Respiration in Plants

Class 11 Biology: Respiration in Plants - Quick Revision Notes

### Introduction to Plant Respiration

• Cellular Respiration: The enzymatic breakdown of complex organic molecules (like glucose) in the presence or absence of oxygen to release energy (ATP) inside living cells.
• Equation of Aerobic Respiration: C6H12O6 + 6O2 -> 6CO2 + 6H2O + Energy (ATP)
• Respiratory Substrate: The organic substance that is oxidized during respiration (e.g., Carbohydrates, Fats, Proteins). Glucose is the most common respiratory substrate.

### Types of Respiration

1. Aerobic Respiration: Takes place in the presence of oxygen; complete breakdown of glucose into $CO_2$ and $H_2O$ with high energy yield (36 or 38 ATP).
2. Anaerobic Respiration (Fermentation): Takes place in the absence of oxygen; incomplete breakdown of glucose into ethanol/lactic acid and $CO_2$ with low energy yield (2 ATP).

### Major Pathways of Respiration

1. Glycolysis (EMP Pathway - Embden-Meyerhof-Parnas Pathway)

• Site: Cytoplasm of the cell.
• Oxygen requirement: Does not require oxygen (common to both aerobic and anaerobic respiration).
• Net Reaction: 1 Glucose (6C) -> 2 Pyruvic Acid (3C)
• Energy Yield:
  • Total ATP produced = 4 ATP
  • ATP consumed = 2 ATP
  • Net Gain = 2 ATP + 2 NADH + $H^+$

2. Fermentation (Anaerobic Respiration)

• Alcoholic Fermentation: Pyruvic acid is converted into Ethanol and $CO_2$ by enzymes pyruvate decarboxylase and alcohol dehydrogenase (e.g., in Yeast).
• Lactic Acid Fermentation: Pyruvic acid is reduced to Lactic acid by lactate dehydrogenase (e.g., in skeletal muscles and some bacteria).
• Energy Yield: Very low; only 2 ATP are gained per glucose molecule.

3. Tricarboxylic Acid Cycle (TCA Cycle / Krebs Cycle)

• Site: Mitochondrial Matrix.
• Link Reaction: Pyruvic acid enters the mitochondria and is converted into Acetyl Coenzyme A (Acetyl CoA) before entering the Krebs cycle.
  • Reaction: 2 Pyruvic Acid + 2 CoA + 2 NAD+ -> 2 Acetyl CoA + 2 CO2 + 2 NADH + 2 H+
• First Product: Citric Acid (hence also called Citric Acid Cycle).
• Net Products per 1 Glucose (2 turns of cycle): 6 NADH + H+, 2 FADH2, 4 CO2, 2 GTP (ATP)

4. Electron Transport System (ETS) and Oxidative Phosphorylation

• Site: Inner Mitochondrial Membrane.
• Mechanism: Electrons from NADH and $FADH_2$ (formed during glycolysis and Krebs cycle) are passed along a series of electron carriers (Complex I to IV).
• Terminal Electron Acceptor: Oxygen ($O_2$), which combines with protons to form water ($H_2O$).
• Oxidative Phosphorylation: The synthesis of ATP from ADP and inorganic phosphate using the energy derived from the proton gradient ($H^+$ gradient across the inner mitochondrial membrane). Driven by ATP synthase (Complex V).

### Respiratory Quotient (RQ)

• Definition: The ratio of the volume of $CO_2$ evolved to the volume of $O_2$ consumed in respiration.

• Formula: RQ = (Volume of CO2 evolved) / (Volume of O2 consumed)

• RQ Values for Different Substrates:

  • Carbohydrates: $1$ (e.g., Glucose: $\frac{6CO_2}{6O_2} = 1$)
  • Fats / Lipids: Less than $1$ (approx. $0.7$, e.g., Tripalmitin)
  • Proteins: Around $0.9$
  • Organic Acids: Greater than $1$ (e.g., Malic acid)
  • Anaerobic Respiration: Infinity ($\infty$) (since $O_2$ consumption is zero).

### Energy Yield (ATP Calculation)

• Complete oxidation of one glucose molecule yields 36 to 38 ATP molecules (depending on the shuttle system operating between cytoplasm and mitochondria).
• Substrate-level phosphorylation: Direct synthesis of ATP (e.g., in Glycolysis and Krebs cycle).