Sugars

 Why Do Sugars Appeal to Us? We have sweet receptors on our tongues, which are linked to pleasure centers in our brains. We also have sweet receptors in our guts.

Virtually every cell in every organism uses sugar in a central metabolic pathway—a part of metabolism that’s so essential we can’t live without it. This is why sugar is bundled together to make glycogen in our liver and muscle cells.

But at any given time, we only have about a 24-hour supply of sugar present in our bodies, stored as glycogen. If you did not eat for a day or so, you’d use up all your glycogen. And that can be problematic, especially because our brains strongly prefer glucose to fuel activities. If blood glucose levels fall too low, the resulting hypoglycemia can be dangerous—even fatal.

How Do Cells Make Glucose? Because glucose is so important, when supplies get low, even temporarily, our bodies start making it from other molecules. Cells in the liver and kidney do the work of making glucose to help prevent this from happening when glucose levels drop. Glucose synthesis is called gluconeogenesis, which translates to “new synthesis of glucose.”

Our bodies cannot make glucose from the fatty acids in fat. But our cells can break down proteins and use the amino acids released to make glucose. This is also why people on high-protein, low-carbohydrate diets still get enough glucose.

Glucose can also be converted into other forms. For example, glucose can combine with an amine group to become a glucosamine molecule, which joins with an acetyl group to become N-acetylglucosamine. Long polymers of N-acetylglucosamine contribute to cartilage and produce chitin, which plays important roles in many organisms.

Proteins with oligosaccharides attached to them are called glycoproteins, and each cell membrane has thousands of them.

High blood glucose, as found in diabetes, can damage blood vessels and cause blindness, organ failure, cognitive impairment, and cardiovascular disease. 

Going to all that trouble is worthwhile because ATP works like a battery, storing energy to be used instantaneously by cells. The fully charged battery of ATP can instantaneously discharge its energy and become adenosine diphosphate (ADP). The spent battery of ADP can be recharged to ATP. ATP is a triphosphate, while ADP is a diphosphate, so all that’s going on is the removal and addition of a phosphate. This can happen again and again, using energy from food.