9.6: Summary

In a cell, the nucleus houses DNA (our genetic information), which is packed into chromosomes. Surrounding the nucleus is a fluid called cytoplasm. Floating in the cytoplasm are mitochondria (where oxygen-requiring reactions take place) and ribosomes (for protein synthesis). The cell-covering is called the cell membrane.

The cell membrane is a lineup of lecithin molecules that form a double layer around the cell. Cholesterol is embedded in this membrane, as are a variety of proteins called membrane proteins. Membrane proteins vary and have a variety of functions, e.g., serving as receptors for low-density lipoproteins (LDL) or insulin.

Metabolism refers to the chemical reactions that occur in cells, and can be subdivided into energy-releasing ones and energy-requiring ones. They are interdependent. When ATP energy is needed for muscular activity, or to make protein, etc., the energy-providing nutrients are broken down to provide the needed ATP.

A basic design in the breakdown of energy-providing nutrients is that they break into a few key substances so that they can funnel into the same chemical reactions. For example, carbohydrate, fat, and protein can all be broken down to acetate (a 2-carbon molecule), which can then take several directions. If ATP is needed, acetate can be broken down further to release energy. If ATP isn’t needed just then, acetate can be made into fatty acids and stored as body fat. Acetate can also be used to make other substances like cholesterol.

For glucose and some amino acids, there are two phases of ATP production. The first phase is anaerobic (oxygen not used), which takes place in the cytoplasm of a cell. ATP is made in the systematic breakdown of glucose to pyruvate. Although each molecule of glucose provides only small amounts of ATP, these anaerobic reactions are fast, and ATP can be made quickly, using many molecules of glucose.

The second phase is aerobic (oxygen used), and takes place in the mitochondria. ATP is made when acetate is broken down to carbon dioxide (which we exhale). Although a lot of ATP is made here, these aerobic reactions are relatively slow.

Fatty acids (and some amino acids) are broken down directly to acetate and don’t participate in the anaerobic phase of ATP production. They participate only in the aerobic phase. Without oxygen, fatty acids can’t be used to make ATP.

Glucose is a crucial substance because the brain constantly needs ATP, and under normal circumstances, the brain can use only glucose to make ATP. The liver has the responsibility of providing the brain with glucose via the blood. The liver does this by storing glucose as glycogen when glucose is abundant, and releasing glucose into the blood when glucose is low.

Glucose supply is severely limited during starvation (and other extremely-low-carbohydrate situations). The body must then use certain amino acids to make glucose to feed the brain. (The body can’t make glucose from fatty acids.) This is a rather extreme measure. Body proteins must be broken down to provide those amino acids when there’s no food coming in.

The scarcity of glucose causes a buildup of acetate, causing the acetates to combine to form ketones. The brain then uses some of these ketones as fuel, reducing the brain’s need for glucose. This has obvious survival value, since it lessens the need to break down body proteins. When a person is near death from starvation, the body has to break down proteins from the heart and other key tissues.

Blood-glucose is normally kept within a narrow range by two hormones made by the pancreas: Insulin lowers blood glucose by enabling tissue cells to take in glucose from the blood. Glucagon raises blood glucose by breaking down liver glycogen to glucose, which is then released into the blood.

Blood glucose is abnormally high in diabetes, because cells can’t take in glucose adequately from the blood. In type 1 diabetes, there’s simply a lack of insulin, because the pancreatic cells that make it have been destroyed. This kind of diabetes isn’t related to obesity, usually occurs during childhood or young adulthood, and is controlled with insulin injections, diet, and exercise.

Type 2 diabetes is related to obesity, although a genetic susceptibility is involved. It accounts for about 90-95% of the diabetes in the U.S. The cells can’t take in enough glucose from the blood because of a resistance to insulin action. It’s generally a cell problem rather than an insulin problem.

This diabetes usually occurs in adults over age 40, and is controlled with weight loss (if overweight), diet, exercise, and oral medication (and sometimes insulin injections to try to overcome insulin resistance). One of the more serious health risks of diabetes—insulin-dependent or not—is blood-vessel damage, which can result in serious damage to the tissues served by the vessels.

Hypoglycemia is a condition of low blood sugar accompanied by symptoms that are alleviated by sugar. Aside from hypoglycemia that results when a diabetic injects too much insulin, the most common kind is reactive hypoglycemia. This typically involves a big burst of insulin (thus hypoglycemia) as a reaction to a surge of blood glucose from ingesting a lot of carbohydrate on an empty stomach. The advice is to eat a good diet in frequent and regularly spaced intervals.