Indole-3-acetic acid is a naturally occurring auxin and plant hormone. Its structure combines an indole ring system with an acetic acid side chain; the neutral molecule has formula . Its chemical identity is more specific than the functional class auxin, which also includes other compounds.[ref]
A transport experiment illustrates the difference between applying a signal and delivering it to the responsive cells. Work on Arabidopsis thaliana localized AUX1 within the root apex and connected its function to uptake and redistribution of indole-3-acetic acid. A membrane-permeable synthetic analogue, naphthaleneacetic acid, could bypass the mutant's uptake defect in a gravitropism assay.[ref]
Localization of the auxin permease AUX1 suggests two functionally distinct hormone transport pathways operate in the Arabidopsis root apex by Ranjan Swarup and others. Abstract and Fig. 3, transport interpretation and chemical bypass
Concentration and total amount must also be distinguished. In an illustrative homogeneous solution, at contains of solute by . That calculation specifies an inventory; it does not establish the intracellular concentration, spatial distribution or growth response. Those require measurements of transport and biological activity.
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