Our bodies are an environment just like the outside environment. Just like the way we receive and respond to the signals from our environment, the cells in the body act the same way. Insulin is the hormone that signals cells in our body to take glucose. It is released by the pancreas and is produced as a response to high levels of blood glucose (blood sugar) in the body. The amount produced increases with increase in the levels of blood sugar. After digestion, there is an increase in the levels of glucose in the blood stream. This acts as a signal that stimulates the pancreas to produce insulin. Hence the main purpose of insulin is to reduce the levels of glucose in the blood stream by facilitating transport of glucose into the cells. Glucose acts as a source of energy for the cell (Robin).
To fully understand how glucose enters the cell, there is need to understand the basic structure of a cell. The main components of a cell involved in glucose movement inside a cell are the cell membrane containing the phospholipids bilayers.This is because it is their alteration that allows entrance of glucose transporters.
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The sugars enter the body in complex forms that cannot pass through the semi permeable cell membrane of a cell.i.e.the food that we eat mostly contains sugars in the form of starch and cellulose. These cannot be directly absorbed through the semi permeable membrane of the cell. Digestion of these sugars right from the mouth by the saliva and into the stomach where they are worked on by hormones liberates glucose. This glucose is oxidized in the liver to other forms that can now travel in the body and finally access the muscle cells. The process of glucose oxidation is called glycolysis and it is converted into lactate and Pyruvate.In an inactive cell muscle, it is this pyruvate that enters the cell. The control center of the amount of glucose in the body is the liver. After digestion, the levels of glucose in the body usually go up. And since not all of it can be transported to the cell, the excess is stored in the liver. Hence the liver will produce this glucose for circulation when the levels go down.
Why glucose cannot move without insulin
Though insulin is important for the, movement of glucose across the cell membrane and into the cell, not all cells require insulin e.g. the brain and the liver .The cells that require glucose have receptors whereby insulin binds and thus allow entrance of glucose into the body. Glucose is transported in the cell by elements called glucose transporters. These are present in organelles called vesicles. But these cannot transport glucose without activation by insulin (Silverthorn 2008). Without insulin the levels of blood sugars increases and this can lead to developing of type 2 diabetes.
What insulin does to allow sugar movement?
The general glucose transporter in the body is insulin. Its main function is binding to the cell membrane which opens up the cell for the transfer of glucose from the body. This mechanism makes use of insulin receptors. The insulin receptors consist of two receptor subunits which are located outside the cell membrane and two sub units that penetrate the cell membrane. The extra cellular (outside the cell) and the intracellular (inside the cell) subunits are bonded together chemically. It is the exrracellular sub units that contain a binding site for insulin (Robin). The insulin binds to the extra cellular sub units and this activates a chemical reaction. This chemical reaction travels inside the cells through the linked subunits. Through this mechanism, chemical signals are sent to proteins within the cell. This causes them to change their activity and this initiates the movement of glucose transporters inside the cell membrane. Through this, glucose is transported inside the cell. When the insulin binds to the cell, the vesicles are able to travel rapidly into the cell membrane. Here they are able to fuse with the cell membrane and insert the glucose transporters. When the level of blood glucose is low insulin cease to bind to the cell receptors, the vesicles disengages and they travel back to the cell cytoplasm.
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Energy gradient involved
The transfer of glucose inside the cell membrane involves the process of facilitated diffusion. This is the use of a protein to facilitate the movement of molecules across the membrane .It may involve molecules passing through channels within the protein or the proteins changing shape hence allowing the molecules to pass. The diagram below shows facilitated diffusion:
Fig 1.Facilitated diffusion in a cell membrane. Purves et al.
Once inside, glucose is burned and heat is produced together with adenosine triphosphate (ATP). The process of burning glucose to create energy is accomplished through a process called aerobic metabolism. To complete the process, oxygen is a prerequisite. The oxygen is obtained from the blood and it enters the cell together with the glucose. The glucose is burned using oxygen by the mitochondria (Kapit 122). Heat and adenosine triphosphate (ATP) are released. The ATP produced can be stored or released depending on the requirements of the cell. But the conversion of glucose into energy may or may not require oxygen i.e. it can be an anaerobic metabolism or aerobic metabolism. Those cells without mitochondria e.g. the red blood cells convert glucose into energy without the use of oxygen. Muscle cells require most of the energy in our bodies. They contain mitochondria hence convert energy aerobically. But even when the levels of oxygen are depleted the muscle cells are able to produce energy anaerobically .But anaerobic process produces lactic acid which makes the muscles ache.
Glucose is important in the body for the cell to survive. This is through providing energy to the cell. In doing this, the human body is assured of survival. Insulin produced by the pancreas helps to maintain the levels of glucose at required levels. It is also important in that its activities enable the cell to access the glucose. The amount of insulin produced depends on the levels of glucose in the body. The high the levels of glucose, the more insulin will be produced. Through facilitated diffusion, insulin is able to transport glucose inside the cell.