  Chemical and biochemical reactions can exhibit surprisingly different
behaviours from multiple steady-state solutions to oscillatory solutions and
chaotic behaviours. Such behaviour has been of great interest to researchers
for many decades. The Briggs-Rauscher, Belousov-Zhabotinskii and
Bray-Liebhafsky reactions, for which periodic variations in concentrations can
be visualized by changes in colour, are experimental examples of oscillating
behaviour in chemical systems. These type of systems are modelled by a system
of partial differential equations coupled by a nonlinearity.
  However, analysing the pattern, one may suspect that the dynamic is only
generated by a finite number of spatial Fourier modes. In fluid dynamics, it is
shown that for large times, the solution is determined by a finite number of
spatial Fourier modes, called determining modes. In the article, we first
introduce the concept of determining modes and show that, indeed, it is
sufficient to characterise the dynamic by only a finite number of spatial
Fourier modes.
  In particular, we analyse the exact number of the determining modes of $u$
and $v$, where the couple $(u,v)$ solves the following stochastic system
  \begin{equation*}
  \partial_t{u}(t) = r_1\Delta u(t) -\alpha_1u(t)- \gamma_1u(t)v^2(t) + f(1 -
u(t)) + g(t),\quad
  \partial_t{v}(t) = r_2\Delta v(t) -\alpha_2v(t) + \gamma_2 u(t)v^2(t) +
h(t),\quad
  u(0) = u_0,\;v(0) = v_0,
  \end{equation*}
  where $r_1,r_2,\gamma_1,\gamma_2>0$, $\alpha_1,\alpha_2 \ge 0$ and $g,h$ are
time depending mappings specified later.
