Final answer:
The primary substrate for de novo synthesis of fatty acids in ruminants is acetyl-CoA, which is derived from volatile fatty acids produced from microbial fermentation in the ruminant's stomach, particularly in the rumen.
Step-by-step explanation:
In ruminants, the primary substrate used for the de novo synthesis of fatty acids is acetyl-CoA. Ruminants are animals like cows and goats that have a unique digestive system with specialized stomach compartments such as the rumen, reticulum, omasum, and abomasum, evolved to digest large amounts of plant material. In their complex stomachs, particularly the rumen, microbial fermentation breaks down cellulose into simpler substances that can be used by the animal. Among the products of this digestion are volatile fatty acids (VFAs) such as acetate, propionate, and butyrate, which are absorbed and converted into energy sources like acetyl-CoA for various metabolic processes, including the synthesis of fatty acids.
During the de novo synthesis of fatty acids, acetyl-CoA acts as the source of all carbon atoms in the growing fatty acid chain. The process of fatty acid synthesis generally occurs in the liver and adipose tissue and includes the formation of malonyl-CoA from acetyl-CoA, which is then used in a repeating cycle of reactions to elongate the fatty acid chain, primarily producing palmitic acid in ruminants.