Arachidonic acid concentrations in experimental and physiological settings. Differing sizes of the AA (arachidonic acid) represent differing concentrations. The concentrations range from the nanomolar level for extracellular, unbound, free AA in the physiological setting (right side), up to the high micromolar levels, often used in the absence of binding proteins, in the experimental situation (left side). The ranges on concentrations in practice are considerably more divergent than are represented here by the size of AA. Proteins in the cell membrane may facilitate uptake of extracellular arachidonate and its presentation to a coenzyme A synthetase for esterification. Fatty acid binding proteins (FABP) facilitate transfer and modulate the available concentrations of arachidonic acid within cells. FA transporter = fatty acid transporter.