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What is The important intermediate product of biosynthesis of fatty acid is and explain in detail?
Malonyl CoA 

Acetyl-CoA is the immediate substrate for fatty acid synthesis.
However, acetyl-CoA needs to be converted to the activated form, which will serve as the donor of carbon units to the growing fatty acid chain.
Malonyl-CoA is such activated form; acetyl-CoA is converted to malonyl-CoA by acetyl-CoA carboxylase.