Labelling terms in an equation without using \underbrace

Question

How can I create the descriptors below or above equation terms without displaying the actual underbrace or overbrace?

Answer 1

Is it something like this you want?

 \documentclass[a4paper, 11pt]{book}
\usepackage[utf8]{inputenc}

\usepackage{mathtools}
\usepackage[usestackEOL]{stackengine}

\begin{document}

\[
  U' = \underbracket[0pt]{k_1 U}_{\scriptsize\shortstack{uptake\\ of $U $}}
+\underbracket[0pt]{ k_4U(U_0-U)}_{\scriptsize\shortstack{$U$-dependent\\ biosynthesis of $U$}}
+\underbracket[0pt]{k_5\frac{p_1}{p_1 + U}}_{\scriptsize\shortstack{conversion\\ from $ E\to U $}}
\]

\end{document}