Eukaryotes oxygen requirement

The Krebs cycle is present in virtually all eukaryotic cells that contain mitochondria, but functions only as part of aerobic metabolism (when oxygen is available). This oxygen requirement is owing to the close relationship between the mitochondrial electron transport chain and the Krebs cycle. In the Krebs cycle, four oxidation-reduction reactions occur. A high energy phosphate bond in the form of GTP is also generated. (This high... 

Summary diagram of the breakdown of glucose to carbon dioxide and water in a eukaryotic cell. As depicted here, the process starts with the absorption of glucose at the plasma membrane and its conversion into glucose-6-phosphate. In the cytosol, this six-carbon compound is then broken down by a sequence of enzyme-catalyzed reactions into two molecules of the three-carbon compound pyruvate. After absorption by the mitochondrion, pyruvate is broken down to carbon dioxide and water by a sequence of reactions that requires molecular oxygen. 

Under aerobic conditions, the glycolytic pathway becomes the initial phase of glucose catabolism (fig. 13.2). The other three components of respiratory metabolism are the tricarboxylic acid (TCA) cycle, which is responsible for further oxidation of pyruvate, the electron-transport chain, which is required for the reoxidation of coenzyme molecules at the expense of molecular oxygen, and the oxidative phosphorylation of ADP to ATP, which is driven by a proton gradient generated in the process of electron transport. Overall, this leads to the potential formation of approximately 30 molecules of ATP per molecule of glucose in the typical eukaryotic cell. 

Whatever the real time sequence of eukaryote evolution may be, origin and development of the nucleated cell has implications for the understanding of the evolution of 02 in hydrosphere and atmosphere since even the few anaerobic eukaryotes that presently exist require molecular oxygen for synthesis of some essential biochemicals47 2 Thus all eukaryotic cells are basically aerobic and it is most likely that they evolved from an aerobic protoeukaryote stem cell. 

The anaerobic mode of protein utilization is entirely possible in theory and in practice. Oxygen is not required for protein and nitrogen catabolism until the final stages of amino acid deamination have been reached. Complete anaerobic catabolism of proteins and nitrogen compounds (to the point where the final products C02, HjO and NH3 appear) has been known for a long time in prokaryotic organisms, but in eukaryotes only in parasitic worms, which are obligate anaerobes (von Brand, 1946). However, in recent decades, anaerobic metabolism of proteins has been found in some aquatic... 

Proteobacteria, respectively, which are probably sisters (Fig. 8.1), it is unlikely that these transporters were present in their common ancestor (as sometimes postulated Cavalier-Smith 2000b Amiri et al. 2003) as this ancestor was probably nearly 3 times as ancient as the first eukaryote and such carriers are unknown from more basal free-living members of either phylum. Nor is the idea that mitochondrial enslavement was initiated because of its protective reduction of cytosolic oxygen levels (Andersson et al. 2003) plausible. The host probably already had two oxygen-using organelles (ER and peroxisomes) under its own control thus, any such selective force would be weak or non-existent and not require or explain carrier insertion. 

---