Final answer:
Carnitine is essential for the transport of fatty acids into the mitochondrial matrix for beta-oxidation. It is not a fatty acid or amino acid itself, but rather plays a role in the transportation mechanism by forming a complex with fatty acyl-CoA, facilitating its movement across the mitochondrial membrane.
Step-by-step explanation:
The role of carnitine in cellular metabolism is to facilitate the transport of fatty acids. When fatty acids need to be broken down for energy, they are first activated in the cytoplasm forming fatty acyl-CoA molecules. These molecules, however, cannot cross the mitochondrial membrane readily because the membrane is impermeable to acylated fatty acids. This is where carnitine comes into play.
Carnitine binds with these activated fatty acids to form a complex called fatty acyl carnitine, which is then able to cross the mitochondrial membrane with the help of transport proteins. This transport mechanism involves carnitine acyltransferase I (CPTI) on the outer mitochondrial membrane, which converts fatty acyl-CoA to acyl-carnitine. Through a translocase protein, the acyl-carnitine is shuttled into the mitochondrial matrix, where carnitine acyltransferase II (CPTII) converts it back to fatty acyl-CoA and free carnitine, the latter being ready for reuse. The fatty acyl-CoA enters the process of beta-oxidation, eventually leading to the production of ATP.
To answer the question, carnitine is used to transport fatty acids from the cytosol to the mitochondrial matrix. It is not a 15-carbon fatty acid, not one of the moderately nonpolar amino acids, and while it does facilitate fatty acid movement across membranes generally, its specific role is in the transport to the mitochondrial matrix for beta-oxidation.