Cell Respiration

1. Presented by
Z.Kathambari Barjana
18ZOO15
III - Zoology
E.M.G.Yadava Women's College,
Madurai.
RESPIRATION
E.M.G. Yadava Women’s College, Madurai-625014
An Autonomous Institution – Affiliated to Madurai Kamaraj University
Re-accredited3rd Cycle with Grade A+ & CGPA 3.51 by NAAC
Dr.V.Vijaya
Assistant Professor of Botany
E.M.G. Yadava Women's College
Thirupplai, Madurai-14

2. RESPIRATION
•Respiration is the biological oxidation in which organic compounds are oxidised into Carbon dioxide, and
water with the release of energy
•It occurs in the cytoplasm and mitochondria of plant cells.
•It is also called cellular respiration or biological oxidation.
• It is a catabolism.
•The main aim of respiration is to release energy.
•The organic compounds utilized in respiration are called respiratory substrates.
•The respiratory substrates include
✓ Glucose
✓ Amino acids
✓ Glycerol and
✓ Fatty acids
•Respiration involves the following steps.
1. Glycolysis
2. Oxidative decarboxylation
3. Krebs cycle
4. Electron transport and oxidative phosphorylation.

3. CELLULAR RESPIRATION
•In respiration, cellular substances (carbohydrates, proteins and lipids) are oxidized in the presence of
oxygen to release energy.
•Therefore, it is also known as cellular oxidation.
•It is purely a biological process.
•It releases metabolic energy as heat, which in turn is stored in the form of ATP.
FLOATING RESPIRATION
•Respiration that utilizes carbohydrates and fats as the respiratory substrates is called floating
respiration.
PROTOPLASMIC RESPIRATION
•Proteins are used only in the absence of carbohydrates and lipids.
•Respiration that uses proteins as respiratory substrates is called protoplasmic respiration.
TYPES OF RESPIRATION

4. •Respiration is the biological oxidation.
•It occurs inside the cell. So it is also called cellular respiration.
•Anaerobic respiration occurs inside the cytosol of cytoplasm.
•Fermentation also occurs inside the cytoplasm
•Aerobic respiration occurs both in the cytoplasm and mitochondria
•The first phase of aerobic respiration is glycolysis. It occurs in the
cytoplasm.
•Krebs cycle occurs in the matrix of the mitochondria
•Electron transport occurs in the inner membrane of mitochondria
•Oxidative phosphorylation occurs in the F1 particles.
SITE OF RESPIRATION

5. GLYCOLYSIS
•Glycolysis is the central pathway for the glucose catabolism in which glucose (6-carbon compound) is converted into
pyruvate (3-carbon compound) through a sequence of 10 steps.
•Glycolysis takes place in both aerobic and anaerobic organisms and is the first step towards the
metabolism of glucose.
•The glycolytic sequence of reactions differs from one species to the other in the mechanism of its
regulation and the subsequent metabolic fate of the pyruvate formed at the end of the process.
•In aerobic organisms, glycolysis is the prelude to the citric acid cycle and the electron transport chain,
which together release most of the energy contained in glucose.
•It is also referred to as Embden-Meyerhof-Parnas or EMP pathway, in honor of the pioneer workers in
the field.

6. SITE OF GLYCOLYSIS
• Glycolysis takes place in the cytoplasm of virtually all the cells of the body.
There are two types of glycolysis.
1.Aerobic Glycolysis:From the word aerobic, meaning with the presence of oxygen. It occurs when oxygen
is sufficient. Final product is pyruvate along with the production of eight ATP molecules.
2.Anaerobic Glycolysis: This type of glycolysis takes place in the absence of oxygen. Final product is
lactate along with the production of two ATP molecules.
TYPES OF GLYCOLYSIS

7. •In most kinds of cells, the enzymes that catalyze glycolytic reactions are
present in the extra-mitochondrial fraction of the cell in the cytosol.
•One common characteristic in all the enzymes involved in glycolysis is
that nearly all of them require Mg2+.
•The following are the enzymes that catalyze different steps throughout
the process of glycolysis
1.Hexokinase
2.Phosphoglucoisomerase
3.Phosphofructokinase
4.Aldolase
5.Phosphotriose isomerase
6.Glyceraldehyde 3-phosphate dehydrogenase
7.Phosphoglycerate kinase
8.Phosphoglycerate mutase
9.Enolase
10.Pyruvate kinase
GLYCOLYSIS ENZYMES

8. GLYCOLYSIS

9. The glycolytic pathway can be divided into two phases
1.Preparatory Phase/Glucose Activation Phase
2. Payoff Phase/Energy Extraction Phase
•This phase is also called energy extraction phase. During this phase, conversion of glyceraldehyde-3-phophate to
pyruvate and the coupled formation of ATP take place.
•Because Glucose is split to yield two molecules of D-Glyceraldehyde-3-phosphate, each step in the payoff phase
occurs twice per molecule of glucose.
• The steps after 5 constitute payoff phase.
•This phase is also called glucose activation phase. In the preparatory phase of glycolysis, two molecules of ATP
are invested and the hexose chain is cleaved into two triose phosphates.
•During this, phosphorylation of glucose and it’s conversion to glyceraldehyde-3-phosphate take place.
• The steps 1, 2, 3, 4 and 5 together are called as the preparatory phase.
PHASES OF GLYCOLYTIC PATHWAY

10. Step 1- Phosphorylation of glucose
•In the first step of glycolysis, the glucose is initiated or primed for the
subsequent steps by phosphorylation at the C6 carbon.
•The process involves the transfer of phosphate from the ATP to glucose
forming Glucose-6-phosphate in the presence of the enzyme hexokinase
and glucokinase (in animals and microbes).
•This step is also accompanied by considerable loss of energy as heat.
Step 2- Isomerization of Glucose-6-phosphate
•Glucose 6-phosphate is reversibly isomerized to fructose 6-phosphate by
the enzyme phosphohexoisomerase/phosphoglucoisomerase.
•This reaction involves a shift of the carbonyl oxygen from C1 to C2, thus
converting an aldose into a ketose.
STEPS OF GLYCOLYSIS

11. Step 3- Phosphorylation of fructose-6-phosphate
•This step is the second priming step of glycolysis, where fructose-6-
phosphate is converted into fructose-1,6-bisphosphate in the
presence of the enzyme phosphofructokinase.
•Like in Step 1, the phosphate is transferred from ATP while some
amount of energy is lost in the form of heat as well.
Step 4- Cleavage of fructose 1, 6-diphosphate
•This step involves the unique cleavage of the C-C bond in the
fructose 1, 6-bisphosphate.
•The enzyme fructose diphosphate aldolase catalyzes the
cleavage of fructose 1,6-bisphosphate between C3 and C4
resulting in two different triose phosphates: glyceraldehyde 3-
phosphate (an aldose) and dihydroxyacetone phosphate (a
ketose).
•The remaining steps in glycolysis involve three-carbon units,
rather than six carbon units.

12. Step 5- Isomerization of dihydroxyacetone phosphate
•Glyceraldehyde 3-phosphate can be readily degraded in the subsequent steps of
glycolysis, but dihydroxyacetone phosphate cannot be. Thus, it is isomerized
into glyceraldehyde 3-phosphate instead.
•In this step, dihydroxyacetone phosphate is isomerized into glyceraldehyde 3-
phosphate in the presence of the enzyme triose phosphate isomerase.
•This reaction completes the first phase of glycolysis.
Step 6- Oxidative Phosphorylation of Glyceraldehyde 3-phosphate
•Step 6 is one of the three energy-conserving or forming steps of glycolysis.
•The glyceraldehyde 3-phosphate is converted into 1,3-bisphosphoglycerate by the
enzyme glyceraldehyde 3-phosphate dehydrogenase (phosphoglyceraldehyde
dehydrogenase).
•In this process, NAD+ is reduced to coenzyme NADH by the H– from
glyceraldehydes 3-phosphate.
•Since two moles of glyceraldehyde 3-phosphate are formed from one mole of
glucose, two NADH are generated in this step.

13. Step 7- Transfer of phosphate from 1, 3-diphosphoglycerate to ADP
•This step is the ATP-generating step of glycolysis.
•It involves the transfer of phosphate group from the 1, 3-
bisphosphoglycerate to ADP by the enzyme phosphoglycerate kinase,
thus producing ATP and 3-phosphoglycerate.
•Since two moles of 1, 3-bisphosphoglycerate are formed from one
mole of glucose, two ATPs are generated in this step.
Step 8- Isomerization of 3-phosphoglycerate
•The 3-phosphoglycerate is converted into 2-
phosphoglycerate due to the shift of phosphoryl
group from C3 to C2, by the enzyme phosphoglycerate
mutase.
•This is a reversible isomerization reaction.

14. Step 9- Dehydration 2-phosphoglycerate
•In this step, the 2-phosphoglycerate is dehydrated by the action of enolase
(phosphopyruvate hydratase) to phosphoenolpyruvate.
•This is also an irreversible reaction where two moles of water are lost.
Step 10- Transfer of phosphate from phosphoenolpyruvate
•This is the second energy-generating step of glycolysis.
•Phosphoenolpyruvate is converted into an enol form of pyruvate by
the enzyme pyruvate kinase.
•The enol pyruvate, however, rearranges rapidly and non-
enzymatically to yield the keto form of pyruvate (i.e. ketopyruvate).
The keto form predominates at pH 7.0
•The enzyme catalyzes the transfer of a phosphoryl group from
phosphoenolpyruvate to ADP, thus forming ATP.

15. Result of Glycolysis
The overall process of glycolysis results in the following events:
1.Glucose is oxidized into pyruvate.
2.NAD+ is reduced to NADH.
3.ADP is phosphorylated into ATP.
Fates of Pyruvate
Depending on the organism and the metabolic conditions, the pyruvate takes one of the following three
essential routes:
1. Oxidation of pyruvate
•In aerobic organisms, the pyruvate is then moved to the mitochondria where it is oxidized into the acetyl group of
acetyl-coenzyme A (acetyl Co-A).
•This process involves the release of one mole of Carbon dioxide.
•Later, the acetyl CoA is completely oxidized into Carbon dioxide and water by entering the citric acid cycle.
•This pathway follows glycolysis in aerobic organisms and plants.

16. 2. Lactic acid fermentation
•In conditions where the oxygen is insufficient, like in the skeletal muscle cells, the pyruvate cannot be
oxidized due to lack of oxygen.
•Under such conditions, the pyruvate is reduced to lactate by the process of anaerobic glycolysis.
•Lactate production from glucose also occurs in other anaerobic organisms by the process of lactic acid
fermentation.
3. Alcoholic Fermentation
•In some microbes like brewer’s yeast, the pyruvate formed from glucose is converted anaerobically into
ethanol and CO2.
•This is considered the most ancient form of the metabolism of glucose, as observed in conditions where the
oxygen concentration is low.

17. Oxidative Decarboxylation of pyruvate to Acetyl CoA
OXIDATIVE DECARBOXYLATION
•The oxidative decarboxylation of pyruvate forms a link between glycolysis and the citric acid cycle.
•In this process, the pyruvate derived from glycolysis is oxidatively decarboxylated to acetyl CoA and CO2
catalyzed by the pyruvate dehydrogenase complex in the mitochondrial matrix in eukaryotes and in the
cytoplasm of the prokaryotes.
•From one molecule of glucose, two molecules of
pyruvate are formed, each of which forms one
acetyl CoA along with one NADH by the end of
the pyruvate oxidation.
•The acetyl CoA formed from pyruvate oxidation,
fatty acid metabolism, and amino acid pathway then
enter the citric acid cycle.

18. KREBS CYCLE
Krebs cycle / Citric acid cycle / TCA Cycle
•The Krebs cycle, also known as the citric acid cycle or TCA cycle is a series of reactions that take place in the
mitochondria resulting in oxidation of acetyl CoA to release carbon dioxide and hydrogen atoms that later lead to
the formation of water.
•This cycle is termed the citric acid cycle as the first metabolic intermediate formed in the cycle is citric acid.
•This cycle is also termed tricarboxylic acid (TCA) because it was then not certain whether citric acid or some
other tricarboxylic acid (g., isocitric acid) was the first product of the cycle.
•This cycle only occurs under aerobic conditions as energy-rich molecules like NAD+ and FAD can only be
retrieved from their reduced form once they transfer electrons to molecular oxygen.
•The citric acid cycle is the final common pathway for the oxidation of all biomolecules; proteins, fatty acids,
carbohydrates. Molecules from other cycles and pathways enter this cycle through Acetyl CoA.

19. •The citric acid cycle is a cyclic sequence of reactions formed of 8 enzyme-mediated reactions.
•This cycle is also particularly important as it provides electrons/ high-energy molecules to the electron
transport chain for the production of ATPs and water.
•Pyruvate formed at the end of glycolysis is first oxidized into Acetyl CoA which then enters the citric acid cycle.
•The citric acid cycle in eukaryotes takes place in the mitochondria while in prokaryotes, it takes place in the
cytoplasm.
•The pyruvate formed in the cytoplasm (from glycolysis) is brought into the mitochondria where further
reactions take place.
•The different enzymes involved in the citric acid cycle are located either in the inner membrane or in the
matrix space of the mitochondria.
SITE OF KREBS CYCLE

20. KREBS CYCLE ENZYMES
The following are the enzymes that catalyze
different steps throughout the process of the
citric acid cycle:
1. Citrate synthase
2. Aconitase
3. Isocitrate dehydrogenase
4. α-ketoglutarate
5. Succinyl-CoA synthetase
6. Succinate dehydrogenase
7. Fumarase
8. Malate dehydrogenase

21. KREBS CYCLE
KREBS CYCLE PRODUCTS
At each turn of the cycle
✓ 3 NADH,
✓ 1 FADH2,
✓ 1 GTP (or ATP),
✓ 2 CO2

22. KREBS CYCLE STEPS
Step 1: Condensation of acetyl CoA with oxaloacetate
•The first step of the citric acid cycle is the joining of the four-carbon compound oxaloacetate (OAA) and a two-
carbon compound acetyl CoA.
•The oxaloacetate reacts with the acetyl group of the acetyl CoA and water, resulting in the formation of a six-
carbon compound citric acid, CoA.
•The reaction is catalyzed by the enzyme citrate
synthase that condenses the methyl group of acetyl
CoA and the carbonyl group of oxaloacetate resulting in
citryl-CoA which is later cleaved to free coenzyme A
and to form citrate.

23. Step 2: Isomerization of citrate into isocitrate
•Now, for further metabolism, citrate is converted into isocitrate through the formation of intermediate
cis-aconitase.
•This reaction is a reversible reaction catalyzed by the enzyme (aconitase).
•This reaction takes place by a two-step process where the first step involves dehydration of citrate to
cis-aconitase, followed by the second step involving rehydration of cis-aconitase into isocitrate.

24. Step 3: Oxidative decarboxylations of isocitrate
•The third step of the citric acid cycle is the first of the four oxidation-reduction reactions in this cycle.
•Isocitrate is oxidatively decarboxylated to form a five-carbon compound, α-ketoglutarate catalyzed by the
enzyme isocitrate dehydrogenase.
•This reaction, like the second reaction, is a two-step reaction.
•In the first step, isocitrate is dehydrogenated to oxalosuccinate while the second step involves the
decarboxylation of oxalosuccinate to α-ketoglutarate.
•Both the reactions are irreversible and
catalyzed by the same enzyme.
•The first step, however, results in the
formation of NADH while the second
step involves the release of CO2.

25. Step 4: Oxidative decarboxylation of α-ketoglutarate
•This step is another one of the oxidation-reduction reactions where α-ketoglutarate is oxidatively
decarboxylated to form a four-carbon compound, succinyl-CoA, and CO2.
•The reaction is irreversible and catalyzed by the enzyme complex α-ketoglutarate dehydrogenase found in
the mitochondrial space.
•This reaction is similar to the oxidative decarboxylation of pyruvate involving the reduction of NAD+ into
NADH.

26. Step 5: Conversion of succinyl-CoA into succinate
•In the next step, succinyl-CoA undergoes an energy-conserving reaction in which succinyl-CoA is cleaved to
form succinate.
•This reaction is accompanied by phosphorylation of guanosine diphosphate (GDP) to guanosine
triphosphate (GTP).
•The GTP thus formed then readily transfers its terminal phosphate group to ADP forming an ATP molecule.
•The reaction is catalyzed by the enzyme, succinyl-CoA synthase.

27. Step 6: Dehydration of succinate to fumarate
•Here, the succinate formed from succinyl-CoA is dehydrogenated to fumarate catalyzed by the enzyme
complex succinate dehydrogenase found in the intramitochondrial space.
•This is the only dehydrogenation step in the citric acid cycle in which NAD+ doesn’t participate.
•Instead, another high-energy electron carrier, flavin adenine dinucleotide (FAD) acts as the hydrogen
acceptor resulting in the formation of FADH2.
•The FADH2 then enters the electron transport chain via the complex II transferring the electrons to
ubiquinone, finally forming 2ATPs.

28. Step 7: Hydration of fumarate to malate
•The fumarate is reversibly hydrated to form L-malate in the presence of the enzyme fumarate
hydratase.
•As it is a reversible reaction, the formation of L-malate involves hydration, whereas the formation of
fumarate involves dehydration.

29. Step 8: Dehydrogenation of L-malate to oxaloacetate
•The last step of the citric acid cycle is also an oxidation-reduction reaction where L-malate is dehydrogenated
to oxaloacetate in the presence of L-malate dehydrogenase, which is present in the mitochondrial matrix.
•This is a reversible reaction involving oxidation of L-malate and reduction of NAD+ into NADH.
•Oxaloacetate thus formed, allows the repetition of the cycle and NADH formed participates in the oxidative
phosphorylation.
•This reaction completes the cycle.

30. SIGNIFICANCE OF KREBS CYCLE
i. Role in Central metabolic pathway:
•TCA cycle is a final common metabolic pathway of carbohydrates, fatty acids and aminoacids.
•At first all these biomolecules are catabolized by their separate metabolic pathways to generate acetyl-coA then
acetyl-coA enters TCA cycle for further metabolism in aerobic condition.
•TCA is more efficient in energy conservation than other pathways of metabolism.
ii. TCA is an amphibolic pathway:
•It plays role in both catabolism and anabolism.
Catabolic role:
•TCA is a catabolic pathway because it oxidizes acetyl-coA completely into CO2 and H2O and releases large
amount of energy.

31. •TCA is an anabolic pathway because it provides precursors for biosynthesis of other molecules in cells.
• Such as citrate, α-ketoglutarate, succinylcoA and oxaloacetate act as precursors for biosynthesis of various molecules.
•Glucose, purine and pyrimidine are synthesized from oxaloacetate.
•Fatty acids and steroids are synthesized from succinylcoA.
•Some aminoacids, purine and pyrimidine are synthesized from α-ketoglutarate.
•NAD+ and FAD are electron acceptors in the TCA cycle. These are regenerated by Electron transport chain
which requires oxygen as final electron acceptor.
•Hence overall TCA and ETC are aerobic process.
iii. Citric acid cycle is an aerobic process:
Anabolic role:

32. •Reducing equivalent NADH, FADH2 generated during glycolysis and the link between glycolysis and Kreb’s cycle
are used to synthesize ATP by a process is called oxidative phosphorylation (OP).
•Oxidative phosphorylation involves two components-
• Electron transport chain
• ATP synthase.
•The flow of electrons from the reducing equivalence across the electron transport chain generates proton motive
force (PMF).
•The energy stored in proton motive force is used to drive the synthesis of ATP.
•ATP synthase utilizes this proton motive force to drive the synthesis of ATP.
OXIDATIVE PHOSPHORYLATION

33. •Electron transport chain consists of the series of electron carriers arranged asymmetrically in the membrane.
•The membrane may be either cytoplasmic membrane as in the case of bacteria or inner mitochondrial membrane
as in case of eukaryotes.
•The electron carriers are sequentially arranged and get reduced as they accept electron from the previous carrier
and oxidized as they pass electron to the succeeding carrier.
•The different electron carriers are:
• NADH dehydrogenase
• Flavoproteins (FMN and FAD)
• UbiquinFAD
• Iron sulfur (Fe) center
• Cytochrome
ELECTRON TRANSPORT CHAIN

34. ELECTRON TRANSPORT CHAIN

35. •Two types of NAD dependent dehydrogenase can feed electron transport chain.
•They are NADH and NADPH.
•NADPH is less common as it is involved in anabolic reactions (biosynthesis).
•NADH dehydrogenase removes two hydrogen atoms from the substrate and donates the hydride ion (H–) to
NAD+ forming NADH and H+ is released in the solution.
•NAD+ accepts two e– and two protons from the substrate during catabolic reaction and transfers to the
electron transport chain.
•NAD+ is then reduced to NADH+ H+.
•Reduced NADH+ H+ transfers its e– and proton to FMN which in turn is reduced to FMNH
• AH2+ NAD+ <——————–>A + NADH + H+
(Reduced substrate) (oxidized substrate) .
• NADH + H+ + FMN <———–> FMNH2+ NAD+
1.NADH Dehydrogenase

36. •Flavoproteins are derived from Vitamin B2 (Riboflavin).
•These are the protein containing FMN and FAD as the prosthetic group which may be covalently bound with the
protein.
•They are capable of accepting electrons and protons but can only donate electrons.
•The protons are expelled outside the membrane.
•FMN accept electron and proton from NADH and get reduced to FMNH2 which in turn channel only e– through
to ubiquinone.
•FAD is the component of succinate dehydrogenase complex.
•It accepts two electron and two protons from succinate and gets reduced to FADH2, in the process succinate is
converted to fumarate.
•FADH2 channels its electron only to FeS center through ubiquinone.
• Succinate+ FAD ____________________> Fumarate + FADH2
2. Flavoprotein

37. •Ubiquinone are omnipresent in nature.
•These are similar in structure and property with Vitamin K.
•In plants, these are found as plastoquinone and in bacteria, these are found as menaquinone.
•These are lipid soluble (hydrophobic) and can diffuse across the membrane and channel electrons between
carriers.
•Ubiquinone can accept electrons as well as protons but transfer only electrons.
•They accept electron from complex 1 and 2.
•They can accept one e– and get converted into semiquinone or two e–s to from quinone.
Ubiquinone

38. •These are non-heme Fe (iron) containing proteins in which the Fe-atom is covalently bonded to Sulphur of
cysteine present in the protein and to the free Sulphur atoms.
•Less commonly found FeS centers known as Reiske iron Sulphur centers have iron bonded to Histidine residue
of the proteins.
•There are different types of iron Sulphur center, simplest type consists of an iron atom, another type known as
2Fe-2S (Fe2S2) and the third one (most commonly found) is 4Fe-4S (Fe4-S4) and comprises the ferredoxin.
•FeS center consists of Fe-atoms which can interconnect between ferrous and ferric form as they accept and
donate electrons respectively.
•They are capable of receiving and donating electrons only.
•They form the components of all four complexes.
4. FeS Centre

39. •Cytochromes are the proteins with characteristic absorption of visible lights due to the presence of heme
containing Fe as co-factor.
•There are three different types of cytochrome a, b and c.
•Cytochrome a and b are tightly but not covalently linked with their proteins whereas cytochrome c is covalently
bonded with its protein through cysteine.
•Cytochrome ‘a’ has the maximum absorption spectra at 600nm.
•Cytochrome ‘b’ has maximum absorption spectra at 560nm and cytochrome ‘c’ has maximum absorption spectra
at 550nm.
•Cytochromes are capable of accepting and transferring only one e– at a time during which the Fe– atoms
interconvert between ferrous and ferric.
•Cytochrome- Fe2+ <————> Cytochrome- Fe3+ + e–
•Cytochromes are arranged in the order cytochrome ‘b’, cytochrome c1, cytochrome ‘c’ and cytochrome a/a3.
•a/a3 is also known as cytochrome oxidase.
5. Cytochromes

40. •The five electrons carriers are arranged in the form of four complexes.
•Complex I: NADH Quinone oxidoreductase complex (NADH to Quinone)
Note: NADH——->FMN——> FeS—–> Q
•Complex II: Succinate dehydrogenase complex (Succinate to Quinone)
Note: Succinate——> FAD—–> FeS—-> Q
•Complex III: cytochrome bc1 (Ubiquinone to cytochrome c)
Note: UQ2——> cyt bc1—->cyt c
•Complex IV: Cytochrome oxidase (cytc to O2)
Note: cyt c—-> cyt a—–> cyt a3—-> O2
Arrangement of five electron carriers in the
form of four respiratory enzyme complex

41. ELECTRON TRANSPORT CHAIN

42. •This complex is also known as NADH dehydrogenase complex, consists of 42 different polypeptides, including FMN
containing flavoprotein and at least six FeS centers.
•Complex I is ‘L’ shaped with its one arm in the membrane and another arm extending towards the matrix.
•During catabolic reaction, NAD+ is reduced to NADH+ H+ and this NADH + H+ feeds electrons and protons at the
point of origin in the ETC.
•Both e– and protons are transported to FMN which is then reduced to FMNH2.
•FMNH2 transfers only e– to FeS center whereas protons are extruded outside the membrane (intermembrane space), in
the process FMNH2 is oxidized back to FMN.
•Electrons flow through FeS centers which alternate between reduced (Fe2+) and oxidized (Fe3+) froms.
•Electrons are finally transferred to ubiquinone, which along with protons obtained by the hydrolysis of water in the
matrix site of the membrane is reduced to UQH2.
Complex I : NADH Dehydrogenase Complex

43. •Complex II is also known as succinate dehydrogenase complex.
•Succinate dehydrogenase complex is located towards the matrix side of the membrane.
•Succinate is oxidized to fumarate as it transfers two e–s and two protons to FAD.
•FAD is reduced to FADH2.
•FAD transfers only electrons through FeS center to quinone.
•Quinone (Q) in presence of protons is reduced to QH2.
•Complex II consists of covalently linked FAD containing flavoprotein and two FeS centers.
Complex II : Succinate Dehydrogenase
Complex

44. •Ubiquinone are hydrophobic, lipid soluble molecules capable of diffusing across the membrane.
•Electrons are channeled from complex I and complex II to cytochrome bc1.
•Ubiquinone undergo two rounds of oxidation, one towards the enzyme site on the inner membrane site of the
membrane where two electrons are transferred across cyt c1 to cyt c.
•Another oxidation occurs towards the site of membrane containing cyt b where again 2 electrons are passed to cyt bc
and cyt bH.
•During these two oxidation reactions, four protons are expelled outside the membrane and 2UQH2 is oxidized to
2UQ.
•One of the UQ diffuse towards the matrix site of the membrane where it receives two electrons flowing through
cytochrome b1.
•This UQ along with two protons obtained from the hydrolysis of water in the matrix site of the membrane is reduced
to UQH2, thus completing the Q-cycle.
Complex III : Cytochrome bc1

45. •It is also called as cytochrome oxidase.
•Cytochrome c undergoes oxidation in the side of the membrane facing the intermembrane space and O2 is
reduced in the matrix side of the membrane to H2O.
•Complex IV consists of iron containing heme-a and heme-a3.
•Along with iron atoms, cytochrome oxidase also consists of Cu A and Cu B.
•Cu A is closely but not intimately associated with heme ‘a’ and Cu B is intimately associated with heme a3.
•Electrons from cytochrome c flows to Cu A and then to heme ‘a’ and then to heme a3 and then to Cu B and
then finally to Oxygen.
• Cytochrome c —> Cu A —–> Heme a—–> heme a3—->Cu B—> O2
Complex IV : Cytochrome Oxidase

46. •The copper atoms interconvert between cuprous (reduced) and cupric (oxidized).
•Electrons from Cu B and heme a3 is transferred to O2 forming O–-O– bridge.
•Two more electrons are pass through O–-O– resulting in breakage of O–-O– bridge forming O2
– and O2-.
•Two protons are supplied from the matrix side forming OH– and OH–.
•Now, addition of two more proton from matrix side resulting in formation of two molecule of water (2H2O).

47. •Chemiosmotic theory given by Peter Mitchell (1961) in the widely accepted mechanism of ATP
generation.
•According to this theory electron and proton channel into the membrane from the reducing equivalence
flows through a series of electron carriers, electrons flow from NADH through FMN, Q, cytochrome
and finally to O2
•The extension of protons creates a slight positivity/acidity to the outer side of membrane.
•Reduction of quinones and O2 to water requires protons which are provided by the hydrolysis of water in the
matrix side of the membrane.
• The proton motive force tends to drive the proteins through ATP synthase into the inner side of
the membrane, the consequence of which is ATP production.
•ATP synthase consists of two components, transmembrane ion conducting subunit called Fo and
cytoplasmic multiprotein subunit called F1 which is responsible for ATP production
ATP Synthesis

48. •F1 catalyzes the reversible reaction in which ADP is phosphorylated to ATP.
ADP + Pi <————->ATP
•Proton motive force driven H+ through Fo causes the rotation of C-protein of the subunit.
•Rotation of c generates torque.
•This torque is transmitted through gamma (γ) and epsilon (ε) subunit to β-subunit of F1 resulting in its
conformational change.
•This conformational change in β-subunit allows binding of ADP with inorganic phosphate (Pi).
•Binding of ADP and Pi results in production of ATP and β-subunit original conformation is regained.

49. ENERGETIC VALUE OF AEROBIC RESPIRATION

50. SIGNIFICANCE OF RESPIRATION
• Respiration is important because it produces energy that is essential for the normal functioning of the body.
• Respiration provides cells with oxygen and expels toxic carbon dioxide.
• Some energy released by respiration is also in the form of heat. This heat energy contributes significantly in
the warm-blooded animals towards the body.
• Respiration provide energy for biosynthesis of macromolecules like – carbohydrates, lipids, proteins, etc.
that are required by cells.
• For the products such as alcohol, antibiotics, vitamins, organic acids, bakery and dairy products, tanned leather,
etc. anaerobic respiration is helpful in the industrial production.