaction spontaneous बन जायगा I ऐसा living
cells में cell की metabolism की आवश्यकतानुसार बहुत बार होता है I ΔG°' एक biochemical reaction की
दिशा क
े वल उन परिस्थितियों में दर्शा सकता है जब इसका मान ΔG क
े समान हो I इसीलिए ΔG°' अक
े ले एक
biochemical reaction की दिशा का पूर्वानुमान नहीं कर सकता बल्कि यह ΔG ही है जो हमेशा एक
biochemical reaction की दिशा का पूर्वानुमान कर सकता है I]
Q. 16. Fill in the blanks:
(i) A system which permits the exchange of matter as well as energy with its surroundings is
called _____ system.
(ii) A system which allows the flow of energy but not of matter is called a ____ system
(iii) A system which does not permit the exchange of either energy or matter is called an
_______ system.
(iv) Biological organisms are _____ systems.
(v) Human beings are _______ systems.
(vi) The portion of the universe which we intend to study is called a _____.
(vii) The system and its surroundings constitute the _______.
(viii) The first law of thermodynamics is also referred to as the law of _________ of ______.
(ix) The reactions which are accompanied by the release of energy are called ________
reactions.
(x) The chemical reaction in which the heat is absorbed is called an ________ reaction.
(xi) The temperature of the surroundings ______ in an exothermic reaction.
(xii) The temperature of the surroundings ______ in an endothermic reaction.
(xiii) For a spontaneous reaction the ∆G should be _____.
(xiv) In a chemical reaction the entropy is maximum at ________.
(xv) The contractile units of myofibrils are called _______.
(xvi) Reaction PEP + ADP ----> Pyruvate + ATP is an example of _______ ____
phosphorylation.
(xvii) Oxidation of NADH and FADH2 coupled with synthesis of ATP from ADP and Pi through
electron transport system is an example of _______ phosphorylation.
(xviii) The thick filaments of muscles are composed of _______ protein and thin filaments
composed of ______ protein.
(xix) Chemiosmotic hypothesis is for ______ phosphorylation.
(xx) _____ ions are required for muscle contraction.
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14. Ans. (i) open, (ii) closed, (iii) isolated system, (iv) open, (v) open, (vi) system, (vii) universe, (viii)
conservation, energy, (ix) exothermic, (x) endothermic, (xi) increases, (xii) decreases, (xiii)
negative, (xiv) equilibrium, (xv) sarcomeres, (xvi) substrate level, (xvii) oxidative, (xviii) myosin,
actin, (xix) oxidative, (xx) calcium
REFERENCES:
1. IGNOU, CHE - 9 Biochemistry, Block 3
2. Lehninger Principles of biochemistry, seventh edition ; David L. Nelson & Michael M. Cox.
Disclaimer : The pictures given in the text have been downloaded from Google images and I am
thankful to the persons who have uploaded these pictures.
Dr. P. K. Nigam (Retired Biochemist)
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