Ans. D. State 4a. Respiratory Control Ensures a Constant Supply of ATP. THE rate of respiration of mitochondria can be controlled by the availability of ADP. This is because oxidation and phosphorylation are tightly coupled, ie, oxidation cannot proceed via the respiratory chain without concomitant phosphorylation of ADP.b. Table shows the five conditions controlling the rate of respiration in mitochondria. Most cells in the resting state are in state 4, and respiration is controlled by the availability of ADP. When work is performed, ATP is converted to ADP, allowing more respiration to occur, which in turn replenishes the store of ATP. Under certain conditions, the concentration of inorganic phosphate can also affect the rate of functioning of the respiratory chain,c. As respiration increases (as in exercise), the cell approaches state 3 or state 5 when either the capacity of the respiratory chain becomes saturated or the PO2 decreases below the Km for heme a3 - There is also the possibility that the ADP/ATP transporter, which facilitates entry of cytosolic ADP into and ATP out of the mitochondrion, becomes rate-limiting.States of Respiratory ControlConditions Limiting the Rate of RespirationState 1Availability of ADP and substrateState 2Availability of substrate onlyState 3The capacity of the respiratory chain itself, when all substrates and components are present in saturating amountsState 4Availability of ADP onlyState 5Availability of oxygen onlyThus, the manner in which biologic oxidative processes allow the free energy resulting from the oxidation of foodstuffs to become available and to be captured is stepwise, efficient, and controlled--rather than explosive, inefficient, and uncontrolled, as in many non-biologic processes. The remaining free energy that is not captured as high-energy phosphate is liberated as heat. This need not be considered "wasted," since it ensures that the respiratory system as a whole is sufficiently exergonic to be removed from equilibrium, allowing continuous unidirectional flow and constant provision of ATP. It also contributes to maintenance of body temperature.