Human Biochemistry, Volume 1 - Murray R. 1993

Bioenergetics and Metabolism of Carbohydrates and Lipids
Oxidative Phosphorylation and Mitochondrial Transport Systems
Respiratory Control

Mitochondrial Respiration rate can be controlled by the concentration of ADP. This is because oxidation and phosphorylation are tightly coupled, meaning the Respiratory Chain cannot function unless it is accompanied by the phosphorylation of ADP. Chance and Williams proposed considering 5 states in which The rate of mitochondrial respiration is limited by specific factors (Table 13.1).

Typically, most resting Cells reside in state 4, where the respiration rate is determined by the availability of ADP. The energy required to perform work is supplied through The conversion of ATP to ADP; this creates conditions for an increase in respiration rate, which in turn leads to the replenishment of ATP reserves (Fig. 13.7). Obviously, under certain conditions, the concentration of inorganic phosphate can also influence the rate of the respiratory chain. As the respiration rate increases (triggered, for example, by physical exertion), The Cell approaches state 3 or state 5: either the capacity of the respiratory chain becomes exhausted, or the P_O level drops below the K_m value for cytochrome a₃. The rate-limiting factor may also turn out to be the ATP/ADP translocase (see 138), which ensures the influx of ADP from the Cytosol into the Mitochondria.

Thus, the mechanism by which the Free energy of food oxidation is captured is stepwise, efficient (40–45%), and regulated, rather than explosive, inefficient, and uncontrolled. The portion of free energy that is not captured in the form of high-energy phosphates is released as heat. This does not mean at all that it is wasted—in warm-blooded animals, it is used to maintain body Temperature.

Class="center">Table 13.1. States of respiratory control