Plant glucides are made up of association in different ways of five simple sugars, called monosaccharides, of which glucose is the most important source of energy for the human body. Glucose molecule can have different configurations, and when they are linked together to form polysaccharides, these macromolecules properties will be influenced  by the type of incorporated glucose. Thus, while alpha-D-glucose and beta-D-glucose have a very similar structure, they differ in their biochemical properties. While beta-D-glucose is found in the cellulose, an insoluble fiber, alpha-D-glucose forms the starch, the usual reserve material of plants.

Starch is a polysaccharide composed of many glucose molecules linked in chains, and is found as granular form in different parts of plants. Due to stereochemistry differences  not all starch types are digestible. While digestible starch should form the majority intake of energy, the starch resistant to human digestive enzymes play yet an important role in maintaining the intestinal bacterial flora. Since there is no cellulase enzyme in the human digestive tract, the cellulose can not be digested; however it has an important role, on which we will return to.

Digestive enzymes unfolds starch up to monosaccharides called glucose or dextrose, the most important source of energy for humans.

Another important monosaccharide fructose or levulose, which is found in fruits, in some plants and honey.

When two monosaccharides are connected, a disaccharide is generated, such as lactose, which is composed of a alpha-glucose molecule and one beta-galactose. Lactose is the only animal diglucide. It is found in mammalian milk, the ratio of 4,5-6%. Sucrose, or the ordinary sugar on our tables, is also a disaccharide consisting of one molecule of alpha-glucose and one beta-fructose.

The most common form of sugar in the nature and at the same time the most important source of energy for humans and animals is starch, which is found in the leaves, roots, seeds and tubers.

When are linked 3 to 9 monosaccharide molecules, we talk about oligosaccharides, as raffinose and stachyose. When are linked  ten, hundreds or thousands of glucose molecule, we have polysaccharides. Starch comprises all polysaccharides that can be completely digested in the human digestive tube, while the non-starch polysaccharides contains  those that can’t be digested and absorbed completely. Non-starch polysaccharides, like cellulose, hemicellulose and pectin are also known as fibers or ballast substances.

In order to be absorbed in the digestive tract, glucides from most foods must be loosened in their molecules constituent, which is carried out by enzymes. Glucides digestion already starts in the mouth, which is why is recommended the thorough chewing of food. The salivary glands secrets ptyalin or amylase that attacks the starch by hundreds to thousands of glucose molecules, turning it into disaccharide maltose, which consists of two molecules of alpha-glucose. Because amylase was first found in malt extracts (germinated barley), disaccharide obtained through their action was called ” malt sugar” or maltose.

Gastric acidity under pH 4 inactivates ptyalin, so the starch digestion in the stomach can continue, as long as the pH is not less than 4. Animal proteins determines an accentuated increase of the gastric acidity, thereby preventing the starch digestion. For ex; for the egg white digestion is needed that gastric acidity to reach pH 1.5, while proteins from cereals and vegetables do not need an environment so acid.

After leaving the stomach, glucide digestion is favored alkaline environment of the duodenum and the rest of the small bowel, leading to glucose that can be absorbed and stored in the liver and in muscle cells as glycogen, as energy reserve. If needed, the glycogen transformes again in glucose.

The body needs a constant glucose contribution not only as the main source of energy, but also for operation in conditions optimal of organs such as the brain and nerve cells. Therefore, glycemia, (blood glucose), is controlled very rigorous by two hormones: insulin, hypoglycaemic, encouraging the transformation of glucose into a warehouse form of energy, for example glycogen or fat, and glucagon, with the opposite effect, that of increasing blood sugar.

Categories: Health, Nutrition
Tags: food, glucides, starch

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