Acylcarnitine

Correct Answer: B. transports a fatty acid.

The diagram (top left corner) shows long chain fatty acids going through the plasma membrane converted to acyl CoA within the mitochondrion (between the mitochondrial double membranes) where it combines with carnitine (catalyzed by CPT I) to produce acylcarnitine, which can cross the inner mitochondrial membrane (IMM). Thus the figure suggests carnitine attaches to a fatty acid thus the fatty acid is a part of the acylcarnitine molecule.

Note that, according to the diagram, carnitine and acylcarnitine are not membrane transporters but rather they are transported through membrane transporters (i.e. “Transporter” and “Translocase”, respectively).

Acylcarnitine’s “job” seems over as it is converted back to carnitine and acyl CoA which, as fatty acids are known to do, undergoes beta-oxidation. In summary, acylcarnitine transported long-chain fatty acids across the IMM.

Background information: A membrane transport protein (= transporter) helps move ions or molecules across the plasma membrane. Transporters are integral (= intrinsic) membrane proteins, thus they span the membrane engaging in carrier-mediated transport (without the need for energy: facilitated diffusion; with the need for energy: active transport).

Most biology MCAT questions do not require discrete knowledge but rather the skill to efficiently interpret graphs, tables and/or diagrams – often without previous specific knowledge. To summarize the diagram provided: (1) Fatty acids -> Acyl CoA which attaches to Carnitine; (2) Acyl CoA + Carnitine -> Acylcarnitine which crosses the IMM through the membrane transporter Translocase then -> Acyl CoA + Carnitine is regenerated; (3) Acyl CoA has now crossed the IMM and then can be oxidized.

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