4.6: Introduction to Carbohydrates
Carbohydrates, proteins, and fats are the primary macronutrients in the human diet. However, carbohydrates are the most favored source of energy in the body. They can be found in a wide variety of foods, including whole grains, fruit, and vegetables, in various forms, such as sugars, starch, and dietary fiber. Based on their structure, carbohydrates are classified into three main classes— monosaccharides, disaccharides, and polysaccharides. The body's cells can only utilize simple monosaccharides, such as glucose. The disaccharides like sucrose and lactose need to be broken down into monomer forms to be used by the body. Similarly, polysaccharides, such as starch— a glucose polymer, must undergo enzymatic hydrolysis to be utilized as a source of energy.
Glucose Utilization
Glucose is the body's primary fuel and a key molecule for energy generation. The primary energy reserve like carbohydrates and fats are ultimately converted to glucose to be utilized by the body. The cells then use a set of metabolic reactions, including glycolysis, the citric acid cycle, and the electron transport chain pathway, to completely oxidize glucose and generate energy. At the end of the entire process, the catabolism of one glucose molecule generates 30-32 ATP molecules.
Starch Utilization
Starch is a polysaccharide composed of glucose monomers. It is a mixture of amylose and amylopectin polymers. Amylose is a linear polymer, while amylopectin is a branched polymer. Plants synthesize starch as a reserve carbohydrate. Potatoes and grains are significant sources of starch in the human diet. Enzyme α-amylase present in the saliva is able to break down amylose and amylopectin into short-chain malto-oligosaccharides in the mouth. In the next stage, when these oligosaccharide molecules reach the duodenum, the pancreatic amylase breaks them down into maltose or maltotriose. Later, maltase-glucoamylase and sucrase-isomaltase on the brush border cells of the intestine break down these disaccharides and trisaccharides into glucose monomers. Finally, glucose molecules are absorbed through the cell lining and transported through the whole body via the blood.
