Respiration in Plants

Respiration in Plants

Introduction

  • All cells require energy to facilitate physical and chemical changes.

  • Energy-requiring changes include biosynthesis, movement, uptake of materials, and development.

  • Phototrophs absorb light energy from the sun, converting low-energy inorganic molecules like carbon dioxide into energy-rich compounds during photosynthesis.

  • Chemotrophs oxidize high-energy molecules synthesized by phototrophs to obtain their energy.

  • Respiration is distinct from gaseous exchange; it denotes biological oxidation of organic molecules, releasing energy. Gaseous exchange refers to oxygen intake and carbon dioxide release.

Meaning and Characteristics of Respiration

  • The term "respiration" was introduced by Dutrochet.

  • It derives from the Latin word "respirate," meaning "to breathe."

  • Respiration is a catabolic, exergonic process but is essentially an amphibolic process.

  • It involves oxidation of organic compounds to release free energy utilized for ATP synthesis.

  • Oxidation substrates (respiratory substrates) typically include carbohydrates, fats, organic acids, or proteins.

  • Cellular Respiration: Flow of electrons from a reduced coenzyme to an electron acceptor leads to ATP production.

  • Energy is released as a controlled, stepwise process to minimize waste.

Differences Between Respiration and Combustion

Respiration

  1. Biochemical, occurs within living cells.

  2. Under biological control.

  3. Energy released in stages; only some energy lost as heat.

  4. Light not typically emitted.

  5. Enzymes catalyze each step.

  6. Multiple intermediates formed.

Combustion

  1. Physiochemical, non-cellular process.

  2. Uncontrolled process.

  3. Energy released in one step; most energy lost as heat.

  4. Light emitted.

  5. No enzymatic involvement.

  6. No intermediates produced.

Gaseous Exchange in Plants

  • Plants uptake oxygen and release carbon dioxide during respiration using all parts (roots, leaves, stems).

  • Specialized structures like stomata and lenticels facilitate gaseous exchange.

  • Nighttime respiration produces more carbon dioxide than oxygen, but daytime photosynthesis produces significantly more oxygen.

Respiratory Substrates

  • Carbohydrates (glucose) are the preferred respiratory substrates.

  • Fats are used infrequently; proteins are only used when carbohydrates and fats are depleted.

  • Products of glucose oxidation include CO₂, H₂O, and energy:
    C6H{12}O6 + 6O2
    ightarrow 6CO2 + 6H2O + energy

Types of Respiration

  • Respiration is categorized based on substrate type and oxygen presence:

    • Aerobic Respiration: Complete oxidation of substrates using oxygen.

    • Anaerobic Respiration: Incomplete oxidation of substrates without oxygen, resulting in lesser energy output.

Aerobic Respiration

  • Occurs in mitochondria, requiring oxygen and producing high energy (686 kcal per glucose).

  • Reaction:
    C6H{12}O6 + 6O2
    ightarrow 6CO2 + 6H2O + 36 ATP

Anaerobic Respiration

  • Takes place without oxygen, yielding lower ATP (2 ATP molecules).

  • Types:

    • Alcoholic Fermentation (in yeast and some bacteria):

    • Lactic Acid Fermentation (in human muscles):

Mechanism of Respiration

  • Stages:

    1. Glycolysis (occurs in cytoplasm).

    2. Pyruvate oxidation (connects glycolysis to TCA cycle).

    3. Krebs cycle (TCA cycle).

    4. Electron transport chain (ETS).

    5. ATP synthesis via oxidative phosphorylation.

Glycolysis

  • Glycolysis converts glucose to pyruvate in ten enzymatic steps, producing 2 ATP and 2 NADH:
    C6H{12}O_6 + 2 ADP + 2 NAD^+
    ightarrow 2 Pyruvate + 2 ATP + 2 NADH

  • Occurs in cytoplasm and is anaerobic.

  • Regulated by key enzymes: hexokinase, phosphofructokinase, pyruvate kinase.

Krebs Cycle

  • Located in mitochondrial matrix; processes Acetyl-CoA into energy carriers (NADH, FADH₂) and releases CO₂:
    Acetyl-CoA + 3 NAD^+ + FAD + GDP
    ightarrow 2 CO2 + CoA + 3 NADH + FADH2 + GTP

  • Net yield: 6 NADH, 2 FADH₂, 2 ATP (in eukaryotes); ATP/GTP via substrate-level phosphorylation.

Oxidative Phosphorylation

  • ETS: Transfers electrons from NADH and FADH₂ to O₂ forming water while generating a proton gradient.

  • ATP Synthase: Uses proton flow to synthesize ATP from ADP and inorganic phosphate.

  • Approximate yields: 36-38 ATP from complete aerobic oxidation of glucose.

Respiratory Quotient (RQ)

  • Defined as the ratio of CO₂ produced to O₂ consumed.

  • RQ Values:

    • Carbohydrates: 1.0

    • Fats: < 1 (usually ~0.7)

    • Proteins: ~0.9

Summary

  • Respiratory pathways integrate synthesis and decomposition (amphibolic pathways).

  • Cellular respiration involves glycolysis (cytoplasm), TCA cycle (mitochondria), and proton gradient driven ATP synthesis via oxidative phosphorylation.

Important Notes

  • Energy yields vary; optimal conditions yield 36-38 ATP per glucose molecule.

  • Intermediates from the Krebs cycle can be diverted for anabolic processes.

The above notes cover cellular respiration in depth, outlining key processes, characteristics, and metabolic byproducts essential for understanding plant physiology and metabolism.