Amphibolic Pathways & Substrate Entry — NEET Biology

Glucose is the favoured substrate for respiration. All carbohydrates are usually first converted into glucose before they are used for respiration. Other substrates can also be respired, as has been mentioned earlier, but glucose is the most favoured substrate. But what does the cell do if it has to respire something other than glucose? Also, another important question is: how does the plant cell know that it has to perform photosynthesis when it needs to perform respiration? In order to understand respiration of other substrates, let us go to to see the points of entry of different substrates in the respiratory pathway. Fats would need to be broken down into glycerol and fatty acids first. If fatty acids were to be respired they would first be degraded to acetyl CoA and enter the pathway. Glycerol would enter the pathway after being converted to PGAL. Since fatty acids would be broken down to acetyl CoA before entering the respiratory pathway, a very important question that arises is: what would happen if the acetyl CoA is not respired but is converted to fatty acids? This could happen when the respiratory substrates like glucose or fats are in excess, or when the organism has a high energy charge. In such a case acetyl CoA instead of entering the TCA cycle gets converted to fatty acids for the synthesis of the said substrates. Hence, fatty acids would be broken down to acetyl CoA before entering the respiratory pathway which is used as a substrate. But when the organism needs to synthesise fatty acids, acetyl CoA would be withdrawn from the respiratory pathway and enter the pathway for synthesis of fatty acids. Similarly, during breakdown and synthesis of proteins too, respiratory intermediates form the link. Breaking down of proteins within the living organism is catabolism, and synthesis is anabolism. Because the respiratory pathway is involved in both anabolism and catabolism, it would hence be better to consider the respiratory pathway as an amphibolic pathway rather than as a catabolic one.