Respiration in Plants

What to remember

• Cellular respiration is the stepwise oxidation of food, mainly glucose, to release energy as ATP. Glycolysis happens in the cytoplasm and gives 2 net ATP and 2 NADH per glucose. It is common to aerobic and anaerobic respiration.
• In aerobic respiration, pyruvate enters the mitochondrial matrix. It is changed to acetyl CoA and enters the Krebs cycle. NADH and FADH₂ are oxidised in the electron transport system on the inner mitochondrial membrane, and oxygen is the final electron acceptor.
• The theoretical net gain is 38 ATP per glucose in aerobic respiration, against only 2 ATP in fermentation.
• Respiration is an amphibolic pathway: it breaks down carbohydrates, fats and proteins, and it also supplies intermediates for synthesis. The respiratory quotient (RQ) is about 1 for carbohydrates, about 0.7 for fats and about 0.9 for proteins.

Do plants breathe and what is respiration

Plants have no special breathing organs. Each part of the plant takes care of its own gas exchange. Oxygen enters and carbon dioxide leaves through stomata in leaves, lenticels in stems and the surface of roots. Plants need much less gas exchange than animals, and the distance gases travel inside the plant is short. Each living cell is close to the surface or to an air space. In leaves, photosynthesis gives off oxygen inside the leaf, which also helps.

Cellular respiration is the oxidation of food materials, such as carbohydrates, fats and proteins, in the cell. The energy released is trapped in ATP. The most common respiratory substrate is glucose. The energy is released in small steps under the control of enzymes, so it can be stored. If glucose were burnt in one step, it would give off heat and light.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

Respiration can be aerobic (with oxygen) or anaerobic (without oxygen).

Glycolysis

Glycolysis is also called the EMP pathway, after Gustav Embden, Otto Meyerhof and J. Parnas. It takes place in the cytoplasm. It is the first stage of all respiration, aerobic or anaerobic. In plants, glucose comes from sucrose, which is split into glucose and fructose by the enzyme invertase.

One glucose (6-carbon) changes in ten steps into two molecules of pyruvic acid (3-carbon).

Stage	Main events	Energy change
Preparatory (steps 1 to 3)	Glucose is phosphorylated to glucose 6-phosphate (enzyme hexokinase), changed to fructose 6-phosphate, and phosphorylated again to fructose 1,6-bisphosphate	2 ATP are used
Splitting	Fructose 1,6-bisphosphate splits into dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (PGAL). They can be changed into each other	None
Oxidation	PGAL is oxidised and phosphorylated to 1,3-bisphosphoglyceric acid (BPGA). NAD⁺ is reduced to NADH + H⁺	2 NADH per glucose
Pay-off	BPGA → 3-phosphoglyceric acid (PGA) → 2-PGA → phosphoenolpyruvate (PEP) → pyruvic acid	4 ATP are made by substrate-level phosphorylation (at BPGA → PGA and at PEP → pyruvate)

Net gain from one glucose: 2 ATP (4 made, 2 used), 2 NADH + 2H⁺ and 2 pyruvic acid. Substrate-level phosphorylation means that ATP is made by direct transfer of a phosphate group from a substrate, without an electron transport chain.

The fate of pyruvic acid depends on the cell and on the availability of oxygen. It can go into fermentation (lactic acid or alcohol) or into aerobic respiration.

Fermentation

When oxygen is absent, pyruvic acid is not oxidised further in the mitochondria. In fermentation, NADH is reoxidised to NAD⁺, so that glycolysis can keep going. Without NAD⁺ glycolysis would stop.

Feature	Alcoholic fermentation	Lactic acid fermentation
Organism or tissue	Yeast	Some bacteria, and muscle cells in animals when oxygen is short
Steps	Pyruvic acid → acetaldehyde + CO₂ (enzyme pyruvic acid decarboxylase). Then acetaldehyde + NADH + H⁺ → ethanol + NAD⁺ (enzyme alcohol dehydrogenase)	Pyruvic acid + NADH + H⁺ → lactic acid + NAD⁺ (enzyme lactic acid dehydrogenase)
CO₂ released	Yes	No
End product	Ethanol and CO₂	Lactic acid

Fermentation releases only a small part of the energy in glucose. Less than about 7 percent of the energy is released, and not all of it is stored in ATP. The end products are harmful. Yeast cells are poisoned when the alcohol concentration rises to about 13 percent.

Aerobic respiration

Aerobic respiration needs oxygen. It is the complete oxidation of the substrate to CO₂ and H₂O, and it releases much more energy. It takes place in the mitochondria. The final steps need oxygen.

Pyruvate oxidation (link reaction). Pyruvic acid from glycolysis moves from the cytoplasm into the mitochondrial matrix. There it is oxidatively decarboxylated by pyruvate dehydrogenase. This needs NAD⁺, coenzyme A and Mg²⁺.

Pyruvic acid + CoA + NAD⁺ → acetyl CoA + CO₂ + NADH + H⁺

Two molecules of pyruvic acid from one glucose give 2 NADH, 2 CO₂ and 2 acetyl CoA.

Krebs cycle (TCA cycle). It takes place in the mitochondrial matrix. It was worked out by Hans Krebs. Acetyl CoA joins oxaloacetic acid (OAA, 4-carbon) to make citric acid (6-carbon). The enzyme is citrate synthase. The cycle then goes as follows:

Citric acid → isocitric acid → α-ketoglutaric acid → succinyl CoA → succinic acid → fumaric acid → malic acid → OAA

Step	Event	Product
Isocitric acid → α-ketoglutaric acid	Oxidative decarboxylation	CO₂, NADH
α-Ketoglutaric acid → succinyl CoA	Oxidative decarboxylation	CO₂, NADH
Succinyl CoA → succinic acid	Substrate-level phosphorylation	GTP, which gives ATP
Succinic acid → fumaric acid	Oxidation	FADH₂
Malic acid → OAA	Oxidation	NADH

Products for one turn (one acetyl CoA): 3 NADH, 1 FADH₂, 1 GTP (or ATP) and 2 CO₂. Products for one glucose (two turns): 6 NADH, 2 FADH₂, 2 ATP and 4 CO₂.

Electron transport system (ETS). The ETS is in the inner mitochondrial membrane. Electrons from NADH and FADH₂ pass along a series of carriers, and finally reach oxygen.

Component	Role
Complex I (NADH dehydrogenase)	Takes electrons from NADH (made in the matrix) and passes them to ubiquinone
Complex II (succinate dehydrogenase)	Takes electrons from FADH₂ and passes them to ubiquinone
Ubiquinone (UQ)	A mobile carrier in the membrane. It receives electrons from complex I and II and passes them to complex III
Complex III (cytochrome bc₁)	Passes electrons to cytochrome c
Cytochrome c	A small mobile protein on the outer surface of the inner membrane. It carries electrons from complex III to complex IV
Complex IV (cytochrome c oxidase)	Has cytochrome a and a₃ and two copper centres. It passes electrons to oxygen, which is reduced to water
Complex V (ATP synthase)	Has two parts, F₀ (a channel in the membrane) and F₁ (the head that makes ATP)

Flow: NADH → complex I → ubiquinone → complex III → cytochrome c → complex IV → O₂

Oxygen is the final electron acceptor. It is reduced to water. Oxygen acts as the terminal acceptor. Without it, the chain would stop.

Oxidative phosphorylation. As electrons pass along the chain, protons are pumped from the matrix into the intermembrane space. This builds a proton gradient. Protons then return to the matrix through the F₀ channel of complex V. The energy is used by F₁ to make ATP from ADP and Pi. One NADH gives about 3 ATP. One FADH₂ gives about 2 ATP.

Respiratory balance sheet

The theoretical balance sheet for one glucose is as follows.

Stage	Product	ATP
Glycolysis	2 ATP directly	2
Glycolysis	2 NADH	6
Pyruvate oxidation	2 NADH	6
Krebs cycle	2 ATP (as GTP) directly	2
Krebs cycle	6 NADH	18
Krebs cycle	2 FADH₂	4
Total		38

The value of 38 ATP is a theoretical figure. It rests on some assumptions: the pathway is a sequential, orderly process (glycolysis, then Krebs cycle, then ETS); the NADH made in glycolysis is moved into the mitochondria and oxidised; no intermediate is taken out of the pathway to make other compounds; and only glucose is respired. These assumptions are not true in a living cell, so the real gain is lower. The cell works as a series of pathways that run at the same time.

Feature	Fermentation	Aerobic respiration
Oxygen	Not needed	Needed
Site	Cytoplasm	Cytoplasm (glycolysis) and mitochondria
Breakdown of glucose	Partial	Complete
End products	Ethanol and CO₂ or lactic acid	CO₂ and H₂O
Net ATP per glucose	2	38 (theoretical)
NADH is reoxidised by	Pyruvic acid or acetaldehyde (in the same pathway)	The ETS, with O₂ as acceptor

Amphibolic pathway and respiratory quotient

Glucose is the usual substrate, but fats and proteins can also be respired. Fats are broken to glycerol and fatty acids. Glycerol is changed to PGAL and enters glycolysis. Fatty acids are changed to acetyl CoA, which enters the Krebs cycle. Proteins are broken to amino acids. After deamination (removal of the amino group), the amino acids enter the cycle at the stage of pyruvic acid, acetyl CoA or a Krebs cycle acid.

The respiratory pathway is amphibolic. It is catabolic, because it breaks down substrates. It is also anabolic, because its intermediates are used to make other compounds. For example, acetyl CoA is used to make fatty acids, and Krebs cycle acids are used to make amino acids. So the pathway is not purely catabolic.

The respiratory quotient (RQ), or respiratory ratio, is the volume of CO₂ released divided by the volume of O₂ used in respiration.

RQ = volume of CO₂ evolved ÷ volume of O₂ consumed

Substrate	Example	RQ
Carbohydrate	Glucose: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O	1.0
Fat	Tripalmitin: 2(C₅₁H₉₈O₆) + 145O₂ → 102CO₂ + 98H₂O + energy	102 ÷ 145 = about 0.7
Protein	Amino acids as substrate	About 0.9
Organic acid	Malic acid: C₄H₆O₅ + 3O₂ → 4CO₂ + 3H₂O	4 ÷ 3 = about 1.33, which is more than 1

Fats have less oxygen than carbohydrates, so they need more oxygen to be oxidised. So their RQ is less than 1. Organic acids have more oxygen, so they need less oxygen and their RQ is more than 1. In a plant the RQ is often a mixture, because many substrates are respired together.

NEET traps

• 1. Glycolysis uses 2 ATP and makes 4 ATP. The net gain is 2 ATP, not 4.
• 2. Glycolysis is in the cytoplasm. The Krebs cycle is in the mitochondrial matrix. The ETS is in the inner membrane.
• 3. The pyruvate dehydrogenase step gives NADH and CO₂ but no ATP. It is not part of the Krebs cycle.
• 4. The Krebs cycle makes 3 NADH, 1 FADH₂ and 1 GTP for each acetyl CoA. For one glucose, double each number.
• 5. Alcoholic fermentation releases CO₂. Lactic acid fermentation does not.
• 6. FADH₂ enters the chain at complex II and gives about 2 ATP. NADH enters at complex I and gives about 3 ATP.
• 7. Cytochrome c is a small mobile protein. Ubiquinone is a mobile carrier in the membrane lipid. Do not swap them.
• 8. The 38 ATP figure is theoretical. Real cells give less, because the assumptions behind it do not hold.
• 9. The RQ of fats is about 0.7 (less than 1). The RQ of organic acids is more than 1. The RQ of carbohydrates is 1.
• 10. The respiratory pathway is amphibolic, not only catabolic.

One-liners

• 1. The EMP pathway is another name for glycolysis.
• 2. Glycolysis converts one glucose into two pyruvic acid molecules.
• 3. ATP in glycolysis is made by substrate-level phosphorylation.
• 4. Invertase splits sucrose into glucose and fructose.
• 5. Yeast is poisoned when the alcohol level reaches about 13 percent.
• 6. Fermentation releases less than about 7 percent of the energy in glucose.
• 7. Pyruvate dehydrogenase needs NAD⁺, coenzyme A and Mg²⁺.
• 8. Citric acid is the first product of the Krebs cycle.
• 9. Succinyl CoA is changed to succinic acid with the formation of GTP.
• 10. Complex IV is cytochrome c oxidase.
• 11. Oxygen is the final acceptor and is reduced to water.
• 12. Complex V is ATP synthase, with the F₀ channel and the F₁ head.
• 13. The theoretical net gain is 38 ATP per glucose.
• 14. Fats and proteins enter the pathway as acetyl CoA, PGAL or other intermediates.
• 15. The RQ of glucose is 1, of tripalmitin about 0.7 and of malic acid about 1.33.

Practice questions

1. Glycolysis takes place in the

   1. stroma of the chloroplast
   2. inner mitochondrial membrane
   3. mitochondrial matrix
   4. cytoplasm
   Answer

   D. cytoplasm

   Glycolysis occurs in the cytoplasm of the cell.

2. The net gain of ATP from glycolysis of one glucose molecule is

   1. 6
   2. 2
   3. 8
   4. 4
   Answer

   B. 2

   Two ATP are used and four are made, so the net gain is 2 ATP.

3. In plants, sucrose is split into glucose and fructose by the enzyme

   1. invertase
   2. amylase
   3. hexokinase
   4. lactic acid dehydrogenase
   Answer

   A. invertase

   Invertase splits sucrose.

4. ATP is made in glycolysis by

   1. oxidative phosphorylation
   2. chemiosmosis in the mitochondria
   3. substrate-level phosphorylation
   4. photophosphorylation
   Answer

   C. substrate-level phosphorylation

   A phosphate group is transferred directly from a substrate, as in BPGA → PGA and PEP → pyruvate.

5. The end products of alcoholic fermentation are

   1. lactic acid only
   2. ethanol and CO₂
   3. ethanol only
   4. lactic acid and CO₂
   Answer

   B. ethanol and CO₂

   Pyruvic acid is converted to acetaldehyde and CO₂, and then to ethanol.

6. Which enzyme converts acetaldehyde to ethanol in yeast?

   1. Pyruvic acid decarboxylase
   2. Hexokinase
   3. Lactic acid dehydrogenase
   4. Alcohol dehydrogenase
   Answer

   D. Alcohol dehydrogenase

   Alcohol dehydrogenase uses NADH to reduce acetaldehyde to ethanol.

7. The Krebs cycle takes place in the

   1. mitochondrial matrix
   2. cytoplasm
   3. inner mitochondrial membrane
   4. intermembrane space
   Answer

   A. mitochondrial matrix

   The Krebs cycle enzymes are in the matrix.

8. When acetyl CoA joins oxaloacetic acid in the Krebs cycle, the first product is

   1. malic acid
   2. isocitric acid
   3. citric acid
   4. succinic acid
   Answer

   C. citric acid

   Citrate synthase makes citric acid (6-carbon) from acetyl CoA and OAA.

9. Substrate-level phosphorylation in the Krebs cycle occurs during the conversion of

   1. malic acid to oxaloacetic acid
   2. succinyl CoA to succinic acid
   3. isocitric acid to α-ketoglutaric acid
   4. fumaric acid to malic acid
   Answer

   B. succinyl CoA to succinic acid

   This step forms GTP, which gives ATP.

10. Which complex of the electron transport system contains cytochromes a and a₃ and two copper centres?

    1. Complex II
    2. Complex I
    3. Complex IV
    4. Complex III
    Answer

    C. Complex IV

    Complex IV is cytochrome c oxidase.

11. The final acceptor of electrons in the respiratory electron transport system is

    1. ubiquinone
    2. NAD⁺
    3. cytochrome c
    4. oxygen
    Answer

    D. oxygen

    Oxygen is reduced to water at complex IV.

12. FADH₂ gives its electrons to the respiratory chain through

    1. complex II
    2. complex III
    3. complex I
    4. complex V
    Answer

    A. complex II

    Complex II (succinate dehydrogenase) receives electrons from FADH₂.

13. The respiratory quotient of carbohydrates is

    1. 0.9
    2. 0.7
    3. 1
    4. more than 1
    Answer

    C. 1

    For glucose, 6 CO₂ are released for 6 O₂ used.

14. The respiratory quotient of tripalmitin is about

    1. 1.0
    2. 0.9
    3. 1.3
    4. 0.7
    Answer

    D. 0.7

    102 CO₂ are released for 145 O₂, which is about 0.7.

15. Which set of factors is needed for the oxidative decarboxylation of pyruvic acid by pyruvate dehydrogenase?

    1. NAD⁺, coenzyme A and Mg²⁺
    2. NADP⁺, ATP and Mn²⁺
    3. FAD, coenzyme A and Zn²⁺
    4. NAD⁺, ADP and Fe²⁺
    Answer

    A. NAD⁺, coenzyme A and Mg²⁺

    Pyruvate dehydrogenase needs NAD⁺, coenzyme A and Mg²⁺.

16. Given below are two statements: Statement I: Glycolysis takes place in the cytoplasm. Statement II: Glycolysis needs oxygen. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    Glycolysis is in the cytoplasm and is common to aerobic and anaerobic respiration, so it does not need oxygen.

17. Given below are two statements: Statement I: In glycolysis, four ATP are made and two are used for each glucose. Statement II: The net gain of glycolysis is 2 ATP. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    A. Both Statement I and Statement II are correct

    4 made minus 2 used gives a net gain of 2 ATP.

18. Given below are two statements: Statement I: Lactic acid fermentation releases CO₂. Statement II: Alcoholic fermentation releases CO₂. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    D. Statement I is incorrect but Statement II is correct

    Lactic acid fermentation gives only lactic acid; alcoholic fermentation gives ethanol and CO₂.

19. Given below are two statements: Statement I: The oxidation of pyruvic acid to acetyl CoA takes place in the cytoplasm. Statement II: This step makes ATP directly. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    B. Both Statement I and Statement II are incorrect

    It takes place in the mitochondrial matrix and gives NADH and CO₂, not ATP.

20. Given below are two statements: Statement I: Cytochrome c carries electrons from complex III to complex IV. Statement II: Ubiquinone receives electrons from complex I and complex II. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    A. Both Statement I and Statement II are correct

    Both statements describe mobile carriers of the chain.

21. Given below are two statements: Statement I: The 38 ATP per glucose is a theoretical value. Statement II: It assumes that intermediates are drawn out of the pathway to make other compounds. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    The value assumes that no intermediate is removed for synthesis.

22. Which of the following statements are correct? 1. Fats enter respiration as glycerol, changed to PGAL, and as fatty acids, changed to acetyl CoA. 2. Amino acids from proteins are deaminated before they enter the pathway. 3. The respiratory pathway is only catabolic.

    1. 1 and 2 only
    2. 2 and 3 only
    3. 1 and 3 only
    4. 1, 2 and 3
    Answer

    A. 1 and 2 only

    The pathway is amphibolic, so statement 3 is wrong.

23. Which of the following statements about the respiratory quotient are correct? 1. Carbohydrates have an RQ of 1. 2. Fats have an RQ of more than 1. 3. Organic acids have an RQ of more than 1.

    1. 1 and 2 only
    2. 2 and 3 only
    3. 1 and 3 only
    4. 1, 2 and 3
    Answer

    C. 1 and 3 only

    Fats have an RQ of less than 1, so statement 2 is wrong.

24. Which of the following statements about the Krebs cycle are correct? 1. One turn gives 3 NADH and 1 FADH₂. 2. Two CO₂ molecules are released in one turn. 3. The cycle takes place in the cytoplasm.

    1. 2 and 3 only
    2. 1, 2 and 3
    3. 1 and 3 only
    4. 1 and 2 only
    Answer

    D. 1 and 2 only

    The cycle is in the mitochondrial matrix, so statement 3 is wrong.

25. Given below are two statements: Statement I: Yeast is poisoned when the alcohol concentration reaches about 13 percent. Statement II: Fermentation releases more than half of the energy in glucose. Choose the correct answer.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    Fermentation releases less than about 7 percent of the energy in glucose.

26. Assertion (A): Glycolysis can continue during fermentation only if NAD⁺ is regenerated. Reason (R): NADH is reoxidised to NAD⁺ when pyruvic acid or acetaldehyde is reduced.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    A. Both A and R are true and R is the correct explanation of A

    Reduction of pyruvic acid or acetaldehyde regenerates NAD⁺ for glycolysis.

27. Assertion (A): The Krebs cycle takes place in the mitochondrial matrix. Reason (R): Glycolysis takes place in the cytoplasm.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    B. Both A and R are true but R is not the correct explanation of A

    Both are true, but one does not explain the other.

28. Assertion (A): Oxygen is the final electron acceptor of the respiratory chain. Reason (R): Oxygen is oxidised to water at complex IV.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    C. A is true but R is false

    Oxygen is reduced, not oxidised, to water.

29. Assertion (A): The respiratory pathway is only catabolic. Reason (R): Intermediates of the pathway are used to make fatty acids and amino acids.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    D. A is false but R is true

    The pathway is amphibolic; the statement about intermediates is true.

30. Assertion (A): The respiratory quotient of fats is less than 1. Reason (R): Fats contain less oxygen than carbohydrates, so they need more oxygen to be oxidised.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    A. Both A and R are true and R is the correct explanation of A

    More O₂ is used than CO₂ released, so the ratio is below 1.

31. Assertion (A): A net gain of 38 ATP per glucose is obtained in every living cell. Reason (R): The value of 38 ATP is based on assumptions.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    D. A is false but R is true

    The value is theoretical and real cells give less.

32. Match List I with List II. List I: a. Hexokinase b. Invertase c. Alcohol dehydrogenase d. Pyruvate dehydrogenase List II: i. Splits sucrose into glucose and fructose ii. Pyruvic acid to acetyl CoA iii. Glucose to glucose 6-phosphate iv. Acetaldehyde to ethanol

    1. a-i, b-iii, c-iv, d-ii
    2. a-iii, b-i, c-iv, d-ii
    3. a-iii, b-i, c-ii, d-iv
    4. a-iv, b-i, c-iii, d-ii
    Answer

    B. a-iii, b-i, c-iv, d-ii

    Hexokinase phosphorylates glucose; invertase splits sucrose; alcohol dehydrogenase forms ethanol; pyruvate dehydrogenase forms acetyl CoA.

33. Match List I with List II. List I: a. Succinyl CoA to succinic acid b. Succinic acid to fumaric acid c. Isocitric acid to α-ketoglutaric acid d. Acetyl CoA joining OAA List II: i. Citric acid formed ii. GTP formed iii. FADH₂ formed iv. CO₂ and NADH formed

    1. a-iv, b-iii, c-ii, d-i
    2. a-ii, b-iii, c-iv, d-i
    3. a-iii, b-ii, c-iv, d-i
    4. a-ii, b-iii, c-i, d-iv
    Answer

    B. a-ii, b-iii, c-iv, d-i

    GTP at succinyl CoA step; FADH₂ at succinic acid step; CO₂ and NADH at isocitric step; citric acid from acetyl CoA and OAA.

34. Match List I with List II. List I: a. Complex I b. Complex II c. Complex IV d. Complex V List II: i. Cytochrome c oxidase ii. ATP synthase iii. NADH dehydrogenase iv. Succinate dehydrogenase

    1. a-iii, b-iv, c-ii, d-i
    2. a-i, b-iv, c-iii, d-ii
    3. a-iv, b-iii, c-i, d-ii
    4. a-iii, b-iv, c-i, d-ii
    Answer

    D. a-iii, b-iv, c-i, d-ii

    I is NADH dehydrogenase, II succinate dehydrogenase, IV cytochrome c oxidase, V ATP synthase.

35. Match List I with List II. List I: a. Glucose b. Tripalmitin c. Protein d. Malic acid List II: i. About 0.9 ii. 1.0 iii. About 1.33 iv. About 0.7

    1. a-ii, b-i, c-iv, d-iii
    2. a-ii, b-iv, c-i, d-iii
    3. a-ii, b-iv, c-iii, d-i
    4. a-iv, b-ii, c-i, d-iii
    Answer

    B. a-ii, b-iv, c-i, d-iii

    Glucose 1.0, tripalmitin about 0.7, protein about 0.9, malic acid about 1.33.

36. Match List I with List II. List I: a. ATP from 2 NADH of glycolysis b. ATP from 6 NADH of the Krebs cycle c. ATP from 2 FADH₂ of the Krebs cycle d. Total theoretical ATP per glucose List II: i. 4 ii. 38 iii. 18 iv. 6

    1. a-iv, b-iii, c-i, d-ii
    2. a-iv, b-iii, c-ii, d-i
    3. a-i, b-iii, c-iv, d-ii
    4. a-iii, b-iv, c-i, d-ii
    Answer

    A. a-iv, b-iii, c-i, d-ii

    2 NADH give 6 ATP, 6 NADH give 18, 2 FADH₂ give 4, and the total is 38.

37. Match List I with List II. List I: a. Alcoholic fermentation b. Lactic acid fermentation c. Aerobic respiration d. Glycolysis List II: i. Pyruvic acid to lactic acid ii. Ends in CO₂ and H₂O iii. Glucose to two pyruvic acid molecules iv. Gives ethanol and CO₂

    1. a-ii, b-i, c-iv, d-iii
    2. a-iv, b-i, c-ii, d-iii
    3. a-i, b-iv, c-ii, d-iii
    4. a-iv, b-i, c-iii, d-ii
    Answer

    B. a-iv, b-i, c-ii, d-iii

    Each process is matched with its end product or step.

38. Yeast in a closed vessel with plenty of glucose stops fermenting when ethanol rises to about 13 percent. What is the reason?

    1. NAD⁺ has become too abundant
    2. Glucose has become toxic
    3. The yeast cells are poisoned by the alcohol
    4. Oxygen has entered the vessel
    Answer

    C. The yeast cells are poisoned by the alcohol

    Yeast cells are poisoned when alcohol reaches about 13 percent.

39. A plant tissue releases 4 volumes of CO₂ for every 3 volumes of O₂ it uses. Which substrate is most likely being respired?

    1. Glucose
    2. A pure fat
    3. Tripalmitin
    4. An organic acid such as malic acid
    Answer

    D. An organic acid such as malic acid

    The RQ is about 1.33, which is more than 1 and fits an organic acid.

40. A poison stops electron flow at complex IV in a mitochondrion. What happens next?

    1. NADH cannot be reoxidised and ATP synthesis by the chain stops
    2. Oxygen is used faster
    3. Glycolysis produces 38 ATP
    4. The Krebs cycle makes more ATP
    Answer

    A. NADH cannot be reoxidised and ATP synthesis by the chain stops

    Without electron flow to oxygen, the chain stops and NAD⁺ is not regenerated.

41. How many ATP molecules are made by substrate-level phosphorylation alone, in glycolysis (net) and in the Krebs cycle, for one glucose molecule?

    1. 4
    2. 2
    3. 6
    4. 38
    Answer

    A. 4

    Glycolysis gives 2 net ATP and the Krebs cycle gives 2 ATP (as GTP).

42. A muscle cell is short of oxygen during heavy exercise. Which compound accumulates?

    1. Citric acid
    2. Lactic acid
    3. Ethanol
    4. Acetyl CoA
    Answer

    B. Lactic acid

    Pyruvic acid is reduced to lactic acid by lactic acid dehydrogenase.

43. How many CO₂ molecules are released in total by the oxidation of pyruvic acid and the Krebs cycle for one glucose molecule?

    1. 4
    2. 6
    3. 2
    4. 8
    Answer

    B. 6

    The link reaction gives 2 CO₂ and the Krebs cycle gives 4 CO₂.

44. In a cell, NAD⁺ cannot be regenerated. Which process stops first?

    1. Formation of 2 ATP by hexokinase
    2. Reduction of oxygen to water only
    3. Hydrolysis of sucrose by invertase
    4. Glycolysis, because the oxidation of PGAL needs NAD⁺
    Answer

    D. Glycolysis, because the oxidation of PGAL needs NAD⁺

    NAD⁺ is needed for the oxidation of PGAL in glycolysis, which is why fermentation regenerates it.