The Interrelation between the Effects of 2,4-Dinitrophenol and of Substrate Concentration on the Rate of Exogenous Respiration

1. The specific activity of the CO₂ evolved by sunflower hypocotylsegments supplied with labelled glucose or glutamic acid isrelated to the external concentration by a rectangular hyperbolaover a concentration range of approx. 2–60 mM. The overallrespiration rate over this range is approximately 15 per cent.above that of a buffer control, and rises only slightly withincreasing external concentration. 2. The curve for total exogenous substrate respired is alsoclose to a rectangular hyperbola over this concentration range. 3. 2,4-dinitrophenol (DNP) reduces the slope of the hyperbolafor specific activity, but not its maximum value. Increasingthe glucose concentration while maintaining the DNP concentrationconstant reduces the degree of inhibition. 4. The effect of DNP on absolute amount of exogenous substraterespired depends on the external concentration of the substrate.At lower concentrations, absolute exogenous respiration is inhibited:at higher concentrations, it is stimulated. 5. The results have been analysed with the object of determiningwhether the respiratory sites lie in the ‘sugar free space’of the cell. Kinetic analysis indicates (a) that exogenous substratesdo not diffuse to these sites through an appreciable diffusionbarrier; (b) that there is no evidence for an active transportstep; and (c) that the data accord well with the view that exogenoussubstrates diffuse freely to the respiratory sites where theycompete on equal terms with endogenous substrates.