Respiration in Plants

Respiratory Substrates: Organic compounds that are oxidised during respiration to release energy. Examples: Carbohydrates, Proteins, Fats, Organic acids.

Most Common Substrate: Glucose (Carbohydrate).

Main Steps:

1. Glycolysis: Breakdown of glucose to Pyruvate (Cytoplasm).
2. Link Reaction: Pyruvate to Acetyl CoA (Mitochondrial Matrix).
3. Krebs’ Cycle (TCA Cycle): Oxidation of Acetyl CoA (Mitochondrial Matrix).
4. Electron Transport System (ETS) & Oxidative Phosphorylation: ATP synthesis (Inner Mitochondrial Membrane).

ETS: A metabolic pathway present in the inner mitochondrial membrane through which electrons pass from one carrier to another.

• Electrons from NADH and FADH$_2$ (produced in glycolysis/Krebs cycle) are passed through complexes I to IV.
• As electrons move, protons ($H^+$) are pumped from the matrix to the intermembrane space, creating a proton gradient.
• Electrons are finally accepted by Oxygen to form metabolic water.
• The energy from the proton gradient is used by ATP Synthase (Complex V) to synthesize ATP (Oxidative Phosphorylation).

(Refer to Q1 for detailed tables. Key points summarized below)

• (a) Aerobic vs Anaerobic: Presence vs Absence of $O_2$; Complete vs Partial breakdown; High vs Low energy.
• (b) Glycolysis vs Fermentation: Glycolysis is the first step (common to both). Fermentation is the anaerobic fate of pyruvate (lactate/alcohol formation). Glycolysis produces Pyruvate; Fermentation reduces it.
• (c) Glycolysis vs Citric Acid Cycle: Linear (Cytoplasm) vs Cyclic (Mitochondria). Partial oxidation vs Complete oxidation (of Acetyl group).

1. Sequential Pathway: Glycolysis $\to$ TCA Cycle $\to$ ETS occur in an orderly fashion.
2. NADH Transfer: NADH synthesised in glycolysis enters mitochondria for phosphorylation.
3. No Intermediates Used: Intermediates in the pathway are not used to synthesise other compounds.
4. Glucose Only: Only glucose is being respired; no other alternative substrates enter the pathway.

Note: These assumptions are not valid in living systems as pathways are interconnected (amphibolic).

Amphibolic Pathway: A pathway involved in both Catabolism (breakdown) and Anabolism (synthesis).

• Catabolic Role: Break down of carbohydrates, fats, and proteins to release energy (ATP).
• Anabolic Role: Intermediates of the pathway are withdrawn to synthesise bio-molecules.
  • Acetyl CoA is withdrawn to synthesize fatty acids/gibberellins.
  • Succinyl CoA is used for chlorophyll/cytochrome synthesis.
  • Alpha-ketoglutaric acid is used for amino acid synthesis.

Since it serves both breakdown and synthesis, it is amphibolic, not just catabolic.

Respiratory Quotient (RQ): The ratio of the volume of $CO_2$ evolved to the volume of $O_2$ consumed in respiration.

Value for Fats: Less than 1 (approx 0.7). Fats require more oxygen for oxidation due to lower oxygen content in their structure.

Oxidative Phosphorylation: The synthesis of ATP from ADP and inorganic phosphate ($Pi$) occurring in the inner mitochondrial membrane, driven by the energy released from the oxidation of NADH and FADH$_2$ via the Electron Transport System (ETS). Oxygen acts as the final electron acceptor.

1. Efficiency: It allows energy to be trapped efficiently in the form of ATP bonds rather than being lost as heat in one go.
2. Control: The rate of energy release can be regulated by enzymes at various steps.
3. Intermediates: Step-wise breakdown produces various intermediates that serve as precursors for the synthesis of other essential biomolecules (Amphibolic nature).
4. Prevention of Damage: Sudden release of large amounts of energy (heat) could damage cell structures.