Small-  and  medium-chain   acyl  CoA  molecules  (up  to  10  carbon  atoms)  are readily capable to cross the inner mitochondrial  membrane by diffusion. Furthermore, longer chain acyl CoAs do not readily cross the inner mitochondrial membrane, and requires a specific transport mechanism.  To achieve this, the longer chain acyls CoAs are conjugated to the polar carnitine molecule that is found in both animals and plants.  This reaction,  catalyzed  by an enzyme on the outer face of the  inner  mitochondrial  membrane  (carnitine  acyltransferase  I),  erased  the CoA set and substitutes it with a carnitine molecule shown in the figure. The acylcarnitine is then transported with the inner mitochondrial membrane by a carnitine or acylcarnitine translocase.

                                                      Figure:  Activation of a fatty acid.