Glucose can be synthesised from all except:
## Core Concept
Glucose synthesis, also known as gluconeogenesis, is a metabolic pathway that results in the generation of glucose from certain non-carbohydrate carbon substrates. This process is crucial for maintaining blood glucose levels during fasting or when glucose is in short supply. The primary substrates for gluconeogenesis include amino acids, lactate, and glycerol.
## Why the Correct Answer is Right
The correct answer, , is not a gluconeogenic precursor. Fatty acids cannot be converted back into glucose through gluconeogenesis because the two-carbon fragments (acetyl-CoA) produced from fatty acid oxidation cannot be used to synthesize glucose in animals, including humans. This is due to the irreversibility of the pyruvate dehydrogenase reaction and the lack of a pathway to convert acetyl-CoA into glucose.
## Why Each Wrong Option is Incorrect
* **Option A:** can be converted into glucose through gluconeogenesis. This amino acid can be transaminated to form oxaloacetate, which can then be converted into glucose.
* **Option B:** can also serve as a gluconeogenic precursor. It can be converted into glucose through gluconeogenesis by first being converted to pyruvate.
* **Option C:** is another gluconeogenic precursor. Glycerol, a byproduct of fat metabolism, can be converted into glycerol-3-phosphate and then into dihydroxyacetone phosphate, which can enter gluconeogenesis.
## Clinical Pearl / High-Yield Fact
A key point to remember is that while fatty acids cannot be used for gluconeogenesis, ketone bodies (which are produced from acetyl-CoA in the liver) can be used as an alternative energy source by the brain and other tissues during fasting, thereby reducing the brain's dependency on glucose.
## Correct Answer: .