Simple sugars and starches are both carbohydrates, and both contain the molecule glucose, which is also called blood sugar. Glucose is a very important biological molecule, as it is the brain's primary source of energy and a significant source of energy for all body cells. The circulatory system helps move glucose out of the digestive tract and into the body cells.
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The major function of the biomolecule glucose is to provide energy to cells. Body cells take up glucose from the blood and chemically burn it, yielding energy molecules that they can use to fulfill cellular functions. Some cells, such as those of the liver and muscles, store glucose and release it under fasting conditions. In their book "Biochemistry," Drs. Mary Campbell and Shawn Farrell describe glucose as the most ubiquitous of the carbohydrate molecules.
To move glucose from the digestive tract, where it is located after a meal, into the body cells, where it's utilized, the glucose has to cross several cell membranes. Since glucose is water soluble while cell membranes are made of fatty material, glucose can't move across cell membranes on its own. Instead, explains Dr. Lauralee Sherwood in her text, "Human Physiology," transporter molecules must ferry it in and out of cells. Glucose does dissolve readily in the bloodstream, however.
Glucose first moves into the bloodstream upon absorption from the intestine. Specialized cellular transporters called sodium-dependent hexose transporters shuttle glucose across the cells that line the intestinal tract, explain Drs. Campbell and Farrell. Once through the intestinal lining, glucose is free to dissolve in the blood, and travels around the body. The intestinal transporters act quickly, such that blood glucose rises rapidly after a carbohydrate-containing meal. The pumping action of the heart then distributes blood glucose absorbed at the intestines to every part of the body.
While glucose in the bloodstream can reach all body cells, it can't enter them--entering cells requires crossing a cell membrane, which glucose can't do on its own. Glucose from the bloodstream enters cells with the help of two proteins. The first, explains Dr. Sherwood, is called a glucose transporter, or GLUT protein. The second is the hormone insulin, which the pancreas releases into the bloodstream to help cells absorb glucose from the blood.
Since cells need insulin to take up glucose from the bloodstream, and since cells need glucose to fulfill their energy needs, it's possible for cells to chemically "starve" even in the presence of plenty of glucose, if insulin is missing. This, explains Dr. Gary Thibodeau in his book, "Anatomy and Physiology," is the medical basis for Type 1 diabetes, or diabetes mellitus. If the pancreas doesn't produce insulin, cells are unable to gain access to glucose in the bloodstream, leading to various symptoms including cellular damage and death.