`star` Amphibolic Pathway
`star` Respiratory Quotient


● `color{violet}"Glucose"` is the `color{violet}"favoured substrate"` for respiration.

● All `color{violet}"carbohydrates"` are usually `color{violet}"first converted into glucose"` before they are used for `color{violet}"respiration"`.

● Other `color{violet}"substrates can also be respired"`, as has been mentioned earlier, but then they do not enter the `color{violet}"respiratory pathway"` at the first step.

● `color{violet}"Fats"` would need to be `color{violet}"broken down"` into `color{violet}"glycerol and fatty acids"` first.

● If `color{violet}"fatty acids"` were to be `color{violet}"respired"` they would first be `color{violet}"degraded to acetyl CoA
"` and enter the pathway.

● `color{violet}"Glycerol"` would enter the pathway after being `color{violet}"converted to PGAL"`.

● The `color{violet}"proteins would be degraded"` by `color{violet}"proteases"` and the `color{violet}"individual amino acids"` (after deamination) depending on their structure would `color{violet}"enter the pathway"` at some stage within the `color{violet}"Krebs’ cycle"` or even as `color{violet}"pyruvate or acetyl CoA"`.

● Since respiration involves `color{violet}"breakdown of substrates"`, the `color{violet}"respiratory process"` has traditionally been considered a `color{violet}"catabolic process"` and the respiratory pathway as a `color{violet}"catabolic pathway"`.

● We have `color{violet}"discussed above"`, at which points in the `color{violet}"respiratory pathway"` `color{violet}"different substrates"` would enter if they were to be `color{violet}"respired and used"` to derive energy.

● What is important to `color{violet}"recognise"` is that it is these `color{violet}"very compounds"` that would be `color{violet}"withdrawn"` from the respiratory pathway for the synthesis of the said substrates.

● Hence, `color{violet}"fatty acids"` would be broken down to `color{violet}"acetyl CoA"` before entering the `color{violet}"respiratory pathway"` when it is used as a substrate.

● But when the organism needs to `color{violet}"synthesise fatty acids"`, `color{violet}"acetyl CoA"` would be withdrawn from the `color{violet}"respiratory pathway"` for it.

● Hence, the `color{violet}"respiratory pathway"` comes into the picture both during `color{violet}"breakdown and synthesis"` of fatty acids.

● Similarly, during `color{violet}"breakdown and synthesis"` of protein too, `color{violet}"respiratory intermediates"` form the link.

● `color{violet}"Breaking down processes"` within the living organism is `color{violet}"catabolism"`, and synthesis is `color{violet}"anabolism."`

● Because the `color{violet}"respiratory pathway"` is involved in both a`color{violet}"nabolism and catabolism"`, it would hence be better to consider the respiratory pathway as an `color{brown}"amphibolic pathway"` rather than as a `color{violet}"catabolic one"`.


● During `color{violet}"aerobic respiration"`, `O_2` is `color{violet}"consumed"` and `CO_2` is `color{violet}"released"`.

● The `color{violet}"ratio of the volume"` of `CO_2` evolved to the volume of `O_2` consumed in `color{violet}"respiration"` is called the `color{Brown}"respiratory quotient"` `color{violet}"(RQ)"` or `color{Brown}"respiratory ratio"`.

● The `color{violet}"respiratory quotient"` depends upon the type of `color{violet}"respiratory substrate"` used during `color{violet}"respiration"`.

● When `color{violet}"carbohydrates are used as substrate"` and are `color{violet}"completely oxidised"`, the `color{Brown}"RQ will be 1"`, because `color{violet}"equal amounts"` of `CO_2` and `_O`2 are evolved and consumed, respectively, as shown in the `color{violet}"equation below"` :

● When `color{violet}"fats are used"` in respiration, the `color{Brown}"RQ is less than 1"`.

● Calculations for a `color{violet}"fatty acid, tripalmitin"`, if used as a substrate is shown:

● When `color{violet}"proteins are respiratory substrate"`s the ratio would be about `color{Brown"0.9"`.

● What is important to recognise is that in `color{violet}"living organisms respiratory substances"` are often more than one; `color{violet}"pure proteins or fats"` are never used as `color{violet}"respiratory substrates"`.