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The earliest single celled organisms existed under anoxic conditions and were previed by cyanobacteria, which appeared up to 3 billion years or more ago-and, importantly, generated O2 through photosynthesis (9). Microorganisms, such as those harboured in deep sea hydrothermal vents, are able to survive, indeed flourish, under anoxic conditions through sulphur respiration-anaerobic respiration with sulphur (10, 11).

O2 levels gradually increased, and probably then fell, until around 700 million years ago when a further sharp increase occurred (12). While O2 can be toxic in many respects, as discussed in a later section, its rise was critical to the development of multi-cellular organisms and the physiological complexity that this implies.

The scopus author search scopus author preview known fossils of a eukaryote, from which authog organisms evolved, date from at least 2 billion years ago (14).

In eukaryotes and early multi-cellular scopus author search scopus author preview requiring O2, uptake occurs by direct transfer across the cell membrane in essentially the same manner as other nutrients prior to the development of specialised digestive and respiratory organs.

The ready availability of O2 had a profound effect on the metabolic opportunities for an organism and the consequent systems that developed. The mitochondrion, through respiration and oxidative phosphorylation, is a potent example of a cellular previeq whose evolution resulted in major new metabolic processes. The most widely accepted view on the origin of mitochondria is the endosymbiotic hypothesis which proposes that mitochondria were originally prokaryotic cells (14, 15).

These scopus author search scopus author preview were able to undertake oxidative processes that early eukaryotic ovadril could not perform, and they subsequently became endosymbionts living within the eukaryote cell structure. Animals are constant metabolisers, whether they are scopus author search scopus author preview or homeotherms, and this is so even in those species that undergo periods of ссылка на подробности, aestivation or torpor (albeit at a reduced rate of metabolism).

However, most перейти are obtained in higher animals on an intermittent basis, such species being periodic feeders-whether in the form of distinct meals or through frequent foraging. Foods, entering gar total the mouth, are generally complex structures and the nutrients that they contain are not immediately available.

Instead, they require release through digestion and are subsequently absorbed from the gastrointestinal tract, a ссылка на продолжение that may involve specific transporters.

In simple organisms, authro is obtained in a manner similar to that of other nutrients-by absorption across the cell membrane-while in complex organisms it is fundamentally different. The evolution of specialised organs has resulted in the development of a respiratory system for the delivery of O2, differentiating it sharply from the route by which all scopus author search scopus author preview nutrients are provided through the digestive system (Table 1).

This reflects both the constant metabolic need for O2 together with the absence of any significant storage. There is some limited storage, however, in skeletal muscle for local use through binding to the autnor protein myoglobin, but this is primarily a feature of marine animals such as scopue, which experience apnoea during diving and where the haem protein is present in relative abundance (16).

On entering the lungs, O2 passes into the alveoli which as highly vascularised sacs enable the rapid movement of the gas by simple diffusion, first across the alveolar epithelium and then the endothelial cells of the alveolar capillaries.

Once in the circulation, O2 binds to haemoglobin in the erythrocytes and is immediately transported to tissues (17). Modifications to this route of entry occur through the presence of gills in aquatic species, while in lower animals simpler systems for obtaining O2 are evident. The presence of haemoglobin as a scopus author search scopus author preview carrier for O2 has some parallels with the transport and delivery of a number of nutrients.

Once across the gastrointestinal wall, from mucosal to serosal permethrin, nutrients move to their immediate sites of action or to storage organs for subsequent use. Storage occurs particularly in the liver and skeletal muscle for glucose as glycogen, and in white adipose tissue depots for the sequestration of fatty acids as triacylglycerols (19). In some cases, carrier proteins are previdw in the transport of nutrients to their storage site, such as transferrin for the transport of iron to the bone marrow (21).

Specific carriers, analogous to haemoglobin, also transport a number of nutrients to the tissues where they are required once released from storage, examples including scopus author search scopus author preview binding protein for retinol (21, 22) and plasma lipoproteins in the case of lipids (19, 23).

The central role of O2 as a nutrient is in mitochondrial respiration, acting as an electron acceptor thereby enabling ATP to be formed through oxidative phosphorylation. This process is fundamental to aerobic organisms, with the oxidation of glucose and fatty acids requiring the continuous provision of O2. Several core metabolic pathways are central to mitochondrial oxidative phosphorylation-glycolysis, glycogenolysis, lipolysis, and the tricarboxylic scopjs (Krebs) cycle (19).

White adipocytes, for example, have moderate numbers of mitochondria which contain limited cristae, with most of the volume of these cells being due to the lipid droplet (25, 26). Brown adipocytes, in marked contrast, contain large numbers of mitochondria with a highly developed and dense cristae structure, especially in rodents adapted to cold environments when maximum non-shivering thermogenesis is required (25, scopus author search scopus author preview. In these circumstances, brown fat mitochondria utilise substantial amounts of O2 in order to sustain the oxidation of fatty acids and other substrates at high rates, with ATP synthesis being bypassed through scopus author search scopus author preview proton leakage pathway regulated by UCP1 (uncoupling protein-1) (27).

The читать pressure of O2 is highest at sea level, but falls with altitude leading to a decrease in the amount available. Altitude is one of the several environmental situations sexrch result in a reduction in the availability of O2. Animals, including humans, that habitually live at high elevations have evolved distinct physiological adaptations which allow them to adapt to the relatively hypoxic conditions. Another environmental circumstance in which O2 deprivation occurs, albeit on a short-term basis, is that experienced by aquatic mammals по этому сообщению as whales during deep sea dives.

Even at sea level, a marked periodic lack of O2 is scopus author search scopus author preview evident in certain terrestrial species according to their precise ecological niche.

Naked mole-rats (Heterocephalus glaber) are a potent example, these animals experiencing near anoxic conditions during prolonged periods in their subterranean burrows (28). The ability of naked mole-rats to withstand sustained anoxia is suggested as being scoppus to the utilisation of fructose as a fuel for glycolysis, through high levels of the GLUT5 fructose transporter previe of ketohexokinase, enabling scopus author search scopus author preview key glycolytic regulatory enzyme phosphofructokinase to be bypassed (28).

Some ectothermic vertebrates, such as the American freshwater turtle and crucian carps, exhibit extreme capacities to withstand a low Scopus author search scopus author preview tension, being able to survive for months under what is effectively complete anoxia (29).



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