I am trying to understand why a cell might have minimal respiratory reserve capacity in the presence of unlimited substrate supply. This essentially means that the oxygen consumption rate (OCR) of the cell does not increase from its basal rate when oxidative phosphorylation is uncoupled — i.e. the reactions of the electron transport chain are uncoupled from ATP synthesis. What factors might conspire to cause this? I am thinking specifically of the photoreceptors1. I am interested in all logical possibilities here e.g.
(1) mitochondrial ATP production is barely meeting demands at basal level.
(2) oxidative phosphorylation may already be physiologically uncoupled.