The Respiratory Balance Sheet
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The net gain of ATP from each glucose molecule is calculated based on the following assumptions:
- All steps in Glycolysis, TCA cycle, and ETS occur sequentially and orderly.
- The NADH synthesized in glycolysis is transferred into mitochondria and undergoes oxidative phosphorylation.
- Intermediates in the pathway are not used to synthesize other compounds.
- Only glucose is being respired. Other alternative substrates do not enter the pathway at any stage.
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Such assumptions are not valid because:
- All pathways work simultaneously and do not occur one after another.
- Substrates enter and are withdrawn from the pathways as needed.
- ATP is utilized as needed.
- Enzymatic rates are controlled by multiple mechanisms.
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Such calculations are useful to appreciate the efficiency of the living system in extracting and storing energy.
Net gain of ATP molecules from one glucose molecule:
| Process | Yield | ATP Produced |
|---|---|---|
| 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 FADH₂ | 4 ATP | |
| 2 GTP | 2 ATP | |
| Total | 38 ATP |
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Two ATP molecules are spent transporting 2 NADH molecules formed during glycolysis to the mitochondria. Hence, the net gain is 36 ATP molecules.
Comparison between Fermentation and Aerobic Respiration:
| Fermentation | Aerobic Respiration |
|---|---|
| Partial breakdown of glucose. | Complete breakdown of glucose to CO₂ and H₂O. |
| Net gain of only 2 ATP. | Net gain of 36 ATP. |
| NADH is oxidized to NAD⁺ rather slowly. | NADH is oxidized to NAD⁺ very vigorously. |
Amphibolic Pathway
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Glucose is the favored substrate for respiration. All carbohydrates are first converted to glucose for respiration. Other substrates are also respired.
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Fats break down into glycerol and fatty acids. Fatty acids are degraded to acetyl CoA and enter the pathway. Glycerol is converted to PGAL and enters the pathway.
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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.
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The respiratory pathway is generally considered a catabolic pathway. However, it involves both anabolism (synthesis) and catabolism (breakdown). Thus, it is better called an amphibolic pathway.
For example, fatty acids break down to acetyl CoA before entering the respiratory pathway. When the organism needs to synthesize fatty acids, acetyl CoA is withdrawn from the respiratory pathway. Similarly, respiratory intermediates are involved in the breakdown and synthesis of proteins.
Respiratory Quotient (RQ) or Respiratory Ratio
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The respiratory quotient (RQ) is the ratio of the volume of CO₂ evolved to the volume of O₂ consumed in respiration.
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RQ depends on the type of respiratory substrate.
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RQ for carbohydrates = 1, because equal amounts of CO₂ and O₂ are evolved and consumed, respectively:
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RQ for fats less than 1. Calculations for a fatty acid (e.g., tripalmitin) are shown:
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RQ for proteins = 0.9.
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In living organisms, respiratory substrates are often a mix of carbohydrates, fats, and proteins. Pure proteins or fats are rarely used as respiratory substrates.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
RQ = 6CO₂ / 6O₂ = 1
2(C₅₁H₉₈O₆) + 145O₂ → 102CO₂ + 98H₂O + energy
RQ = 102CO₂ / 145O₂ ≈ 0.7