The body is constantly fuelled by glucose. Glucose is an essential source of energy for the brain. It is also fundamental energy source for working muscles. There are three primary source of blood glucose as mentioned by Champe and colleagues (2007), these are diet, degradation of glycogen and gluconeogenesis. The food intake gives a random source of glucose. On the other hand, gluconeogenesis provides a continuing supply of glucose (Champe etal 2007). However, the process cannot keep up immediately if the body is losing so much glucose.
Thus, the body store mobilizable glucose called ‘glycogen that is quickly released in the liver and kidneys (Champe et al. 2007). ’ Glycogen Glycogen in muscle tissues is primarily responsible for providing energy source for muscles during exercise or intense work. Glycogen is commonly stored in the liver and the skeletal muscles (Houston 2006). It provides the muscles a fuel reserve during muscle contraction that results to the synthesis of adenosine triphosphate (ATP). Glycogen is primarily made up of polymerized glucose (as shown below).
Excess glycogen is converted into triglycerides or fats by the liver, which are stored in different boy tissues. Fats can also be ingested from foods (Houston, 2006). They are broken into non-esterified fatty acids, which are carried by albumin into the cells. The cells can use these fatty acids as source of energy or convert them back to triglycerides that will be deposited to different tissues for later use. The fats are especially stored in certain fat cells called adipocytes though they may also be present in muscle cells (Houston 2006).
Fats are water insoluble. They are made of glycerol and three fatty acids (Martin and Coe 1997).
Degradation and Storage When glucagons or epinephrine binds with membrane receptors of the cells, it signals the need for glycogen (Champe et al. 2007). The ? -1,4 linkage of the glycogen undergoes phosphorylysis, through the enzyme phosphorylase at the outset of glycogen degradation (Stryer et al. 2006). This linkage is more often cited as the phosphorylitic cleavage and he breakdown process is known as glycogenolysis (Houston 2006).
The process involves the separation of glucose with the glycogen molecule up to four residues. The enzyme 4:4 transferase will act and transfer a ‘three-residue section’ which allow the -1,6-glucosidase to remove the ? -1,6 linkage through the process of hydrolysis (Stryer et al, 2006). The phosphorylazaion forms a glucose-1-phosphate, which is converted into glucose-6-phosphate. When it enters the liver, it is then converted into glucose and send to the blood (Houston 2006). When glycogenolysis happens in the muscle tissue, the glucose is transferred into the glycolitic pathway. Glycogen is stored largely in the liver (Figure 1. 1 A)
Brown and colleagues explained that glucose synthesis happens by ‘adding one glucose unit at a time to an existing molecule through a -1,4 glycosidic bond and then rearranging the glycogen chains to make branches which are necessary to make glycogen water soluble. ’ Triglycerides are degraded through a process known as lipolysis. Lipase, an enzyme activated by cAMP-sensitive protein kinases promotes the hydrolysis of fat and break it down into fatty acids and glycerol (Smith et al. 2004). Smith and colleagues (2004) demonstrated that the degradation of fatty acid occurs in the mitochondria and transported via carnitine shuttle that undergoes a sequence of oxidative reactions removing carbon each time in the form of acetyl CoA.
The synthesis of triglycerides happens in the cytoplasm wherein carbon is added in the form of malonyl CoA. The major storage site for triglycerides are the adipose tissues which contains 77% of the total body energy as indicated in Figure 1. 1 A. Insulin increase upon food intake when there is high concentration of glucose in the blood. Its main role is to promote the removal of glucose from the blood and encourage storage of excess glucose into glycogen and fatty acids into triglycerides. Insulin is the main factor that inhibits the continuation of lipolysis (Smith et al.2004).
As the glucose absorbed in the intestine decrease, glucagons is secreted into the blood, which encourage catabolism of fat and glycogen and increase the glucose level. The low levels of blood glucose also results to an increase of epinephrine and norepinephrine released by the adrenal glands (Stryer, et al, 2006). These two functions to increase glucose production through catabolism. Figure 1. 1 B. A summary of fuel metabolism and reactions.
The above table (Figure 1.1B) provides a concise representation of the different metabolic processes in the body. The basic relationship that can be drawn out from this table is that synthesis reduces the amount of fatty acids or glucose in the blood while degradation increases their amount. The synthesis state happens as the body absorbs glucose and fats from food or from the body systems. On the other hand, degradation happens as the body undergoes the postabsorptive state wherein glycogen and triglycerides are consumed due to the absence or lower concentration of glucose in the blood (Sherwood 2005).
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