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
CSIR NET UNIT 6 Notes
- Photosynthesis
Overall process – Carbon reduction
CO2 ——–> Organic Carbohydrates - Respiration
Overall process – Carbon oxidation
Organic Carbohydrates ———-> CO2 + Energy
Respiration
- 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
- Glycolysis (NADH produced)
Pentose Phosphate Pathway (NADPH produced) - Citric Acid Cycle (NADH, FADH2 produced)
- Oxidative Phosphorylation (e- from NADH and related species from steps 1, 2, and 3 are transferred to O2 , leading to the synthesis of ATP)
Glycolysis
- 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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