Tuesday, September 15, 2020

Respiration in Plants - Notes | Class 11 | Part 5: Respiratory Balance Sheet, Amphibolic Pathway, RQ

14. RESPIRATION IN PLANTS 

THE RESPIRATORY BALANCE SHEET


Net gain of ATP from each glucose molecule is calculated based on the following assumptions:All steps in Glycolysis, TCA cycle & ETS occur sequentially and orderly.
  • The NADH synthesised in glycolysis is transferred into mitochondria and undergoes oxidative phosphorylation.
  • Intermediates in the pathway are not used to synthesise other compounds.
  • Only glucose is being respired. Other alternative substrates are not entered in the pathway at any stages.
Such assumptions are not valid because,
  • All pathways work simultaneously and do not take place one after another.
  • Substrates enter the pathways and are withdrawn from it as and when necessary.
  • ATP is utilized as and when needed.
  • Enzymatic rates are controlled by multiple means.
Such calculations are useful to appreciate the efficiency of the living system in extraction and storing energy. 
Net gain of ATP molecules from one glucose molecule

Glycolysis

2 ATP directly

2 ATP

2 molecules of NADH

6 ATP

Oxidative decarboxylation

2 NADH

6 ATP

TCA cycle

6 NADH

18 ATP

2 FADH

4 ATP

2 GTP

2 ATP

Total

38 ATP


2 ATP molecules are spent for transporting 2 NADH molecules formed during glycolysis to the mitochondria. Hence the net gain = 36 ATP molecules.

Comparison b/w fermentation & aerobic respiration

Fermentation

Aerobic respiration

Partial breakdown of glucose.

Complete breakdown of glucose to CO2 & H2O.

Net gain of only 2 ATP.

Net gain of 36 ATP.

NADH is oxidised to NAD+ rather slowly.

NADH is oxidised to NAD+ very vigorously.

 

AMPHIBOLIC PATHWAY


Glucose is the favoured substrate for respiration. So, all carbohydrates are first converted to glucose for respiration.

Other substrates are also respired. 



Fats breakdown into glycerol & fatty acids. Fatty acids are degraded to acetyl CoA and enter the pathway. Glycerol is converted to PGAL and enters the pathway.

Proteins are degraded by proteases into amino acids. Each amino acid (after deamination) enters the pathway at some stage in the Krebs’ cycle or as pyruvate or acetyl CoA.

The respiratory pathway is generally considered as a catabolic pathway. But it involves both anabolism (synthesis) and catabolism (breakdown). So it is better called as an amphibolic pathway.

E.g. Fatty acids breakdown to acetyl CoA before entering the respiratory pathway. But when the organism needs to synthesise fatty acids, acetyl CoA withdraw from the respiratory pathway.

Similarly, during breakdown and synthesis of protein, respiratory intermediates are involved.

RESPIRATORY QUOTIENT (RQ) OR RESPIRATORY RATIO


It is the ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration. 


RQ depends upon the type of respiratory substrate.

RQ for carbohydrates= 1, because equal amounts of CO2 and O2 are evolved and consumed, respectively.

C6H12O6 + 6O2 → 6CO2 + 6 H2O + energy


RQ for fats = < 1. Calculations for a fatty acid, (e.g. tripalmitin) are shown:

2 (C51H98O6) + 145O2 → 102 CO2 + 98 H2O + energy

RQ for proteins = 0.9.

In living organisms, respiratory substances are often more than one. Pure proteins or fats are never used as respiratory substrates.
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