4.2 Respiration in Plants

Introduction to Respiration

Respiration is a metabolic process that occurs in all living organisms, including plants, to release energy from organic molecules (like glucose) for various cellular activities. This energy is primarily stored in the form of ATP (adenosine triphosphate).

Exchange of Gases

In plants, the exchange of gases (oxygen and carbon dioxide) primarily occurs through:

• Stomata: Small pores on the surface of leaves, regulated by guard cells.
• Lenticels: Pores on the bark of woody stems and roots.
• General surface of roots: Younger roots can absorb oxygen from the soil.

Plants respire throughout the day and night. During the day, photosynthesis produces oxygen, some of which is used for respiration. At night, only respiration occurs, leading to a net release of carbon dioxide.

Types of Respiration

Respiration can be broadly classified into two types based on the presence or absence of oxygen:

1. Aerobic Respiration: Occurs in the presence of oxygen. It is a complete oxidation of organic substances, releasing a large amount of energy.
2. Anaerobic Respiration (Fermentation): Occurs in the absence of oxygen. It is an incomplete oxidation of organic substances, releasing a smaller amount of energy.

Cellular Respiration: An Overview

Cellular respiration is a complex process that involves several stages:

1. Glycolysis: Occurs in the cytoplasm.
2. Krebs Cycle (TCA Cycle): Occurs in the mitochondrial matrix.
3. Electron Transport System (ETS) / Oxidative Phosphorylation: Occurs on the inner mitochondrial membrane.

Glycolysis (EMP Pathway)

Glycolysis is the first stage of both aerobic and anaerobic respiration. It is a 10-step process that breaks down one molecule of glucose (a 6-carbon compound) into two molecules of pyruvate (a 3-carbon compound).

Location: Cytoplasm

Key Steps (Flowchart Idea):

Glucose (6C)
↓ (ATP used)
Glucose-6-phosphate
↓
Fructose-6-phosphate
↓ (ATP used)
Fructose-1,6-bisphosphate
↓
Dihydroxyacetone phosphate + Glyceraldehyde-3-phosphate
↓ (interconversion)
2 x Glyceraldehyde-3-phosphate
↓ (NADH produced)
2 x 1,3-Bisphosphoglycerate
↓ (ATP produced)
2 x 3-Phosphoglycerate
↓
2 x 2-Phosphoglycerate
↓
2 x Phosphoenolpyruvate
↓ (ATP produced)
2 x Pyruvate (3C)

Net Products of Glycolysis (per glucose molecule):

• 2 molecules of Pyruvate
• 2 molecules of ATP (net gain, 4 produced - 2 consumed)
• 2 molecules of NADH

Fermentation (Anaerobic Respiration)

Fermentation occurs when oxygen is not available after glycolysis. Pyruvate is converted into different products to regenerate NAD+ from NADH, allowing glycolysis to continue.

Brief Idea of Fermentation:

• Alcoholic Fermentation: Occurs in yeast and some bacteria. Pyruvate is converted to acetaldehyde, and then to ethanol, releasing CO₂. Pyruvate → Acetaldehyde + CO₂ Acetaldehyde + NADH → Ethanol + NAD+

• Lactic Acid Fermentation: Occurs in some bacteria and animal muscle cells under anaerobic conditions. Pyruvate is directly converted to lactic acid. Pyruvate + NADH → Lactic Acid + NAD+

Energy Yield: Fermentation yields very little energy (only the 2 ATP from glycolysis) compared to aerobic respiration.

Krebs Cycle (TCA Cycle / Citric Acid Cycle)

The Krebs cycle is the central metabolic pathway in aerobic respiration. Before entering the Krebs cycle, pyruvate (from glycolysis) is converted to Acetyl-CoA in a process called oxidative decarboxylation.

Location: Mitochondrial matrix

Oxidative Decarboxylation of Pyruvate:

Pyruvate + CoA + NAD+ → Acetyl-CoA + CO₂ + NADH

Key Steps of Krebs Cycle (Flowchart Idea - per Acetyl-CoA):

Acetyl-CoA (2C) + Oxaloacetate (4C)
↓
Citrate (6C)
↓
Isocitrate (6C)
↓ (CO₂ released, NADH produced)
α-Ketoglutarate (5C)
↓ (CO₂ released, NADH produced, ATP/GTP produced)
Succinyl-CoA (4C)
↓
Succinate (4C)
↓ (FADH₂ produced)
Fumarate (4C)
↓
Malate (4C)
↓ (NADH produced)
Oxaloacetate (4C) (regenerated)

Net Products of Krebs Cycle (per glucose molecule, i.e., 2 Acetyl-CoA):

• 6 molecules of NADH
• 2 molecules of FADH₂
• 2 molecules of ATP (or GTP)
• 4 molecules of CO₂

Electron Transport System (ETS) and Oxidative Phosphorylation

This is the final stage of aerobic respiration, where the majority of ATP is produced.

Location: Inner mitochondrial membrane

Brief Idea of ETS (Flowchart Idea):

NADH and FADH₂ (produced in glycolysis and Krebs cycle) donate their electrons to a series of protein complexes (Complex I, II, III, IV) embedded in the inner mitochondrial membrane.

Electrons move from a higher energy level to a lower energy level through these complexes.

As electrons move, protons (H+) are pumped from the mitochondrial matrix into the intermembrane space, creating a proton gradient.

Oxygen acts as the final electron acceptor, combining with electrons and protons to form water.

Oxidative Phosphorylation:

Definition: The process of ATP synthesis that occurs when the energy released from the oxidation of NADH and FADH₂ (via the electron transport chain) is used to generate a proton gradient, which then drives the synthesis of ATP by ATP synthase.

Mechanism: The proton gradient created by the ETS drives protons back into the mitochondrial matrix through ATP synthase (Complex V). This flow of protons powers the synthesis of ATP from ADP and inorganic phosphate.

Energy Relations: Number of ATP Molecules Generated

The theoretical maximum ATP yield from the complete aerobic respiration of one glucose molecule is approximately 30-32 ATP molecules.

Note: The exact number of ATPs can vary due to different shuttle systems for NADH from glycolysis into the mitochondria and the efficiency of proton pumping.

Amphibolic Pathways

Definition: Metabolic pathways that can function in both catabolism (breakdown of molecules) and anabolism (synthesis of molecules). They are central to metabolism, allowing for the interconversion of different types of molecules.

Brief Idea of Amphibolic Pathway:

Cellular respiration, particularly the Krebs cycle, is a prime example of an amphibolic pathway.

• Catabolic Role: The Krebs cycle breaks down Acetyl-CoA to produce ATP, NADH, and FADH₂.
• Anabolic Role: Intermediates of the Krebs cycle can be used as precursors for the synthesis of other molecules:
  • α-Ketoglutarate: Can be used to synthesize amino acids.
  • Oxaloacetate: Can be used to synthesize amino acids and glucose (via gluconeogenesis).
  • Succinyl-CoA: Can be used for chlorophyll and heme synthesis.

This dual nature allows the cell to efficiently manage its resources, breaking down molecules when energy is needed and building up molecules when precursors are available.

Respiratory Quotient (RQ)

Definition: The ratio of the volume of carbon dioxide (CO₂) evolved to the volume of oxygen (O₂) consumed during respiration.

RQ = Volume of CO₂ evolved / Volume of O₂ consumed

Significance: RQ values provide information about the type of respiratory substrate being oxidized.

RQ Values of Different Substrates:

• Carbohydrates: RQ = 1 Example: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O RQ = 6CO₂ / 6O₂ = 1

• Fats: RQ < 1 (typically around 0.7) Fats have less oxygen relative to carbon and hydrogen, so they require more oxygen for complete oxidation, leading to a smaller RQ. Example: 2C₅₁H₉₈O₆ + 145O₂ → 102CO₂ + 98H₂O RQ = 102CO₂ / 145O₂ ≈ 0.7

• Proteins: RQ < 1 (typically around 0.8-0.9) Proteins are complex and their exact RQ depends on the specific amino acids being respired.

• Organic Acids: RQ > 1 Organic acids are rich in oxygen, so they require less external oxygen for oxidation, leading to an RQ greater than 1. Example: 4C₂H₂O₅ (Oxalic Acid) + O₂ → 8CO₂ + 4H₂O RQ = 8CO₂ / 1O₂ = 8

• Anaerobic Respiration: RQ = Infinity (∞) In anaerobic respiration, CO₂ is evolved but no O₂ is consumed. Example (Alcoholic Fermentation): C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ RQ = 2CO₂ / 0O₂ = ∞