NCERT Solutions for Class 11 Biology Chapter 12: Respiration in Plants

(a) Respiration vs. Combustion

(b) Glycolysis vs. Krebs’ cycle

(c) Aerobic respiration vs. Fermentation

Respiratory substrates are the organic compounds that are oxidized during the process of respiration to release energy. These compounds are broken down to liberate energy, which is then stored in the form of ATP.

While several types of organic molecules can act as respiratory substrates, the most common respiratory substrate is glucose.

Other substrates include:

• Fats
• Proteins
• Organic acids

Glycolysis is a 10-step process that breaks down one molecule of glucose into two molecules of pyruvic acid. This occurs in the cytoplasm.

Schematic Representation of Glycolysis:

1. Glucose (6C) → (Uses 1 ATP) → Glucose-6-phosphate (6C)
2. Glucose-6-phosphate (6C) → Fructose-6-phosphate (6C)
3. Fructose-6-phosphate (6C) → (Uses 1 ATP) → Fructose-1, 6-bisphosphate (6C)
4. Fructose-1, 6-bisphosphate (6C) splits into:
   • Glyceraldehyde-3-phosphate (PGAL) (3C)
   • Dihydroxyacetone phosphate (DHAP) (3C) (which converts to PGAL)
5. 2 x Glyceraldehyde-3-phosphate (3C) → (Produces 2 NADH) → 2 x 1, 3-bisphosphoglycerate (3C)
6. 2 x 1, 3-bisphosphoglycerate (3C) → (Produces 2 ATP) → 2 x 3-phosphoglycerate (3C)
7. 2 x 3-phosphoglycerate (3C) → 2 x 2-phosphoglycerate (3C)
8. 2 x 2-phosphoglycerate (3C) → (Releases 2 H₂O) → 2 x Phosphoenolpyruvate (PEP) (3C)
9. 2 x Phosphoenolpyruvate (PEP) (3C) → (Produces 2 ATP) → 2 x Pyruvic acid (3C)

Net Gain from Glycolysis: 2 ATP and 2 NADH.

Aerobic respiration is the complete oxidation of organic food in the presence of oxygen to release energy. The main steps are:

1. Glycolysis: The breakdown of glucose into two molecules of pyruvic acid.
   • Location: Cytoplasm.
2. Oxidative Decarboxylation (Link Reaction): The conversion of pyruvic acid into acetyl-CoA.
   • Location: Mitochondrial Matrix.
3. Krebs' Cycle / TCA Cycle: The complete oxidation of acetyl-CoA to CO₂ and H₂O.
   • Location: Mitochondrial Matrix.
4. Electron Transport System (ETS) and Oxidative Phosphorylation: The synthesis of ATP using energy from electron carriers.
   • Location: Inner Mitochondrial Membrane.

The Krebs' Cycle begins with acetyl-CoA. For every one molecule of glucose, the cycle turns twice.

Schematic Representation (one turn):

1. Acetyl-CoA (2C) + Oxaloacetic acid (4C) → Citric acid (6C)
2. Citric acid (6C) → Isocitrate (6C)
3. Isocitrate (6C) → α-ketoglutaric acid (5C) + NADH + CO₂
4. α-ketoglutaric acid (5C) → Succinyl-CoA (4C) + NADH + CO₂
5. Succinyl-CoA (4C) → Succinic acid (4C) + ATP (via GTP)
6. Succinic acid (4C) → Fumaric acid (4C) + FADH₂
7. Fumaric acid (4C) → Malic acid (4C)
8. Malic acid (4C) → Oxaloacetic acid (4C) + NADH (Regenerated)

Net Result per Glucose Molecule (2 turns): 2 ATP, 6 NADH, 2 FADH₂, 4 CO₂

The Electron Transport System (ETS), located in the inner mitochondrial membrane, uses the energy from NADH and FADH₂ to generate ATP.

Mechanism of ETS:

1. Electron Donation: NADH (at Complex I) and FADH₂ (at Complex II) donate high-energy electrons.
2. Electron Flow: Electrons move through a series of protein complexes (I, II, III, IV) and mobile carriers (UQ, Cytochrome c).
3. Proton Pumping: As electrons flow, Complexes I, III, and IV pump protons (H⁺) from the matrix to the intermembrane space, creating a proton gradient.
4. Role of Oxygen: Oxygen acts as the final electron acceptor at Complex IV, forming water (H₂O).
5. ATP Synthesis: Protons flow back into the matrix through ATP Synthase (Complex V). This flow powers the synthesis of ATP from ADP.

Oxidation of 1 NADH produces 3 ATP, and 1 FADH₂ produces 2 ATP.

(a) Aerobic respiration vs. Anaerobic respiration

(b) Glycolysis vs. Fermentation

The calculation of 38 ATP per glucose is a theoretical maximum based on these assumptions:

1. Sequential Pathway: The pathway is sequential and orderly (Glycolysis → Krebs' → ETS) with no crossovers.
2. Complete Transfer: All NADH synthesized is transferred into the mitochondria for oxidation.
3. No Intermediate Use: No intermediates are withdrawn from the pathway for other syntheses.
4. Complete Oxidation: Only glucose is respired; no other substrates enter the pathway.
5. Maximum Efficiency: The system works at 100% efficiency (1 NADH = 3 ATP; 1 FADH₂ = 2 ATP).

The respiratory pathway is called an amphibolic pathway because it involves both catabolism (breakdown) and anabolism (synthesis).

• Catabolic Role: The primary function is to break down substrates (carbohydrates, fats, proteins) to release energy.
• Anabolic Role: Intermediates of the pathway are used as precursors for synthesizing other biomolecules. For example:
  • Acetyl-CoA is used to synthesize fatty acids.
  • α-ketoglutaric acid is used to synthesize amino acids.
  • Succinyl-CoA is used to synthesize chlorophyll.

Thus, it serves as a central metabolic hub linking breakdown and synthesis processes.

RQ (Respiratory Quotient) is the ratio of the volume of carbon dioxide (CO₂) evolved to the volume of oxygen (O₂) consumed during respiration.

Formula: RQ = Volume of CO₂ evolved / Volume of O₂ consumed

The RQ value depends on the respiratory substrate.

• For Fats: The RQ is less than 1 (around 0.7). This is because fats are poor in oxygen and require more O₂ for complete oxidation compared to the amount of CO₂ evolved. For example, for tripalmitin: 2(C₅₁H₉₈O₆) + 145O₂ → 102CO₂ + 98H₂O
RQ = 102CO₂ / 145O₂ = 0.7
• For Carbohydrates: RQ is 1.
• For Proteins: RQ is about 0.9.

Oxidative phosphorylation is the process of synthesizing ATP using the energy released from the oxidation of reduced coenzymes (NADH and FADH₂) during the electron transport system (ETS).

It occurs in the inner mitochondrial membrane. It's named "oxidative" for the oxidation of NADH/FADH₂ and "phosphorylation" for the addition of phosphate to ADP to form ATP. The energy for this process comes from the proton motive force generated by the ETS.

The overall equation for aerobic respiration is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP)

Based on this, the end products are:

1. Carbon Dioxide (CO₂): Released during the link reaction and Krebs' cycle.
2. Water (H₂O): Formed at the end of the ETS.
3. Energy (ATP): Synthesized primarily via oxidative phosphorylation.

NAD⁺ (Nicotinamide Adenine Dinucleotide) acts as an electron carrier. It accepts electrons during glycolysis and the Krebs' cycle to become reduced to NADH. This NADH then transports these high-energy electrons to the Electron Transport System (ETS), where they are used to generate ATP.

Substrate-level phosphorylation is the direct synthesis of ATP by transferring a phosphate group from a high-energy substrate molecule to ADP. It does not involve the ETS.

Example: The conversion of Phosphoenolpyruvate (PEP) to Pyruvic acid in glycolysis, or the conversion of Succinyl-CoA to Succinic acid in the Krebs' cycle.

In the absence of oxygen, pyruvic acid in muscle cells undergoes lactic acid fermentation. The enzyme lactate dehydrogenase reduces pyruvic acid to lactic acid, using the NADH produced during glycolysis. This regenerates NAD⁺, allowing glycolysis to continue producing a small amount of ATP.

Glycolysis is the partial oxidation of glucose into two molecules of pyruvic acid.

Location: It occurs in the cytoplasm of all living cells.

Key Steps:

1. Preparatory Phase (Energy Investment):
   • Glucose is phosphorylated twice, using 2 ATP molecules, to form Fructose-1, 6-bisphosphate.
   • This 6-carbon molecule is then split into two 3-carbon molecules (PGAL and DHAP).
2. Payoff Phase (Energy Generation):
   • Each 3-carbon molecule is oxidized, producing 4 ATP through substrate-level phosphorylation and 2 NADH.
   • The final product is two molecules of pyruvic acid.

Net Products (from one glucose):

• 2 molecules of Pyruvic Acid
• 2 molecules of ATP (4 produced - 2 consumed)
• 2 molecules of NADH

The chemiosmotic hypothesis explains how ATP is synthesized during oxidative phosphorylation.

1. Creation of a Proton Gradient: As electrons pass along the ETS, energy is used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space.
2. Proton Motive Force: This gradient creates a "proton motive force," a form of stored energy.
3. Role of ATP Synthase (Complex V): Protons flow back into the matrix down their gradient through a channel in ATP synthase.
4. ATP Synthesis: The flow of protons through ATP synthase causes a conformational change that drives the synthesis of ATP by combining ADP and inorganic phosphate (Pi).

i. What metabolic process is the yeast carrying out?

The yeast is carrying out anaerobic respiration, specifically alcoholic fermentation.

ii. Name the gas being produced and the alcohol formed.

The gas is Carbon Dioxide (CO₂), and the alcohol is Ethanol (C₂H₅OH).

iii. What is the net ATP gain in this process? Why is it so low?

The net ATP gain is only 2 ATP molecules. It is low because glucose is only partially oxidized, and most of the energy remains stored in ethanol.