= Solution
<Hexokinase> consumes exported <ATP> in the reaction
$$
\mathrm{glucose+ATP\longrightarrow glucose\ 6\!\! -\!phosphate+ADP}.
$$
The <adenine nucleotide translocator> imports regenerated <ADP>, permitting <ATP synthase> to use the <proton-motive force>. Increased proton return reduces the opposing gradient and allows faster activity of the <electron transport chain> and oxygen consumption: this is <respiratory control>. The first <hexokinase> additions therefore increase demand and respiration. \b[By about the fourth addition, demand no longer limits the measured flux]: the respiratory and transport system has reached its available capacity, so more <hexokinase> cannot increase it. This does not imply that <hexokinase> ceases functioning, and its zero incremental control at maximal demand does not make it dispensable for maintaining that demand.
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