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A tiny hemeprotein called the cytochrome complex, or cyt c, is found loosly connected to the mitochondrion's inner membrane. It is a member of the protein family known as cytochrome C and is crucial for cell death.

We must compare $∆{\mathrm{ξ}}^{o^{\text{'}}}$of reactions to ascertain which reaction takes place. A reaction with a bigger $∆{\mathrm{ξ}}^{o^{\text{'}}}$ will take place because of the differences in $∆{\mathrm{ξ}}^{o^{\text{'}}}$ between the reactions.

Following the following processes, an electron is transferred from cytochrome c's reduced form to cytochrome a and cytochrome b's oxidised forms:

$$\begin{gathered} cyctochromec\left(F{e}^{2+}\right)+cyctochromea\left(F{e}^{3+}\right)→cyctochromec\left(F{e}^{3+}\right)+cyctochromea\left(F{e}^{2+}\right) \\ cyctochromec\left(F{e}^{2+}\right)+cyctochromea\left(F{e}^{3+}\right)→cyctochromec\left(F{e}^{3+}\right)+cyctochromea\left(F{e}^{2+}\right) \end{gathered}$$

Since cytochrome c is oxidised, $∆{\mathrm{ξ}}^{o^{\text{'}}}$ is the difference between typical reduction potentials of processes. $∆{\mathrm{ξ}}^{o^{\text{'}}}$of the reaction has a negative sign. Hence, cytochrome c will donate an electron to a reduced form with a higher $∆{\mathrm{ξ}}^{o^{\text{'}}}$ value, in this case cytochrome a.