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CSIR UNIT 6 Notes – System Physiology – Plant: Respiration

Students from Non-Botany background might think to skip this UNIT but one must not, as it has a weightage of almost 30 marks or more in the CSIR NET Life Science Exam. The topics in this UNIT are not very difficult. One can find Topics from UNIT 1 repeated here like Electron Transport Chain, nitrogen metabolism, etc. It’s advisable not to skip this UNIT.

Reference Book – Plant Physiology, by Lincoln Taiz and Eduardo Zeiger


  • Photosynthesis
    Overall process – Carbon reduction
    CO2 ——–> Organic Carbohydrates
  • Respiration
    Overall process – Carbon oxidation
    Organic Carbohydrates ———-> CO2 + Energy


  • Biological process whereby the energy stored in carbohydrates is released in a step-wise, controlled manner.
  • The energy released is coupled to the synthesis of ATP.
  • ATP is essential for plant cell maintenance, growth, and development

Respiration: 3 Major Steps CSIR UNIT 6 Notes

  1. Glycolysis (NADH produced)
    Pentose Phosphate Pathway (NADPH produced)
  2. Citric Acid Cycle (NADH, FADH2 produced)
  3. Oxidative Phosphorylation (e- from NADH and related species from steps 1, 2, and 3 are transferred to O2 , leading to the synthesis of ATP)


  • glykos, “sugar,” and lysis, “splitting”
  • Source: Sucrose, Starch
  • 1st Part: ATP input

  • 2nd Part: Generation of ATP and Reducing Equivalents
  • At the end of the glycolytic sequence, plants have alternative pathways for metabolizing PEP
  • End Products: Pyruvate and Malate
  • Malate can be stored in the vacuole or transported to the mitochondrion where it enters the citric acid cycle

Glycolysis Stoichiometry

 If glucose is the source

  • 2 ATPs consumed
  • 2 NADH produced
  • 4 ATP produced
  • Net production: 2 ATP and 2 NADH

If Sucrose is the source

  • 4 ATPs consumed
  • 4 NADH produced
  • 8 ATP produced
  • Net production: 4 ATP and 4 NADH

Fermentative Metabolism

In absence of O2 citric acid cycle and oxidative phosphorylation cannot occur
• In absence of O2, Fermentation Regenerates the NAD+ Needed for Glycolysis
• 2 Pathways:
✓Lactate dehydrogenase forms Lactate (High acid production)
✓Pyruvate decarboxylase and Alcohol dehydrogenase form Ethanol with the release of CO2
• Regeneration of NAD+
• Low O2 (hypoxic) or zero O2 (anoxic) conditions
• Eg. Flooded or water-logged soil
• Ethanol is thought to be less toxic compared to lactate


The efficiency of Fermentative Metabolism

• Glucose to Ethanol: net synthesis of 2 ATP
• Sucrose to Ethanol: net synthesis of 4 ATP
• Efficiency of Anaerobic fermentation compared to aerobic respiration: 4-6%
• Most energy remains in the form of lactate/ethanol
• High rate of glycolysis required to sustain ATP production
• Pasteur Effect: increased glycolysis when yeast switch from aerobic respiration to anaerobic alcoholic fermentation

Pentose Phosphate Pathway (PPP) …. CONTD

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