= Indole-3-acetic acid
<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 $\mathrm{C_{10}H_9NO_2}$. Its chemical identity is more specific than the functional class <auxin>, which also includes other compounds.<Indole-3-acetic acid in ChEBI>[CHEBI:16411, Definition, Formula and biological roles]{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.<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]{ref}
Concentration and total amount must also be distinguished. In an illustrative homogeneous solution, $1\,\mathrm{mL}$ at $1\,\mathrm{\mu mol\,L^{-1}}$ contains $1\,\mathrm{nmol}$ of solute by $n=cV$. 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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