Final answer:
In fed ruminants, gluconeogenesis primarily occurs in the liver and uses substrates like lactate, glycerol, and glucogenic amino acids, especially alanine, for the synthesis of new glucose molecules, as fatty acids cannot be directly converted into carbohydrates.
Step-by-step explanation:
In fed ruminants, the primary substrate used for gluconeogenesis is not fatty acids but rather substances such as lactate, alanine, and glycerol. During periods of fasting, starvation, or low carbohydrate intake, glucose levels in the blood decrease to a point where it becomes necessary to synthesize new glucose. While certain organisms can convert fatty acids to carbohydrates via the glyoxylate cycle, ruminants, like other higher animals, lack this pathway. Hence, the use of fatty acids as gluconeogenic substrates is not possible. Instead, ruminants rely on the gluconeogenesis pathway, which is primarily active in the liver, to produce glucose from non-carbohydrate precursors such as lactate, glycerol, and glucogenic amino acids like alanine.
The process serves the vital function of maintaining blood glucose levels, particularly in glucose-dependent organs such as the brain. The gluconeogenic amino acids play a significant role in this process, as they can be converted to substrates such as alanine, which enters the gluconeogenic pathway. In ruminants that are continually feeding, such as cows, the process of gluconeogenesis is ongoing to ensure that there is a consistent supply of glucose for the body's needs.