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.
- 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- Substrates enter the pathways and are withdrawn from it as and when necessary.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.
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.
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 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
In living organisms, respiratory substances are often more than one. Pure proteins or fats are never used as respiratory substrates.
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