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Respiration In Plants

Respiration in Plants - Complete NEET Guide with Diagrams & Practice Questions

Table of Contents

  1. Introduction
  2. Key Concepts
  3. Important Formulas & Equations
  4. Memory Techniques (Mnemonics)
  5. Previous Year Questions (NEET)
  6. Key Takeaways for Quick Revision

Introduction

Every living organism, from the smallest microbe to the largest plant, requires a constant supply of energy to perform life-sustaining activities. While plants are masters of capturing light energy via photosynthesis, they must convert this stored chemical energy into a usable form—ATP. This process is cellular respiration. For NEET aspirants, this chapter is crucial as it lays the foundation for understanding energy metabolism in all living systems. You can typically expect 1-2 questions from this topic, often focusing on the sites, products, and net energy gain of its various stages.

This guide will break down the intricate pathways of respiration, starting from the universal process of glycolysis to the complex machinery of the Electron Transport System. We will explore the differences between aerobic and anaerobic respiration, calculate the net ATP yield, and understand why this pathway is central to all metabolism.

Key Concepts

1. Cellular Respiration: The Basics

Cellular respiration is the mechanism of breaking down food materials (respiratory substrates) within the cell to release energy and trap it in the form of ATP.

  • Respiratory Substrates: Compounds that are oxidized during respiration. The most common is glucose. Fats, proteins, and organic acids can also be used.
  • Do Plants Breathe? Yes, but they lack specialized respiratory organs like animals. Gas exchange occurs through stomata in leaves and lenticels in stems. Each plant part typically takes care of its own gas exchange needs.
  • Overall Equation for Aerobic Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP + Heat)

2. Glycolysis: The Universal First Step (EMP Pathway)

Glycolysis ('splitting of sugar') is the partial oxidation of glucose into two molecules of pyruvic acid. It is a universal pathway found in the cytoplasm of all living organisms, both aerobic and anaerobic.

  • Process: A ten-step enzymatic pathway.

    1. Energy Investment Phase: Two ATP molecules are consumed to phosphorylate glucose and then fructose-6-phosphate.
    2. Splitting Phase: Fructose-1,6-bisphosphate (6C) is split into two 3-carbon molecules: PGAL and DHAP (which is converted to PGAL).
    3. Energy Generation Phase: The two PGAL molecules are oxidized, yielding 4 ATP (via substrate-level phosphorylation) and 2 NADH + H⁺.

  • Net Gain from one Glucose molecule:

    • 2 Pyruvic Acid (3C molecules)
    • 2 ATP (4 produced - 2 consumed)
    • 2 NADH + 2H⁺

Figure: The Glycolysis Pathway (EMP Pathway). This flowchart shows the 10 steps, highlighting the points of ATP consumption (steps 1 & 3) and ATP/NADH production (steps 7, 10 for ATP; step 6 for NADH).

3. Fermentation: The Anaerobic Fate of Pyruvate

In the absence of oxygen, some organisms perform fermentation to regenerate the NAD⁺ required for glycolysis to continue. It is an incomplete oxidation of glucose.

  • Location: Cytoplasm.

  • Types of Fermentation:

    1. Alcoholic Fermentation: Occurs in yeast. Pyruvic acid is first decarboxylated to acetaldehyde, which is then reduced by NADH to form ethanol and CO₂.
    2. Lactic Acid Fermentation: Occurs in some bacteria and animal muscle cells during strenuous exercise. Pyruvic acid is directly reduced by NADH to form lactic acid.

  • Net ATP Gain: Only 2 ATP (from glycolysis), as no further ATP is produced during fermentation itself.

Figure: Major pathways of anaerobic respiration, showing pyruvic acid being converted to either Lactic acid or Ethanol + CO₂, with the purpose of regenerating NAD⁺ from NADH.

4. Aerobic Respiration: The Complete Oxidation

This process occurs in the presence of oxygen and involves the complete oxidation of pyruvic acid into CO₂ and H₂O, releasing a large amount of energy.

  • Location: Mitochondria.
The Link Reaction: Gateway to the Krebs Cycle

Before entering the Krebs cycle, pyruvic acid is transported into the mitochondrial matrix where it undergoes oxidative decarboxylation.

  • Process: Pyruvic acid (3C) is converted into Acetyl-CoA (2C).
  • Enzyme: Pyruvate dehydrogenase complex.
  • Products (per glucose, i.e., 2 pyruvic acid):
    • 2 Acetyl-CoA
    • 2 CO₂
    • 2 NADH + 2H⁺
Krebs Cycle (TCA Cycle)

The Tricarboxylic Acid (TCA) or Citric Acid Cycle occurs in the mitochondrial matrix.

  • Process:

    1. Acetyl-CoA (2C) combines with oxaloacetic acid (OAA, 4C) to form citric acid (6C).
    2. The cycle involves a series of reactions where citric acid is oxidized, releasing CO₂ and generating high-energy molecules.
    3. OAA is regenerated at the end of the cycle.

  • Products (per glucose, i.e., 2 turns of the cycle):

    • 4 CO₂
    • 6 NADH + 6H⁺
    • 2 FADH₂
    • 2 ATP (via substrate-level phosphorylation of GTP)

Figure: The Krebs Cycle (Citric Acid Cycle). This diagram shows the cyclic pathway starting with Acetyl-CoA + OAA, with key points of NADH, FADH₂, ATP, and CO₂ production.

Electron Transport System (ETS) & Oxidative Phosphorylation

This is the final stage of aerobic respiration, occurring on the inner mitochondrial membrane.

  • Process:

    1. Electrons from NADH and FADH₂ are passed along a series of protein complexes (I to IV).
    2. NADH donates electrons to Complex I, while FADH₂ donates to Complex II.
    3. As electrons move through the chain, energy is released, which is used to pump protons (H⁺) from the matrix into the intermembrane space, creating a proton gradient.
    4. Oxygen acts as the final electron acceptor at Complex IV, combining with protons to form water.
    5. The proton gradient drives H⁺ back into the matrix through Complex V (ATP Synthase). This movement powers the synthesis of ATP from ADP and Pi. This process is called oxidative phosphorylation.

  • Energy Yield:

    • 1 NADH → 3 ATP
    • 1 FADH₂ → 2 ATP

Figure: The Electron Transport System (ETS). This diagram illustrates the arrangement of Complexes I-V on the inner mitochondrial membrane, showing electron flow, proton pumping, and the final synthesis of ATP by ATP synthase.

5. The Respiratory Balance Sheet

A theoretical calculation of the net ATP produced from one molecule of glucose.

PathwayNADHFADH₂ATP (Direct)Total ATP
Glycolysis2022 + (2x3) = 8
Link Reaction2002x3 = 6
Krebs Cycle622(6x3) + (2x2) + 2 = 24
Total102438 ATP

Note: This calculation assumes that NADH from glycolysis is transported into the mitochondria to yield 3 ATP each. In some systems, it yields only 2 ATP, resulting in a total of 36 ATP.

6. Amphibolic Pathway

The respiratory pathway is not purely a catabolic (breakdown) process. It is an amphibolic pathway, meaning it involves both catabolism and anabolism (synthesis).

  • Catabolism: Carbohydrates, fats, and proteins are broken down to release energy.
  • Anabolism: Intermediates from the pathway are withdrawn to synthesize other molecules.
    • Acetyl-CoA can be used to synthesize fatty acids.
    • α-ketoglutaric acid can be used to synthesize amino acids.

Figure: Interrelationship of metabolic pathways showing how fats and proteins can enter the respiratory pathway, and how intermediates can be withdrawn for synthesis, demonstrating its amphibolic nature.

7. Respiratory Quotient (RQ)

RQ is the ratio of the volume of CO₂ evolved to the volume of O₂ consumed during respiration.

  • Formula: RQ = Volume of CO₂ evolved / Volume of O₂ consumed
  • RQ values for different substrates:
    • Carbohydrates: RQ = 1.0 (e.g., C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O; RQ = 6/6 = 1)
    • Fats: RQ < 1 (e.g., Tripalmitin ≈ 0.7) because fats are more reduced and require more O₂ for complete oxidation.
    • Proteins: RQ ≈ 0.9
    • Organic Acids: RQ > 1

Important Formulas & Equations

  • Overall Aerobic Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP
  • Glycolysis Net Reaction: Glucose + 2NAD⁺ + 2ADP + 2Pi → 2 Pyruvate + 2NADH + 2ATP + 2H₂O
  • Link Reaction (x2): 2 Pyruvate + 2NAD⁺ + 2CoA → 2 Acetyl-CoA + 2NADH + 2CO₂
  • Respiratory Quotient (RQ): RQ = (Volume of CO₂ evolved) / (Volume of O₂ consumed)

Memory Techniques (Mnemonics)

  • Krebs Cycle Intermediates: "Citrate Is Krebs' Starting Substrate For Making Oxaloacetate"
    • Citrate, Isocitrate, α-Ketoglutarate, Succinyl-CoA, Succinate, Fumarate, Malate, Oxaloacetate.
  • Glycolysis Products: "Go Paint Pictures" - Glucose -> 2 Pyruvate + 2 Protons (NADH+H⁺).
  • Sites of Respiration:
    • Glycolysis -> Cytoplasm (Goes in the Cell's fluid).
    • Krebs Cycle -> Matrix (Kreeps into the Middle).
    • ETS -> Inner Membrane (Electrons Inside Mitochondria).

Previous Year Questions (NEET)

Q1. What is the role of NAD⁺ in cellular respiration? (NEET 2018) a) It is a nucleotide source for ATP synthesis. b) It functions as the final electron acceptor. c) It functions as an electron carrier. d) It is a final product of anaerobic respiration.

Explanation: During glycolysis and the Krebs cycle, NAD⁺ accepts electrons (and protons) to become NADH. It then carries these high-energy electrons to the Electron Transport System. Therefore, it acts as an electron carrier. Answer: (c) It functions as an electron carrier.

Q2. The number of substrate-level phosphorylations in one turn of citric acid cycle is: (NEET 2020) a) One b) Two c) Three d) Zero

Explanation: Substrate-level phosphorylation is the direct synthesis of ATP (or GTP) from ADP (or GDP). In one turn of the Krebs cycle, one molecule of GTP is produced during the conversion of succinyl-CoA to succinate. This GTP is equivalent to one ATP. Answer: (a) One

Q3. The Respiratory Quotient (RQ) value of tripalmitin is: (NEET 2019) a) 0.9 b) 0.7 c) 0.07 d) 0.09

Explanation: Tripalmitin is a fatty acid. Fats are less oxidized than carbohydrates and require more oxygen for their complete oxidation relative to the amount of CO₂ produced. Their RQ is always less than 1. For tripalmitin, the calculated value is approximately 0.7. Answer: (b) 0.7

Q4. Where is the Electron Transport System (ETS) located in a plant cell? (NEET-II 2016) a) Outer mitochondrial membrane b) Intermembrane space c) Inner mitochondrial membrane d) Mitochondrial matrix

Explanation: The protein complexes (I-V) that constitute the ETS are embedded in the inner mitochondrial membrane, which allows for the pumping of protons into the intermembrane space to create a gradient. Answer: (c) Inner mitochondrial membrane

Key Takeaways for Quick Revision

  • Glycolysis occurs in the cytoplasm and yields 2 ATP and 2 NADH. It is common to both aerobic and anaerobic pathways.
  • Krebs Cycle occurs in the mitochondrial matrix, completely oxidizing Acetyl-CoA to produce NADH, FADH₂, ATP, and CO₂.
  • The Electron Transport System (ETS) is located on the inner mitochondrial membrane.
  • Oxygen is the final electron acceptor in the ETS, forming water.
  • Oxidative Phosphorylation uses the proton gradient generated by the ETS to synthesize the bulk of ATP.
  • The respiratory pathway is amphibolic, providing intermediates for both catabolic and anabolic processes.
  • Respiratory Quotient (RQ) depends on the substrate: 1 for carbs, <1 for fats, and >1 for organic acids.
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