High Energy Compounds Primary need of the animal body from the nutrients is energy. Free energy is the term used to denote the energy available to do useful work. Anabolic or synthetic reactions in the body utilize energy and are called endergonic reactions. Catabolic or breakdown processes release energy and are called exergonic. Certain intermediate compounds trap free energy released from exergonic reactions and make it available for endergonic reactions. These compounds are high energy compounds which play an important role during metabolic reactions and energy transformation in the body.
Adenosine triphosphate or ATP is one such compound which is formed when energy is released. The formation of ATP is a measure of energy efficiency of nutrients. There are similar high energy phosphate compounds playing the role of energy transformation. Creatine phosphate is another compound present which is present in the skeletal muscles. It is useful in the transfer of high energy phosphate bonds and is used as an energy reserve in the muscular tissue.
Stages in Intermediary Metabolism
There are three stages during the process of nutrient catabolism in the animal body. In the first stage, major nutrients are broken down to their fundamental building blocks such as amino acids, monosugars and fatty acids. This stage is mostly taken care of by the digestive processes in the alimentary canal. The second stage involves the breakdown of these units further into much simpler common metabolites such as Acetyl Coenzyme A (Acetyl CoA). This second stage forms the preparatory stage of intermediary metabolism. The third stage consists of final breakdown of the simple metabolites into carbon dioxide, water and products of excretion along with the release of energy either for direct use or for the formation of high energy compounds such asATP. During anabolic reactions, reversal of these stages results in the formation of macromolecules, major nutrients and cellular constituents with utilization of energy.
Carbohydrate is the chief nutrient providing energy to the animal body. However, it is not stored appreciably in the tissues. Glucose is the chief sugar present in the blood of mammals. In ruminants, the volatile fatty acids which arise on account ofcrude fibre digestion in the rumen form the chief source of energy to the animal. Propionic acid arising from rumen digestion is an important source of glucose. Glucose is stored as glycogen in liver and muscles. Glucose may be derived from noncarbohydrate sources like amino acids and glycerol. In non ruminants like dogs and pigs, the glucose level in blood may be around 100 mg%. In an adult ruminant, it is around 55 to 60 mg%. Glucose in the body may be utilized for the formation ofglycogen, release of free energy by oxidation, synthesis of body fat or for the formation of lactose and other carbohydrate containing metabolites. During an initial breakdown process of glucose, known as glycolysis, under aerobic conditions, glucose, monosugars and glycogen are broken down to the stage of a metabolite called pyruvic acid. Under anaerobic conditions, the pyruvic acid is further converted to lactic acid which on restoration of aerobic conditions is reconverted to pyruvic acid in the liver for furtherbreakdown.
Lipids in the blood are derived from the dietary source or mobilization from body reserves. New lipids are also formed in the liver and transported to blood. The fat in the body is not an inert material, but undergoes active metabolism of breakdown, synthesis, desaturation, saturation and inter-conversion of fatty acids depending upon the body needs and energy status of the body. Acetyl Co A is the building block for fat synthesis. Though the essential fatty acids are not synthesized in the animal body, some inter-conversion can take place if sufficient linoleic acid is present in the diet. Body requirement of essential fatty acids is about 1% of the total energy needs.
Amino acids which are the end products of protein digestion in the small intestine are absorbed into the blood stream through the intestinal wall. Protein breakdown also takes place in the tissues releasing amino acids into the blood pool. Since proteins cannot be stored significantly in the animal body, a steady supply of proteins through the diet is essential. Amino acids are necessary to meet the demands of tissue synthesis, gluconeogenesis, liponeogenesis, ready supply of energy through oxidation and synthesis of specific organic compounds such as enzymes, hormones and other substances of metabolic importance.