= Solution
The <mitochondrial dicarboxylate carrier> is an inner-membrane transport <protein> that exchanges suitable dicarboxylates, including <succinate> and <malate>, with counter-substrates such as <phosphate>. In this experiment, it supplies external <succinate> to the matrix side of <respiratory complex II>; the <inner mitochondrial membrane> does not simply allow unrestricted diffusion of the charged <substrate>. \b[Without adequate uptake, substrate delivery would restrict respiration even if the respiratory enzymes remained intact.] Its essentiality is substrate- and preparation-dependent: it is not the obligatory entry route for every possible respiratory fuel.
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