16 February 2005
Reading, Chapter 22
E. Energy flow at the organism scale
All of your cells need glucose and oxygen to perform aerobic respiration. Your body takes in oxygen continuously but your cells, especially your nerve cells, need glucose continuously as well. The different cells and organs of your body coordinate to provide glucose and oxygen to all while taking into account the constraints of gathering and eating the food that provides the glucose.
3. Between meals
As noted earlier, the nerve cells of your brain must have glucose to make the ATP they need. Brain cells need a steady supply of glucose for this purpose. Confusion, dizziness, and fainting occur if your blood glucose drops too low, a condition often called "hypoglycemia". This presents a challenge because you cannot eat continuously to maintain your blood glucose in the desired range. All animals must be able to function between meals, sometimes for long periods.
Several cells and organs of the digestive system cooperate to maintain blood glucose levels between meals.
a. Liver cells convert stored glycogen back into glucose and release it into the blood to maintain glucose levels between meals.
b. Fat cells in adipose tissue convert fats back into fatty acids and release them into the blood. This does not help nerve cells, since they can't take up fatty acids from the blood for aerobic respiration. The other cells of your body can use fatty acids, however, leaving more glucose for the nerve cells.
c. Muscle cells can contribute to blood glucose but indirectly. The glycogen in muscle cells can be converted back into glucose and used by those cells to make ATP. This reduces the need to draw glucose from the blood but muscle cells cannot release glucose into the blood for other cells to use. Muscle cells can release pyruvate and lactate (from glycolysis) into the blood. This pyruvate and lactate is taken up and converted to glucose by liver cells.
After a prolonged period between meals, muscle cells begin to convert their proteins back into amino acids and release them into the blood. These amino acids are taken up by the liver and converted into glucose, as described earlier. This is a dire situation that occurs only in cases of severe over-training or starvation. When individuals die of starvation or anorexia, it is often loss of protein from the heart muscle that leads to death.
4. Regulation of blood glucose
Your blood glucose is maintained within a fairly narrow range (80 to 100 mg glucose per 100 ml blood). Too little blood glucose (<40 mg /100 ml) causes coma and death. Too much blood glucose (1000 mg/100 ml) has the same effect, though for different reasons.
Your blood glucose level is maintained by two hormones secreted by your pancreas: insulin and glucagon. Your pancreas is a large gland behind your stomach. Hormones are chemical message between different cells in your body that coordinate their activities.
a. If blood glucose is too high
When levels of glucose in your blood are high, the protein hormone insulin is secreted by certain cells in your pancreas. Insulins binds to receptor proteins in membranes of muscle and adipose cells. Insulin causes these cells to increase their uptake of glucose from the blood and its conversion to glycogen or fat. One way that insulin increases glucose uptake is by increasing the number of glucose transport proteins in the membranes of these cells.
b. If blood glucose is too low
When levels of glucose in your blood are too low, different cells in your pancreas secrete a different hormone into your blood, the hormone glucagon. Glucagon is perceived by receptor proteins in the membranes of your liver cells. It causes them to convert stored glycogen back into glucose, which is released into the blood. Glucagon also stimulates the participation of liver cells in the conversion of fats into glucose.
"Adrenaline" is another hormone that stimulates conversion of glycogen to glucose. "Adrenalin" prepares your body to fight or run. Extra blood sugar is useful under such circumstances.
c. Diabetes mellitus
The health condition known as diabetes mellitus results from inappropriate regulation of blood glucose. The most common type of diabetes is Type II diabetes. People with this condition have cells that stop responding to insulin. The pancreas initially tries to compensate by secreting more insulin but eventually, insulin secretion declines. The result is that blood glucose is chronically high, causing high blood pressure and altered metabolism of fats and proteins. If untreated, severe diabetes can cause coma and death by the accumulation of byproducts from the use of fats and proteins for energy by insulin-insensitive cells. Milder cases eventually cause damage to the heart, kidneys, and eyes.
Type II diabetes affects 8% of the US population and 20% of those over 60 years of age. It was the sixth leading cause of death in the US during the year 2000. Costs related to diabetes in the US were $132 billion in 2002. Treatment of diabetes consists mostly of maintaining appropriate blood glucose levels by frequent testing, careful diet, and injections of insulin.
Other types of diabetes include Type I diabetes, in which the insulin-producing cells of the pancreas are mistakenly destroyed by the immune system, and gestational diabetes, in which hormones from the placenta of a pregnant woman render fat and muscle cells insensitive to insulin (this condition reverses when the placenta is lost during delivery).
More information about diabetes can be obtained from the National Institute of Diabetes, Digestive and Kidney Disease (NIDDK), which is one of the National Institutes of Health (NIH).