This is dealt with under nutrition . Fatty acids are highly reduced molecules. Hydrolysis of these two high energy bonds drives the reaction. The energy change is so large that the reaction is irreversible. In other words, the equivalent of 2ATP are used in this activation step.
Activation occurs in the cytosol but oxidation occurs in mitochondria.
Inner mitochondrial membrane is impermeable to long chain acyl CoA molecules so a transport system is required. I on the inner surface of the outer mitochondrial membrane. Acylcarnitine is transported across the inner mitochondrial membrane by carnitine-acylcarnitine translocase in exchange for carnitine. Once in the matrix, acylcarnitine is converted back acyl CoA by carnitine acyltransferase II . The liberated carnitine can be exchanged for another incoming molecule of acylcarnitine.