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Productions rates are described by constitutive relations which are functions of the state variables. In a biological mixture under isothermal conditions, the minimum set of state variables needed to describe reactive mixtures that include a solid matrix are: the (uniform) temperature $\theta$ , the solid matrix deformation gradient $\mathbf{F}$ (or related strain measures), and the molar content $c^{\alpha}$ of the various constituents. This set differs from the classical treatment of chemical kinetics in fluid mixtures by the inclusion of $\mathbf{F}$ and the subset of constituents bound to the solid matrix. To maintain a consistent notation in this section, solid-bound molecular species are described by their molar concentrations and molar supplies which may be related to their referential mass density and referential mass supply according to $$\begin{equation} c^{\sigma}=\frac{\rho_{r}^{\sigma}}{\left(J-\varphi_{r}^{s}\right)M^{\sigma}},\quad\hat{c}^{\sigma}=\frac{\hat{\rho}_{r}^{\sigma}}{\left(J-\varphi_{r}^{s}\right)M^{\sigma}}\,.\label{eq164} \end{equation}$$ Consider a general chemical reaction, $$\begin{equation} \sum\limits _{\alpha}\nu_{R}^{\alpha}\mathcal{E}^{\alpha}\to\sum\limits _{\alpha}\nu_{P}^{\alpha}\mathcal{E}^{\alpha}\,,\label{eq165} \end{equation}$$ where $\mathcal{E}^{\alpha}$ is the chemical species representing constituent $\alpha$ ; $\nu_{R}^{\alpha}$ and $\nu_{P}^{\alpha}$ represent stoichiometric coefficients of the reactants and products, respectively. Since the molar supply of reactants and products is constrained by stoichiometry, it follows that all molar supplies $\hat{c}^{\alpha}$ in a specific chemical reaction may be related to a production rate $\hat{\zeta}$ according to $$\begin{equation} \hat{c}^{\alpha}=\nu^{\alpha}\hat{\zeta}\,,\label{eq166} \end{equation}$$ where $\nu^{\alpha}$ represents the net stoichiometric coefficient for $\mathcal{E}^{\alpha}$ , $$\begin{equation} \nu^{\alpha}=\nu_{P}^{\alpha}-\nu_{R}^{\alpha}\,.\label{eq167} \end{equation}$$ Thus, formulating constitutive relations for $\hat{c}^{\alpha}$ is equivalent to providing a single relation for $\hat{\zeta}\left(\theta,\mathbf{F},c^{\alpha}\right)$ . When the chemical reaction is reversible, $$\begin{equation} \sum\limits _{\alpha}\nu_{R}^{\alpha}\mathcal{E}^{\alpha}\rightleftharpoons\sum\limits _{\alpha}\nu_{P}^{\alpha}\mathcal{E}^{\alpha}\,,\label{eq168} \end{equation}$$ the relations of (2.10.4-3)-(2.10.4-4) still apply but the form of $\hat{\zeta}$ would be different.