Aerobic Respiration

BiologyRespiration in PlantsFor NEET aspirants

Aerobic respiration is the complete oxidation of glucose in the presence of oxygen to CO₂ and water, releasing a large amount of energy. This page covers the oxidative decarboxylation of pyruvate, the Krebs' (TCA) cycle, the electron transport system and oxidative phosphorylation, ATP synthase, the respiratory balance sheet with its 38 ATP, and the amphibolic pathway. It follows the NCERT Class 11 chapter Respiration in Plants. NEET asks sites, complex names, ATP counts and statement traps from aerobic respiration.

On this page1Overview2Pyruvate to acetyl CoA3TCA cycle4Electron transport system5ATP synthase6Balance sheet7Amphibolic pathway8Exam essentials9Quick revision10Solved examples11Practice

Key Points at a Glance

★ Must learn Aerobic respiration: matrix (pyruvate oxidation, TCA cycle) and inner mitochondrial membrane (ETS, ATP synthesis).
Pyruvate + CoA + $NA D^{+}$ to acetyl CoA + $C O_{2}$ + $NADH + H^{+}$ ; enzyme pyruvate dehydrogenase, with $M g^{2 +}$ .
★ Must learn TCA cycle starts as acetyl CoA + oxaloacetic acid (OAA) + water form citric acid (citrate synthase); named after Hans Krebs.
Per turn: 3 $NA D^{+}$ reduced, 1 $FA D^{+}$ reduced, 1 GTP by substrate-level phosphorylation (succinyl-CoA to succinic acid).
★ Must learn ETS complexes: I NADH dehydrogenase, II succinate dehydrogenase, III cytochrome bc₁, IV cytochrome c oxidase, V ATP synthase.
Ubiquinone and cytochrome c are mobile carriers; complex IV has cytochromes a, a₃ and two copper centres.
★ Must learn 1 NADH gives 3 ATP; 1 $FAD H_{2}$ gives 2 ATP. $O_{2}$ is the final hydrogen acceptor.
ATP synthase: F₀ (proton channel) and F₁ (ATP-making headpiece); 4 $H^{+}$ pass per ATP.
★ Must learn Theoretical net gain: 38 ATP per glucose in aerobic respiration.
The respiratory pathway is amphibolic: it serves both catabolism and anabolism.

1. Aerobic Respiration: An Overview

★ Very important Aerobic respiration: the complete oxidation of organic substances in the presence of oxygen, releasing $C O_{2}$ , water and a large amount of the energy present in the substrate.

Aerobic respiration is the most common type of respiration in higher organisms.
In eukaryotes, its steps take place within the mitochondria, and they need $O_{2}$ .
★ Exam imp Pyruvate, formed by the glycolytic breakdown of carbohydrates in the cytosol, is transported from the cytoplasm into the mitochondria.

The two crucial events of aerobic respiration

Complete oxidation of pyruvate by the step-wise removal of all its hydrogen atoms, leaving three molecules of $C O_{2}$ . This takes place in the matrix of the mitochondria.
Passing of electrons, removed as part of the hydrogen atoms, to molecular $O_{2}$ with simultaneous synthesis of ATP. This takes place on the inner mitochondrial membrane.

Stage	Site	Main output
Glycolysis	Cytoplasm	Pyruvic acid, ATP, $NADH + H^{+}$
Oxidative decarboxylation of pyruvate	Mitochondrial matrix	Acetyl CoA, $C O_{2}$ , $NADH + H^{+}$
TCA (Krebs') cycle	Mitochondrial matrix	$C O_{2}$ , $NADH + H^{+}$ , $FAD H_{2}$ , ATP (via GTP)
Electron transport system and oxidative phosphorylation	Inner mitochondrial membrane	ATP and water

Memory Trick

Matrix makes the CO₂; membrane makes the ATP. Removing hydrogen (and releasing $C O_{2}$ ) happens in the matrix; passing electrons to $O_{2}$ and making most ATP happens on the inner membrane.

Key idea

Aerobic respiration has two halves: strip hydrogen from pyruvate in the matrix, then pass its electrons to oxygen on the inner membrane.

2. Oxidative Decarboxylation of Pyruvate

After entering the mitochondrial matrix, pyruvate undergoes oxidative decarboxylation.
★ Exam imp This happens through a complex set of reactions catalysed by pyruvic dehydrogenase (pyruvate dehydrogenase).
The reactions need several coenzymes, including $NA D^{+}$ and Coenzyme A (CoA).

Pyruvic acid + CoA + NA D^{+} M g^{2 +} Pyruvate dehydrogenase Acetyl CoA + C O_{2} + NADH + H^{+}

★ Exam imp Two molecules of NADH are produced from the two molecules of pyruvic acid formed from one glucose.
Acetyl CoA then enters a cyclic pathway, the tricarboxylic acid (TCA) cycle.
★ Exam imp This cycle is more commonly called Krebs' cycle, after Hans Krebs, who first elucidated it.

Tips and Tricks

Break the name apart: oxidative means hydrogen is removed ( $NA D^{+}$ becomes $NADH + H^{+}$ ); decarboxylation means $C O_{2}$ is released. The 3C pyruvate becomes 2C acetyl CoA.

Key idea

Each pyruvate loses one CO₂ and gives one NADH as it becomes acetyl CoA, the fuel of the TCA cycle.

3. Tricarboxylic Acid (TCA) Cycle

Condensation. The acetyl group combines with oxaloacetic acid (OAA) and water to form citric acid. The enzyme is citrate synthase, and a molecule of CoA is released.
Isomerisation. Citrate is isomerised to isocitrate.
Two successive decarboxylations form $α$ -ketoglutaric acid and then succinyl-CoA.
Substrate-level phosphorylation. When succinyl-CoA is converted to succinic acid, a molecule of GTP is synthesised. In a coupled reaction, GTP is converted to GDP with the simultaneous synthesis of ATP from ADP.
Regeneration of OAA. In the remaining steps, succinyl-CoA (via succinic acid and malic acid) is oxidised back to OAA, allowing the cycle to continue.

★ Exam imp There are three points in the cycle where $NA D^{+}$ is reduced to $NADH + H^{+}$ , and one point where $FA D^{+}$ is reduced to $FAD H_{2}$ .
★ Exam imp Continued oxidation of acetyl CoA needs the continued replenishment of oxaloacetic acid, the first member of the cycle.
It also needs the regeneration of $NA D^{+}$ and $FA D^{+}$ from NADH and $FAD H_{2}$ respectively.

Test yourself: hide labels

Figure 1: The citric acid cycle. Carbon count falls 6C to 5C to 4C as two

C O_{2}

leave. Each turn reduces 3

NA D^{+}

and 1

FA D^{+}

and forms 1 GTP; oxaloacetic acid is regenerated to accept the next acetyl CoA.

Memory Trick

Name the cycle's compounds in order with "Our City Keeps Swimming Merrily": Oxaloacetic acid (4C), Citric acid (6C), $α$ -Ketoglutaric acid (5C), Succinic acid (4C), Malic acid (4C), then back to oxaloacetic acid.

3.1 Summary equation of this phase

Pyruvic acid + 4 NA D^{+} + FA D^{+} + 2 H_{2} O + ADP + P_{i}

Mitochondrial matrix 3 C O_{2} + 4 NADH + 4 H^{+} + FAD H_{2} + ATP

So far, glucose has been broken down to release $C O_{2}$ .
Eight molecules of $NADH + H^{+}$ (2 from pyruvate oxidation and 6 from the TCA cycle) and two of $FAD H_{2}$ have been made from one glucose.
Only two molecules of ATP have been made in the TCA cycle.
$O_{2}$ has not yet been used, and the large number of ATP has not yet been made. These come from the electron transport system.

Per glucose (2 pyruvate)	$C O_{2}$	$NADH + H^{+}$	$FAD H_{2}$	ATP (via GTP)
Pyruvate to acetyl CoA	2	2	0	0
TCA cycle (2 turns)	4	6	2	2
Total	6	8	2	2

NEET Focus

The only substrate-level phosphorylation in the TCA cycle is succinyl-CoA to succinic acid (GTP). $FAD H_{2}$ forms at the succinic acid step; $C O_{2}$ leaves at the two decarboxylations before succinyl-CoA. With pyruvate oxidation, each pyruvate releases 3 $C O_{2}$ , so one glucose releases 6.

Quick Recall: tap to check

Name the enzyme that joins acetyl CoA with oxaloacetic acid.

Citrate synthase.

In Figure 1, at which step is GTP formed?

Succinyl-CoA (from

α

-ketoglutaric acid) to succinic acid.

How many

NA D^{+}

and

FA D^{+}

are reduced in one turn of the cycle?

Three

NA D^{+}

and one

FA D^{+}

Which compound must be continuously replenished for the cycle to run?

Oxaloacetic acid, the first member of the cycle.

Key idea

Each turn of the TCA cycle removes two CO₂ and loads 3 NADH and 1 FADH₂ with electrons for the ETS.

4. Electron Transport System and Oxidative Phosphorylation

4.1 Electron transport system (ETS)

The next steps release and use the energy stored in $NADH + H^{+}$ and $FAD H_{2}$ .
★ Exam imp They are oxidised through the electron transport system, and the electrons are passed on to $O_{2}$ , forming $H_{2} O$ .

★ Very important Electron transport system (ETS): the metabolic pathway through which electrons pass from one carrier to another. It is present in the inner mitochondrial membrane.

Complex I. Electrons from NADH, produced in the matrix during the citric acid cycle, are oxidised by NADH dehydrogenase (complex I). The electrons pass to ubiquinone in the inner membrane.
Complex II. Ubiquinone also receives reducing equivalents via $FAD H_{2}$ (complex II), generated during the oxidation of succinate in the citric acid cycle.
Complex III. Reduced ubiquinone (ubiquinol) is oxidised, and its electrons pass to cytochrome c via the cytochrome bc₁ complex (complex III).
Cytochrome c. A small protein attached to the outer surface of the inner membrane. It is a mobile carrier that transfers electrons between complex III and complex IV.
Complex IV. The cytochrome c oxidase complex, containing cytochromes a and a₃ and two copper centres. It passes the electrons to $O_{2}$ , which forms water.
Coupling to complex V. As electrons pass from complex I to IV, the transfer is coupled to ATP synthase (complex V), which makes ATP from ADP and inorganic phosphate.

Test yourself: hide labels

Figure 2: Electron transport system. Electrons from

NADH + H^{+}

(complex I) and from succinate via

FAD H_{2}

(complex II) pass through ubiquinone, complex III, cytochrome c and complex IV to

O_{2}

. Complexes I, III and IV pump 4, 4 and 2

H^{+}

outward; the protons return through ATP synthase to make ATP.

Complex	Name	Role in the chain
I	NADH dehydrogenase	Oxidises NADH; passes electrons to ubiquinone
II	Succinate dehydrogenase	Passes electrons from $FAD H_{2}$ (succinate oxidation) to ubiquinone
III	Cytochrome bc₁ complex	Passes electrons from ubiquinol to cytochrome c
IV	Cytochrome c oxidase	Cytochromes a, a₃ and two copper centres; passes electrons to $O_{2}$
V	ATP synthase	Makes ATP from ADP and inorganic phosphate
Mobile carriers	Ubiquinone; cytochrome c	Ubiquinone in the membrane; cytochrome c on its outer surface

Memory Trick

Complexes I to V: "No Student Can Cheat Always" for NADH dehydrogenase, Succinate dehydrogenase, Cytochrome bc₁, Cytochrome c oxidase and ATP synthase.

★ Must learn The number of ATP made depends on the electron donor: oxidation of one NADH gives 3 ATP; one $FAD H_{2}$ gives 2 ATP.
★ Exam imp Aerobic respiration takes place only in the presence of oxygen, but the role of oxygen is limited to the terminal stage.
★ Exam imp Yet oxygen is vital: it drives the whole process by removing hydrogen from the system. Oxygen is the final hydrogen acceptor.
Oxygen is thus the ultimate acceptor of electrons, and it gets reduced to water.

Photophosphorylation

Energy source: light energy
Builds the proton gradient used for ATP synthesis
Occurs in photosynthesis

Oxidative phosphorylation

Energy source: oxidation-reduction (electron transport)
Builds the proton gradient used for ATP synthesis
Occurs in respiration

Oxidative phosphorylation: synthesis of ATP in respiration using the energy of oxidation-reduction (electron transport) to build the proton gradient. The name contrasts it with photophosphorylation, which uses light energy.

4.2 ATP synthase (complex V)

ATP synthesis linked to a membrane is explained by the chemiosmotic hypothesis, met earlier in photosynthesis.
The energy released during electron transport is used to synthesise ATP with the help of ATP synthase (complex V).
This complex has two major components: F₁ and F₀.
★ Exam imp The F₁ headpiece is a peripheral membrane protein complex; it contains the site for ATP synthesis from ADP and inorganic phosphate.
★ Exam imp F₀ is an integral membrane protein complex; it forms the channel through which protons cross the inner membrane.
★ Exam imp The passage of protons through this channel is coupled to the catalytic site of F₁, which makes ATP.
★ Must learn For each ATP produced, 4 $H^{+}$ pass through F₀ from the intermembrane space to the matrix, down the electrochemical proton gradient.

Test yourself: hide labels

Figure 3: ATP synthase. Four

H^{+}

flow through F₀ from the outer side (inter-membrane space) into the matrix, and F₁ uses this flow to join ADP and P_i into one ATP.

F₀

Integral membrane protein complex
Forms the proton channel
Lies within the inner membrane

F₁

Peripheral membrane protein complex (headpiece)
Contains the ATP synthesis site
Projects into the matrix

Memory Trick

F₀ is the opening; F₁ is the one that makes ATP. Link the subscript 0 with the opening (the proton channel in the membrane) and the subscript 1 with the single headpiece where ADP + P_i become ATP.

Quick Recall: tap to check

Name the mobile carrier between complex III and complex IV.

Cytochrome c, attached to the outer surface of the inner membrane.

In Figure 2, which complex contains cytochromes a and a₃ and two copper centres?

Complex IV, cytochrome c oxidase.

How many protons pass through F₀ for each ATP made?

Four (

4 H^{+}

Which part of ATP synthase holds the site of ATP synthesis?

The F₁ headpiece.

Key idea

Electrons fall from NADH and FADH₂ to O₂ through complexes I to IV; the proton gradient they build drives ATP synthase.

5. The Respiratory Balance Sheet

The net gain of ATP for every glucose oxidised can be calculated.
★ Exam imp In reality, this can remain only a theoretical exercise.
The calculation is possible only on certain assumptions.

The four assumptions

A sequential, orderly pathway functions: one substrate forms the next, and glycolysis, the TCA cycle and the ETS follow one after another.
The NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.
None of the intermediates in the pathway is used to synthesise any other compound.
Only glucose is being respired; no alternative substrate enters the pathway at any intermediate stage.

Why these assumptions fail in a living system

All pathways work simultaneously; they do not take place one after another.
Substrates enter and are withdrawn from the pathways as and when necessary.
ATP is used as and when needed.
Enzymatic rates are controlled by multiple means.
Still, the exercise is useful: it shows the beauty and efficiency of the living system in extracting and storing energy.

★ Very important Net gain in aerobic respiration: 38 ATP per molecule of glucose (theoretical, on the four assumptions).

5.1 How the 38 ATP add up

Stage (per glucose)	ATP directly	NADH (3 ATP each)	$FAD H_{2}$ (2 ATP each)	Total ATP
Glycolysis	2 (net)	2, giving 6	0	8
Pyruvate to acetyl CoA (twice)	0	2, giving 6	0	6
TCA cycle (two turns)	2 (via GTP)	6, giving 18	2, giving 4	24
Total	4	10, giving 30	2, giving 4	38

Memory Trick

38 = 4 + 30 + 4. Four ATP made directly, ten NADH at 3 ATP each, and two $FAD H_{2}$ at 2 ATP each.

5.2 Fermentation and aerobic respiration compared

Feature	Fermentation	Aerobic respiration
Breakdown of glucose	Partial	Complete, to $C O_{2}$ and $H_{2} O$
Net ATP per glucose	Only 2 (glucose to pyruvic acid)	Many more (38 in theory)
Oxidation of NADH to $NA D^{+}$	Rather slow	Very vigorous

Key idea

38 ATP is a theoretical figure: real cells run all pathways at once and draw intermediates away for other uses.

6. Amphibolic Pathway

6.1 Entry of different substrates

★ Exam imp Glucose is the favoured substrate for respiration.
All carbohydrates are usually first converted into glucose before they are respired.
★ Exam imp Other substrates can also be respired, but they do not enter at the first step of the pathway.

Substrate	First broken down to	Enters the respiratory pathway as
Carbohydrates	Simple sugars such as glucose	Glucose, at the start of glycolysis
Fats: fatty acids	Fatty acids and glycerol	Acetyl CoA
Fats: glycerol	Fatty acids and glycerol	PGAL (glyceraldehyde 3-phosphate)
Proteins	Amino acids, by proteases	After deamination: pyruvate, acetyl CoA, or a stage within Krebs' cycle, depending on structure

Test yourself: hide labels

Figure 4: Interrelationship among metabolic pathways. Fats, carbohydrates and proteins enter the respiratory pathway at different points and are broken down to

C O_{2}

and

H_{2} O

. Dashed arrows: amino acids can also enter as acetyl CoA or within Krebs' cycle.

6.2 Why the pathway is amphibolic

Because respiration breaks down substrates, it has traditionally been called a catabolic process, and the respiratory pathway a catabolic pathway.
But the very compounds at which substrates enter are also withdrawn from the pathway to synthesise those substrates.
★ Exam imp Fatty acids are broken down to acetyl CoA when respired; when the organism needs fatty acids, acetyl CoA is withdrawn to make them.
So the respiratory pathway is involved in both the breakdown and the synthesis of fatty acids.
Similarly, respiratory intermediates link the breakdown and synthesis of proteins.
Catabolism: breaking-down processes in the organism. Anabolism: synthesis.

★ Very important Amphibolic pathway: the respiratory pathway is involved in both anabolism and catabolism, so it is better called amphibolic rather than catabolic.

Memory Trick

Amphi- means "both", as in amphibian (an animal living on both land and water). An amphibolic pathway works both ways: breaking down and building up.

Key idea

The same respiratory intermediates are entry points for breakdown and exit points for synthesis, which makes the pathway amphibolic.

7. Exam Essentials

Pairs to Match

List I	List II
Pyruvate dehydrogenase	Pyruvate to acetyl CoA; needs $M g^{2 +}$ , $NA D^{+}$ and CoA
Citrate synthase	Acetyl CoA + OAA + water to citric acid
Hans Krebs	First elucidated the TCA cycle
Succinyl-CoA to succinic acid	GTP; substrate-level phosphorylation
Complex I	NADH dehydrogenase
Complex II	Succinate dehydrogenase; electrons from $FAD H_{2}$
Complex III	Cytochrome bc₁ complex
Complex IV	Cytochrome c oxidase; cytochromes a, a₃, two copper centres
Complex V	ATP synthase
Cytochrome c	Mobile carrier between complexes III and IV
Ubiquinone	Receives electrons from complexes I and II
F₀	Integral protein; proton channel
F₁	Peripheral headpiece; ATP synthesis site
Oxygen	Final hydrogen acceptor
Fatty acids; glycerol; amino acids	Acetyl CoA; PGAL; pyruvate, acetyl CoA or Krebs' cycle

Exceptions

Oxygen acts only at the terminal stage, yet without it the whole process stops.
The GTP (ATP) of the TCA cycle comes from substrate-level phosphorylation, not oxidative phosphorylation.
$FAD H_{2}$ gives 2 ATP, not 3.
Cytochrome c is not embedded in the membrane; it is attached to its outer surface and is mobile.
Electrons from $FAD H_{2}$ enter at complex II, not complex I.
The figure of 38 ATP is only theoretical; its assumptions are not valid in a living system.
Fats and proteins do not enter the pathway at its first step.
The respiratory pathway is not purely catabolic; it is amphibolic.

Numbers to Remember

3 $C O_{2}$ per pyruvate (1 in oxidative decarboxylation, 2 in the TCA cycle); 6 per glucose.
2 NADH per glucose from pyruvate oxidation.
Per turn: 3 NADH, 1 $FAD H_{2}$ , 1 GTP; per glucose, 8 NADH, 2 $FAD H_{2}$ and 2 ATP after the TCA cycle.
3 ATP per NADH; 2 ATP per $FAD H_{2}$ .
4 $H^{+}$ through F₀ for each ATP.
5 complexes (I to V); complex IV has 2 copper centres.
4 assumptions behind the theoretical net gain of 38 ATP per glucose.
Carbon counts: citric acid 6C, $α$ -ketoglutaric acid 5C, succinic, malic and oxaloacetic acids 4C, acetyl CoA 2C.

Tips and Tricks

For any ATP count, first list the carriers: multiply NADH by 3 and $FAD H_{2}$ by 2, then add ATP made directly. Glycolytic NADH counts as 3 ATP only because the balance sheet assumes it enters the mitochondria.

8. Quick Revision

Aerobic respiration completely oxidises substrates to $C O_{2}$ and water in the presence of $O_{2}$ ; in eukaryotes it occurs in mitochondria.
Pyruvate enters the mitochondria; hydrogen removal (leaving 3 $C O_{2}$ ) is in the matrix, electron transfer to $O_{2}$ is on the inner membrane.
Pyruvate dehydrogenase, with $M g^{2 +}$ , $NA D^{+}$ and CoA, converts pyruvate to acetyl CoA, $C O_{2}$ and NADH.
Acetyl CoA + OAA + water form citric acid (citrate synthase); the cycle is named after Hans Krebs.
Citrate to isocitrate, two decarboxylations to $α$ -ketoglutaric acid and succinyl-CoA, then GTP at succinyl-CoA to succinic acid.
Each turn: 3 NADH, 1 $FAD H_{2}$ , 1 GTP; OAA, $NA D^{+}$ and $FA D^{+}$ must be regenerated.
ETS lies in the inner mitochondrial membrane: I (NADH dehydrogenase), II, III (cytochrome bc₁), IV (cytochrome c oxidase).
Ubiquinone carries electrons from I and II to III; cytochrome c carries them from III to IV.
One NADH gives 3 ATP and one $FAD H_{2}$ gives 2 ATP; oxygen is the final hydrogen acceptor.
Oxidative phosphorylation uses oxidation-reduction energy; photophosphorylation uses light energy.
ATP synthase: F₀ (integral, proton channel) and F₁ (peripheral headpiece, ATP site); 4 $H^{+}$ per ATP.
Theoretical net gain is 38 ATP per glucose, based on four assumptions that do not hold in a living system.
Fermentation: partial, 2 ATP, slow NADH oxidation; aerobic: complete, many ATP, vigorous NADH oxidation.
Fatty acids enter as acetyl CoA, glycerol as PGAL, amino acids as pyruvate, acetyl CoA or within Krebs' cycle.
The respiratory pathway serves both catabolism and anabolism, so it is amphibolic.

9. Solved Examples

Solved Example 1

Match List I with List II.
List I: A. Complex I, B. Complex III, C. Complex IV, D. Complex V
List II: I. Cytochrome c oxidase, II. ATP synthase, III. NADH dehydrogenase, IV. Cytochrome bc₁ complex
Choose the correct answer:
(A) A-III, B-IV, C-I, D-II
(B) A-IV, B-III, C-I, D-II
(C) A-III, B-I, C-IV, D-II
(D) A-II, B-IV, C-I, D-III

Solution:

Answer: (A). Complex I is NADH dehydrogenase (III), complex III is the cytochrome bc₁ complex (IV), complex IV is cytochrome c oxidase (I), and complex V is ATP synthase (II).

Solved Example 2

Read the statements about the TCA cycle.
A. It begins with the condensation of acetyl group with oxaloacetic acid and water.
B. GTP is formed when succinyl-CoA is converted to succinic acid.
C.

NA D^{+}

is reduced at four points in each turn.
D. It takes place in the mitochondrial matrix.
E.

FAD H_{2}

gives 3 ATP in the ETS.
Choose the correct answer:
(A) A, B and D only
(B) A, C and D only
(C) B, C and E only
(D) A, B, D and E only

Solution:

Answer: (A). C is wrong: $NA D^{+}$ is reduced at three points per turn (and $FA D^{+}$ at one). E is wrong: one $FAD H_{2}$ gives 2 ATP; one NADH gives 3.

Solved Example 3

Arrange the carriers in the order in which electrons from NADH pass through them.
A. Cytochrome c
B. Ubiquinone
C. Complex I
D. Complex IV
E. Complex III
Choose the correct answer:
(A) C, B, E, A, D
(B) C, E, B, A, D
(C) B, C, E, A, D
(D) C, B, A, E, D

Solution:

Answer: (A). NADH dehydrogenase (complex I) passes electrons to ubiquinone, then to complex III, to cytochrome c, and finally to complex IV and oxygen.

Solved Example 4

Using the balance-sheet values, how many ATP are obtained from the complete oxidation of one molecule of pyruvic acid (from pyruvate oxidation, the TCA cycle and the ETS)?
(A) 12
(B) 15
(C) 18
(D) 38

Solution:

Answer: (B). One pyruvate gives 1 NADH in oxidative decarboxylation and 3 NADH, 1 $FAD H_{2}$ and 1 GTP in the TCA cycle. ATP = 4 NADH $\times$ 3 + 1 $FAD H_{2}$ $\times$ 2 + 1 GTP = 12 + 2 + 1 = 15. Two pyruvates give 30, and glycolysis adds 8, giving 38.

Solved Example 5

Which of the following is NOT an assumption made in calculating the net gain of 38 ATP?
(A) Glycolysis, the TCA cycle and the ETS follow one after another
(B) NADH made in glycolysis enters the mitochondria and is oxidised there
(C) Intermediates are withdrawn to make other compounds
(D) Only glucose is being respired

Solution:

Answer: (C). The calculation assumes that none of the intermediates is used to synthesise other compounds. The other three are actual assumptions.

Solved Example 6

Statement I: The role of oxygen in aerobic respiration is limited to the terminal stage.
Statement II: Oxygen drives the whole process by removing hydrogen from the system as the final hydrogen acceptor.
Choose the correct answer:
(A) Both Statement I and Statement II are correct
(B) Both Statement I and Statement II are incorrect
(C) Statement I is correct but Statement II is incorrect
(D) Statement I is incorrect but Statement II is correct

Solution:

Answer: (A). Oxygen acts only at complex IV, yet by accepting hydrogen it keeps the whole chain, and hence the TCA cycle, running.

10. Practice Questions

Practice Questions

Match List I with List II. List I: A. Fatty acids, B. Glycerol, C. Amino acids (after deamination), D. Carbohydrates. List II: I. Enter as glucose, II. Enter as PGAL, III. Enter as acetyl CoA, IV. Enter as pyruvate, acetyl CoA or within Krebs' cycle. Options:
(A) A-III, B-II, C-IV, D-I
(B) A-II, B-III, C-IV, D-I
(C) A-III, B-IV, C-II, D-I
(D) A-I, B-II, C-IV, D-IIIAnswer: (A). These are the entry points shown in Figure 4.
Which statements about ATP synthase are correct? A. F₁ is an integral membrane protein complex. B. F₀ forms the proton channel. C. Protons flow from the matrix to the intermembrane space through F₀. D. Four protons pass through F₀ per ATP made. Options:
(A) B and D only
(B) A and B only
(C) B, C and D only
(D) A, C and D onlyAnswer: (A). A is wrong: F₁ is peripheral; F₀ is integral. C is wrong: protons flow from the intermembrane space to the matrix.
Arrange in the order they occur in the TCA cycle: A. Succinyl-CoA, B. Citric acid, C. Isocitrate, D. $α$ -ketoglutaric acid, E. Oxaloacetic acid (regenerated). Options:
(A) B, C, D, A, E
(B) B, D, C, A, E
(C) C, B, D, A, E
(D) B, C, A, D, EAnswer: (A). Citric acid, isocitrate, $α$ -ketoglutaric acid, succinyl-CoA, and finally oxaloacetic acid again.
Which of the following is NOT correct about cytochrome c?
(A) It is a small protein
(B) It is attached to the outer surface of the inner membrane
(C) It transfers electrons between complexes III and IV
(D) It is an integral part of complex IAnswer: (D). Cytochrome c is a mobile carrier, not part of complex I.
Statement I: In respiration, the proton gradient for ATP synthesis is produced by the energy of oxidation-reduction. Statement II: For this reason, ATP synthesis in mitochondria is called photophosphorylation. Options:
(A) Both Statement I and Statement II are correct
(B) Both Statement I and Statement II are incorrect
(C) Statement I is correct but Statement II is incorrect
(D) Statement I is incorrect but Statement II is correctAnswer: (C). It is called oxidative phosphorylation; photophosphorylation uses light energy.
How many molecules of $C O_{2}$ are released from one molecule of pyruvic acid during aerobic respiration?
(A) 1
(B) 2
(C) 3
(D) 6Answer: (C). One in oxidative decarboxylation and two in the TCA cycle.
The enzyme pyruvic dehydrogenase needs which coenzymes?
(A) FAD and GTP
(B) $NA D^{+}$ and Coenzyme A
(C) NADP and CoA
(D) ATP and FMNAnswer: (B). The reaction needs several coenzymes, including $NA D^{+}$ and Coenzyme A, with $M g^{2 +}$ .
Differentiate between glycolysis and Krebs' cycle (citric acid cycle).Answer: Glycolysis occurs in the cytoplasm of all organisms, needs no oxygen, partly oxidises glucose to 2 pyruvic acid and gives 2 NADH and net 2 ATP. Krebs' cycle occurs in the mitochondrial matrix, is part of aerobic respiration, completely oxidises acetyl CoA to $C O_{2}$ and gives 3 NADH, 1 $FAD H_{2}$ and 1 GTP per turn.
What are the main steps in aerobic respiration? Where does it take place?Answer: Oxidative decarboxylation of pyruvate and the TCA cycle in the mitochondrial matrix, then the electron transport system and oxidative phosphorylation on the inner mitochondrial membrane. Glycolysis, which precedes them, occurs in the cytoplasm.
Give the schematic representation of an overall view of Krebs' cycle.Answer: Acetyl CoA (2C) + oxaloacetic acid (4C) to citric acid (6C) to isocitrate to $α$ -ketoglutaric acid (5C; $C O_{2}$ , NADH) to succinyl-CoA ( $C O_{2}$ , NADH) to succinic acid (GTP) to malic acid ( $FAD H_{2}$ ) to oxaloacetic acid (NADH). See Figure 1.
Explain ETS.Answer: The ETS is a chain of electron carriers in the inner mitochondrial membrane. NADH is oxidised by complex I and $FAD H_{2}$ via complex II; electrons pass through ubiquinone, complex III, cytochrome c and complex IV to $O_{2}$ , forming water. The energy released is coupled to ATP synthase (complex V).
What are the assumptions made during the calculation of net gain of ATP?Answer: A sequential, orderly pathway (glycolysis, TCA cycle, ETS one after another); glycolytic NADH enters the mitochondria for oxidative phosphorylation; no intermediate is used for other syntheses; only glucose is respired.
Discuss: "The respiratory pathway is an amphibolic pathway."Answer: Substrates such as fatty acids and amino acids enter the pathway as acetyl CoA, pyruvate or TCA intermediates when respired. The same intermediates are withdrawn to synthesise fatty acids and proteins. As the pathway serves both catabolism and anabolism, it is amphibolic.
What is oxidative phosphorylation?Answer: Synthesis of ATP in mitochondria in which the energy of oxidation-reduction in the ETS builds a proton gradient that drives ATP synthase. It differs from photophosphorylation, which uses light energy.

Common Mistakes to Avoid

Watch out

Placing the TCA cycle on the inner membrane. It runs in the matrix; the ETS is on the inner membrane.
Giving 3 ATP for $FAD H_{2}$ . One NADH gives 3 ATP, but one $FAD H_{2}$ gives only 2.
Calling the GTP of the TCA cycle oxidative phosphorylation. It is substrate-level phosphorylation.
Writing complex III as cytochrome c oxidase. Complex III is cytochrome bc₁; complex IV is cytochrome c oxidase.
Saying protons flow through F₀ from the matrix to the intermembrane space. They flow into the matrix, down the gradient.
Counting 6 NADH from the TCA cycle as the total. Per glucose there are 8 NADH after the TCA cycle (2 from pyruvate oxidation), plus 2 from glycolysis.
Treating 38 ATP as the actual yield in cells. It is a theoretical value based on four assumptions.
Calling the respiratory pathway purely catabolic. Its intermediates are also withdrawn for synthesis, so it is amphibolic.

Frequently Asked Questions

Where does aerobic respiration take place in a cell?

Glycolysis occurs in the cytoplasm. Pyruvate then enters the mitochondria, where its oxidative decarboxylation and the TCA cycle take place in the matrix. The electron transport system and ATP synthesis by ATP synthase take place on the inner mitochondrial membrane.

What happens to pyruvate before it enters the Krebs' cycle?

In the mitochondrial matrix, pyruvate undergoes oxidative decarboxylation catalysed by pyruvate dehydrogenase, which needs Mg2+, NAD+ and coenzyme A. Each pyruvate forms acetyl CoA, one carbon dioxide and one NADH + H+. Acetyl CoA then enters the TCA cycle, also called Krebs' cycle.

What are the main products of one turn of the TCA cycle?

Each turn releases two carbon dioxide, reduces three NAD+ to NADH + H+ and one FAD+ to FADH2, and forms one GTP by substrate-level phosphorylation, which is used to make ATP. Oxaloacetic acid is regenerated so that the cycle can accept the next acetyl group.

What are the complexes of the electron transport system?

Complex I is NADH dehydrogenase, complex II passes electrons from FADH2 (succinate dehydrogenase), complex III is the cytochrome bc1 complex, complex IV is cytochrome c oxidase with cytochromes a and a3 and two copper centres, and complex V is ATP synthase. Ubiquinone and cytochrome c are mobile carriers.

Why is oxygen called the final hydrogen acceptor?

At the end of the electron transport system, complex IV passes electrons to oxygen, which combines with hydrogen to form water. By removing hydrogen from the system, oxygen keeps the whole chain running. Its role is limited to this terminal stage, but without it aerobic respiration stops.

How many ATP are produced from one glucose in aerobic respiration?

On theoretical assumptions there is a net gain of 38 ATP. This includes 4 ATP made directly, 10 NADH at 3 ATP each and 2 FADH2 at 2 ATP each. In a living cell the actual figure differs because pathways run together and intermediates are withdrawn.

Why is the respiratory pathway called amphibolic?

Fats, proteins and carbohydrates enter the respiratory pathway at points such as acetyl CoA, PGAL and pyruvate when they are respired. The same compounds are withdrawn from the pathway when the cell synthesises fatty acids and proteins. Since the pathway serves both catabolism and anabolism, it is amphibolic.

Which points of Aerobic Respiration are most asked in NEET?

Any line of the NCERT text can be asked, but the most frequent points are the sites of each stage, the enzymes pyruvate dehydrogenase and citrate synthase, the GTP step, the names of complexes I to V, 3 ATP per NADH and 2 per FADH2, F0 and F1, 4 protons per ATP, the 38 ATP assumptions and the amphibolic pathway.

Previous year questions on Aerobic Respiration

10 questions from past papers, each with a step-by-step solution.

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