Mitochondrial physiology

Internal and external respiration (mt) Mitochondrial catabolic respiration JkO2 is the O2 consumption in the oxidation of fuel substrates (electron donors) and reduction of O2 catalysed by the electron transfer system ETS, which drives the protonmotive force pmF. JkO2 excludes mitochondrial residual oxygen consumption, mt-Rox (). (ce) Cell respiration JrO2 is internal cellular O2 consumption, taking into account all chemical reactions r that consume O2 in the cells. Catabolic cell respiration is the O2 consumption associated with catabolic pathways in the cell, including mitochondrial (mt) catabolism, and: mt-Rox (); non-mt O2 consumption by catabolic reactions, particularly peroxisomal oxidases and microsomal cytochrome P450 systems (); non-mt Rox by reactions unrelated to catabolism (). (ext) External respiration balances internal respiration at steady state, including extracellular Rox () and aerobic respiration by the microbiome (). External O2 is transported from the environment across the respiratory cascade by circulation between tissues and diffusion across cell membranes, to the intracellular compartment. The respiratory quotient RQ is the molar CO2/O2 exchange ratio; combined with the nitrogen quotient N/O2 (mol N given off per mol O2 consumed), the RQ reflects the proportion of carbohydrate, lipid and protein utilized in cell respiration during aerobically balanced steady states. Bicarbonate and CO2 are transported in reverse to the extracellular milieu and the organismic environment. Hemoglobin provides the molecular paradigm for the combined CO2/O2 exchange, as do lungs and gills on the morphological level, but CO2/O2 exchange across the skin and other surfaces is less interdependent, and highly independent in cell respiration.