Respiration in Plants — NCERT Class 11 Biology

(a) Respiration vs Combustion:

(b) Glycolysis vs Krebs cycle:

The main steps in aerobic respiration are:

1. Glycolysis: Glucose (6C) is broken down into 2 molecules of pyruvate (3C). Occurs in the cytoplasm. Net gain: 2 ATP + 2 NADH. 2. Oxidative decarboxylation of pyruvate: Each pyruvate (3C) is converted to acetyl-CoA (2C) + CO₂ + NADH. Occurs in the mitochondrial matrix. Per glucose: 2 CO₂ + 2 NADH. 3. Krebs cycle (TCA cycle): Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). Through a series of reactions, oxaloacetate is regenerated. Per glucose (2 turns): 4 CO₂ + 2 ATP + 6 NADH + 2 FADH₂. Occurs in the mitochondrial matrix. 4. Electron transport chain (ETC) and oxidative phosphorylation: NADH and FADH₂ donate electrons to the ETC on the inner mitochondrial membrane. ATP is synthesised by ATP synthase using the proton gradient. Per glucose: ~32–34 ATP.

Aerobic respiration takes place in the cytoplasm (glycolysis) and mitochondria (Krebs cycle and ETC).

Oxidative phosphorylation is the process of ATP synthesis that is coupled to the electron transport chain (ETC). It occurs on the inner mitochondrial membrane. The steps are:

1. NADH and FADH₂ donate electrons to the electron carriers (Complex I, II, III, IV) of the ETC. 2. As electrons pass through the carriers, energy is released, which is used to pump protons (H⁺) from the mitochondrial matrix to the intermembrane space. 3. This creates a proton gradient (higher H⁺ concentration in the intermembrane space). 4. The protons flow back into the matrix through ATP synthase (F₀–F₁ complex), and the energy of this flow drives the synthesis of ATP from ADP and inorganic phosphate (Pi). 5. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water (H₂O).

This process is also called chemiosmosis (proposed by Peter Mitchell). Approximately 2–3 ATP are synthesised per NADH and approximately 1.5–2 ATP per FADH₂.

The significance of step-wise release of energy in respiration is:

1. Energy is not released all at once — if it were, most energy would be lost as heat, which could damage cells. 2. ATP is synthesised gradually — energy is released in small amounts at each step and stored as ATP, which can be used by the cell as needed. 3. Intermediates are produced — these are used as precursors for the synthesis of other biomolecules (amino acids, fatty acids, etc.). 4. Enzyme-controlled reactions — each step is catalysed by a specific enzyme, allowing regulation of the pathway. 5. Efficiency — about 40% of the energy released is trapped in ATP; the rest is released as heat, which helps maintain body temperature.

When cells oxidise pyruvate without oxygen (anaerobic conditions), fermentation occurs. There are two types:

1. Alcoholic fermentation (in yeast):

• Pyruvate (3C) → Acetaldehyde (2C) + CO₂ (by pyruvate decarboxylase)
• Acetaldehyde + NADH → Ethanol (2C) + NAD⁺ (by alcohol dehydrogenase)
• NAD⁺ is regenerated, allowing glycolysis to continue.
• Net yield: 2 ATP per glucose molecule.

2. Lactic acid fermentation (in muscle cells under strenuous exercise, and in some bacteria):

• Pyruvate + NADH → Lactic acid + NAD⁺ (by lactate dehydrogenase)
• NAD⁺ is regenerated for glycolysis.
• No CO₂ is released.
• Net yield: 2 ATP per glucose molecule.

In both cases, NAD⁺ is regenerated so that glycolysis can continue producing a small amount of ATP (2 per glucose) even in the absence of oxygen.

The respiratory quotient (RQ) of fats is less than 1 because fats require more oxygen for oxidation than the amount of CO₂ produced. Fats have more hydrogen and less oxygen in their molecules compared to carbohydrates, so more O₂ is needed to oxidise the hydrogen to water.

For example, tripalmitin (a fat):

C₅₁H₉₈O₆ + 72.5 O₂ → 51 CO₂ + 49 H₂O

RQ = 51 / 72.5 = 0.70

For glucose (a carbohydrate):

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O

RQ = 6 / 6 = 1.0

Since more O₂ is consumed per molecule of CO₂ released during fat oxidation, the RQ of fats is less than 1.

(See Intext Page 231 Q1 — answered above.)

Respiratory substrates are the organic substances that are oxidised during respiration to release energy. The most common respiratory substrates are:

• Carbohydrates (most common, especially glucose)
• Fats
• Proteins
• Organic acids

The most common respiratory substrate is glucose (C₆H₁₂O₆). It is the preferred substrate because: 1. It can be easily oxidised compared to fats and proteins. 2. It yields energy quickly. 3. It is directly used in glycolysis.

When glucose is unavailable, the body uses fats and then proteins as respiratory substrates.

Glycolysis (Embden-Meyerhof-Parnas pathway):

Energy investment phase: 2 ATP consumed. Energy payoff phase: 4 ATP produced + 2 NADH generated. Net gain per glucose: 2 ATP + 2 NADH.

(See Intext Page 231 Q2 — answered above.)

Krebs cycle (TCA cycle / Citric acid cycle):

1. Acetyl-CoA (2C) + Oxaloacetate (4C) → Citrate (6C) (by citrate synthase) 2. Citrate → Isocitrate (6C) (by aconitase) 3. Isocitrate + NAD⁺ → α-Ketoglutarate (5C) + NADH + CO₂ (by isocitrate dehydrogenase) 4. α-Ketoglutarate + NAD⁺ + CoA → Succinyl-CoA (4C) + NADH + CO₂ (by α-ketoglutarate dehydrogenase) 5. Succinyl-CoA + GDP + Pi → Succinate (4C) + GTP (→ ATP) + CoA (by succinyl-CoA synthetase) 6. Succinate + FAD → Fumarate (4C) + FADH₂ (by succinate dehydrogenase) 7. Fumarate + H₂O → Malate (4C) (by fumarase) 8. Malate + NAD⁺ → Oxaloacetate (4C) + NADH + H⁺ (by malate dehydrogenase)

Per turn of the cycle (per acetyl-CoA): 3 NADH + 1 FADH₂ + 1 ATP + 2 CO₂ Per glucose (2 turns): 6 NADH + 2 FADH₂ + 2 ATP + 4 CO₂

(See Intext Page 236 Q1 — answered above.)

(See Intext Page 236 Q2 — answered above.)

(See Intext Page 240 Q1 — answered above.)

The respiratory pathway (specifically the Krebs cycle) is called an amphibolic pathway because it serves both catabolic and anabolic functions:

Catabolic role:

• The Krebs cycle breaks down acetyl-CoA (derived from carbohydrates, fats, and proteins) to release energy in the form of NADH, FADH₂, and GTP/ATP.
• CO₂ is released as a waste product.
• The electrons from NADH and FADH₂ are used in the ETC to produce more ATP.

Anabolic role:

• The intermediates of the Krebs cycle are withdrawn for the biosynthesis of various compounds:
• α-Ketoglutarate → amino acid glutamate → other amino acids
• Oxaloacetate → amino acid aspartate → pyrimidines
• Succinyl-CoA → porphyrins (haem, chlorophyll)
• Citrate → fatty acids and sterols (in cytoplasm)
• Acetyl-CoA → fatty acids, steroids
• When intermediates are withdrawn, they are replenished by anaplerotic reactions (e.g., pyruvate + CO₂ → oxaloacetate by pyruvate carboxylase).

Since the pathway functions as both a breakdown (catabolic) and synthesis (anabolic) route, it is amphibolic.

Respiratory Quotient (RQ) is defined as the ratio of the volume of CO₂ evolved to the volume of O₂ consumed during respiration:

RQ = Volume of CO₂ evolved / Volume of O₂ consumed

The RQ value for fats is approximately 0.7 (less than 1). This is because fats contain more hydrogen and less oxygen compared to carbohydrates, so more O₂ is required for complete oxidation than the CO₂ produced.

For tripalmitin: RQ = 51/72.5 = 0.70

The common pathway for both aerobic and anaerobic respiration is glycolysis (also called the Embden-Meyerhof-Parnas pathway).

• Glycolysis occurs in the cytoplasm and does not require oxygen.
• It breaks down one molecule of glucose (6C) into two molecules of pyruvate (3C).
• Net energy yield: 2 ATP + 2 NADH per glucose molecule.

After glycolysis, the fate of pyruvate differs: