Question

Researchers conducted two experiments to investigate the enzymatic activity of cellobiase, an enzyme that hydrolyzes cellobiose into glucose.

*Experiment 1*
Reaction mixtures were prepared using a fixed cellobiase concentration of $0.5\ \text{mg/mL}$ and varying initial cellobiose concentrations ($1.0\ \text{mM}$, $2.5\ \text{mM}$, $5.0\ \text{mM}$, and $10.0\ \text{mM}$). All mixtures were incubated at $37^\circ\text{C}$ and $\text{pH}\ 5.0$. The initial rate of glucose production ($v_0$) was measured for each mixture.

*Experiment 2*
Reaction mixtures were prepared using a constant cellobiose concentration of $5.0\ \text{mM}$ and a constant cellobiase concentration of $0.5\ \text{mg/mL}$. The mixtures were incubated at $\text{pH}\ 5.0$ under four different temperatures ($25^\circ\text{C}$, $37^\circ\text{C}$, $50^\circ\text{C}$, and $65^\circ\text{C}$). The initial rate of glucose production ($v_0$) was measured for each mixture.

Based on the descriptions of both experiments, which variable served as the independent variable in Experiment 1 but was maintained as a controlled variable in Experiment 2?