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All living things require a continual flow of energy to assemble molecules and perform biological work. Living cells extract energy from food materials using pathways made up of a large number of complex biological reactions. Living cells rely in the storage of energy in the molecule of adenosine triphosphate (ATP). One of the most demanding processes that requires a continual flow of energy is the process of flying. The flight process requires the coordination of many different factors to ensure the proper supply of fuels to muscles and nerves. This essay reviews our current and emerging understanding of the flight respiration and energetic and outlines different mechanisms implicated in the flying process.
The flight process requires the coordination of many different factors to ensure the proper supply of fuels to muscles and nerves. To achieve that the precise amount of ATP in muscle cells is precisely controlled in parallel with O2 consumption and CO2 disposal.
Flight muscles sustain the highest metabolic rates of all animal tissues. This is especially evident in insects. Insect flight is mainly aerobic and they present the highest rates of aerobic metabolism. In fact, a comparison of oxygen consumption rates of (VO2) of mammals, insects, birds and reptiles indicates that flying insects have the highest rates. According to the experiments indicated in table 1 below it was concluded that: The mass specific oxygen amount that insect flights use is from 3 to 30 times bigger than the locomoting insects in the territory at the same body temperature. Some comparisons of 118 invertebrate flying and running species showed that the aerobic volume of 1-g flier is 28 times that of a 1g runner. This difference is not highly affected by the size of their body Moreover, as it can be seen from the table 1, the flight metabolic amounts of insects with asynchronous muscle exceed, the flight metabolic amounts of insects with synchronous flight muscle. Furthermore, this phenomenon observed for species that can both fly and run as well. As a result, the data for insects shows that the aerobic metabolic amount is greater than the amount of runners (1).Comparing bats and other mammals, evidence supports that aerobic locomotory metabolism is greater in bats than in other mammals. However the differences are smaller than for insects (1).
Table 1: from (1)
The working flight muscle requires high rates of ATP hydrolysis, thus requires to maintain high amounts of fuels to sustain extended flights. The fuels required for aerobic muscle metabolism during flight vary according to the type of insect/bird and the stage of flight activity. One important muscle fuel is the sugar trehalose, synthesized by the body fat and a variety of other precursors. Other fuels include, proline glycerol and ketone bodies.
An essential carbohydrate for flight is trehalose, identified in many insect species. Trehalose can be synthesized in the fat body from monosaccharides of dietary origin, stored glycogen or gluoneogenesis. The enzymatic steps are outlined in the Figure 1 below (2)
By facilitated diffusion, monosaccarides from dietary origin pass the gut wall and are transformed to the non-diffusible trehalose. As a result they trap the carbohydrate and support uptake from the gut without the use of an active transport system. The site of trehalose synthesis in many insects is the fat body. This synthesis occurs via UDP glucose and trehalose-6-phosphate using energy in the form of ATP and UTP. During the transformation of monosaccharides to trehalose, they can be stored as a glycogen or changed during glycolysis and the pentose phosphate pathways in order to supply ATP or precursors for lipid synthesis. In honey bee, Apis mellifera, monosaccharides are oxidized by fly muscle and forming a major flight fuel. The trehalose synthesis is regulated by hormonal regulation of glycogen to trehalose and feedback inhibition by trehalose (2).
The transformation of fat body glycogen to trehalose is very important during flight in some species. The enzyme glycogen phosphorylase stimulate the fat body phosphorylase and increase the synthesis of trehalose which can be stimulated hormonally.
The synthesis of carbohydrate from precursors such amino acids, lactate and glycerol is called gluconeogenesis. The insect species that are sufficient of gluconeogenesis form glycogen with the presence of precursors.
Figure 1 from (2)
Metabolism of amino acids:
The oxidation of amino acids can conduce a small amount of a total fuel that insects use during flight. However the oxidation of the amino acid proline is much more important for some species, due to the fact that it is oxidized at high rates in order to support flight metabolism. In the oxidation of proline in muscle, only a part of the proline is oxidized. During the oxidation of proline, two atoms from the five carbon atoms of proline are transformed to carbon dioxide and the other three atoms can be visible as alanine which contains the nitrogen produced from proline. Moreover, the alanine derived from the flight muscle is transited to the fat body which it is transform again to proline, as well as the extra two carbon atoms required produced by β-oxidation of fatty acids to acetyl-CoA. Furthermore, proline in insect flight muscle contribute in a system which transfer the decreasing equivalents from the cytosol to the mitochondria. In this part, pyrroline 5-carboxylate is decreased to proline at the outlay of NADH in the cytosol, and then the proline is transferred into the mitochondria .There, proline is oxidized by proline oxidase back to pyrroline 5-carboxylate using the molecular oxygen. Then, pyrroline 5-carboxylate returned to the cytosol in order to complete the cycle. The regulation of proline metabolism, takes place in the flight muscle in order to join the oxidation rates to muscle contraction. However, the information for the mechanism for this are not yet clear. Finally, the regulation of proline synthesis is also under hormonal control.
Metabolism of glycerol
Locusts metabolize diacyglycerol into fatty acids and glycerol in flight muscle. Experimental evidence proposed that the role of glycerol is to become esterified to fatty acids and act as transporter for fatty acids as diacylglycerol back to the flight muscle. This is very important during a lot of migratory flights where they supply with carbohydrate when glycerol ends. In the same way, glycerol act as a system for fatty acid transport. Moreover, the amount of glycerol produced during the diacylglycerol breakdown in muscles, is twice more that is needed for the system for fatty acids transport. The surplus is partly oxidized by the muscles and transformed to trehalose in the fat body.
Metabolism of ketone bodies
The information on ketone body metabolism currently remains rather unclear. However, experiments mainly in locusts and cockroaches indicate that insects are able to metabolize ketone bodies, acetoacetate and 3-hydroxybutyrate. These metabolites appear to make a small contribution to flight metabolism. This is supported by multiple experiments evidence. Experiments indicated that acetoacetate in locust hemolyph is markedly increased during flight. In addition, the locust fat body and flight muscle contained the enzyme3-hydroxybutyrate hydrogenase. In fact, the fat body of Periplaneta Americana also contained acetoacetate and 3-hydroxybutyrate. In addition the fat body contained enzymes required for ketone body synthesis (hydroxymethylglutaryl CoA synthase and hydroxymethyl CoA lyase). Other evidence supports that other insect species contained 3-oxoacid CoA transferase and acetoacetyl thiolase in flight muscle. Conclusively, it seems that ketone metabolism may take place in both fat body and in some cases flight muscle.
The movements of the animals can be divided into two basic kinds of exercises: the continuous exercise for long periods of time, and the quick and short activities that are supported by type I and muscle type IIb muscle fibers, respectively. Anaerobic glycolysis is fed by phosphorylated glucose derived from muscle glycogen, which is converted into lactate. The use of anaerobic metabolism during flight has received remarkably little attention. At present, the mechanisms of ATP production during short-term, highpower output flights remain unclear. Insect flight muscle generally has very low levels of lactate dehydrogenase. The flight muscles of some large birds are known to contain high levels of lactate dehydrogenase, and anaerobic metabolism is presumed to be critical for flight in all large flying birds (1,3).
Despite recent advancements about the energetics of the flight process, much remains to be elucidated on the details of the molecular pathways of flight and how the various organs are coordinated and contribute to this important process.