The Measurement of the Fraction of Oxygen Carried by Facilitated Diffusion
Evidence has been presented that the cytochrome P-450 in the endoplasmic reticulum can facilitate the diffusion by oxygen through tissue (Longmuir, 1970). This hypothesis is based on the observation that the respiration rate of liver slices as a function of ambient oxygen tension more closely fits a model equation based on purely passive diffusion. However, the fit is not perfect at higher oxygen tensions, and it seems highly improbable that there can be no passive diffusion proceeding concomitantly. The respiration rate of a slice in which there is only passive diffusion would rise as the square root of the ambient oxygen tension, whereas if transport is purely by carrier, the respiration rate would show “saturation” kinetics. That is, the rate of rise would fall off rapidly, becoming asymptotic to a maximum rate. Thus it seems certain that both passive and facilitated diffusion are proceeding simultaneously. That this is the case can be shown by the application of specific inhibitors to tissue slices.
KeywordsRespiration Rate Oxygen Tension Passive Diffusion Liver Slice Oxygen Fraction
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