Chapter 14

Chapter 14 : Respiration in Plants

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Cellular Respiration


  • It is the process of oxidation / breakdown of food materials within the cell to release energy.  Respiratory substrate to be oxidised during respiration is usually glucose, but these can also be proteins, fats or organic acids. 
  • In plants respiration gas exchange occurs through stomata and lenticels.
  • Overall cellular respiration is:
    C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (36 ATPs)
Types of respiration:

·         Aerobic respiration

·         Anaerobic  respiration

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Mechanism of respiration :

·         Glycolysis – it is common to both aerobic and anaerobic respiration

·         Citric acid cycle / Krebs cycle - Aerobic respiration in mitochondria

·         Electron transport system – in the inner membrane of mitochondria

·         Both aerobic and anaerobic respiration starts with Glycolysis.

·         In aerobic respiration Glycolysis is followed by Citric acid cycle and ETS (both occur in mitochondria).

·         In anaerobic respiration Glycolysis is followed by formation of ethyl alcohol / lactic acid in the cytoplasm.

 
Fermentation :

Incomplete oxidation of pyruvic  acid, under anaerobic respiration forms lactic acid/ ethyl alcohol.  It occurs in bacteria, yeast and striated muscles.

In yeast fermentation:

o   Pyruvic acid → Ethanol + CO2

o   Enzymes involved − Pyruvic acid decarboxylase, Alcohol dehydrogenas.


  • Only 7% of energy of glucose is released during fermentation.
  • Yeasts poison themselves to death when alcohol concentration reaches about 13%.
In bacterial fermentation:

o    Pyruvic acid → Lactic acid.

o    Enzyme involved − Lactate dehydrogenase.

o    While doing severe exercise similar reaction occurs in animal muscles in anaerobic conditions.

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Glycolysis :


  • It is the process of breaking down of glucose to pyruvic acid.
  • It was given by Embden, Meyerhof and Parnas
  • A chain of 10 reactions converts glucose into pyruvate.
  • Net ATPs produced = 4 (produced) − 2 (consumed) = 2 ATPs
  • The pyruvate, so produced, may undergo:
    • Lactic acid fermentation
    • Alcoholic fermentation
    • Aerobic respiration (Krebs cycle)
Aerobic Respiration

Citric acid cycle / Tricarboxylic acid cycle / Kreb’s cycle:


    • TCA cycle – it takes place in the mitochondrial matrix – it is the process of complete oxidation of pyruvate by stepwise removal of all hydrogen atoms, which leaves three molecules of CO2
    • Electron Transport Chain and Oxidative phosphorylation – it takes place in the inner membrane of the mitochondria – it is the process of synthesis of ATP fron NADH2 and FADH2.

Formation of Acetyl Coenzyme A

Krebs Cycle/ Tricarboxylic acid cycle / Citric acid cycle:

Overall equation

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Electron Transport Chain (ETS)


  • NADH2 and FADH2 are oxidised to release the energy stored in them in the form of ATPs.
  • Electrons are passed from one carrier to another, and finally to oxygen, resulting in the formation of water.



  • Oxidation of 1 NADH produces 3 ATPs.
    Oxidation of 1 FADH2 produces 2 ATPs.

Oxidative Phosphorylation

Picture Respiratory Balance Sheet


  • Glucose + 6O2 + 36ADP + 36Pi → 6CO2 + 6H2O + 36ATP
Amphibolic Pathway:  Involved in both anabolism and catabolism



Respiratory Quotient (RQ)


  • It is the ratio of the volume of CO2 evolved to the volume of O2 consumed during respiration.
  • RQ = 1 (When carbohydrate is used as substrate)
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
                                  


·         RQ is less than 1 for fats.

2 C51 H98 O6 +145 O2 - --> 102CO2 + 98H2O + Energy

                         RQ = 102 CO2
                               --------------     = 0.7
                                   145 O2

·         RQ is 0.9 for proteins.

·         RQ is more than 1 for organic acids.

·         RQ is infinite in case of anaerobic resp. because CO2 is evolved but O2 is not consumed.