Digestion is the process that turns chunks of food into small sugars, amino acids, fatty acids and nucleotide components. These small molecules are used by all the cells in the body to make new proteins, nucleic acids, fats, sugars and hence the energy needed to run all the activities of the cell. Without digestive enzymes, there would be no raw materials to keep cells functioning.
Digestive enzymes are crucial for breaking down food, so it can be absorbed by the body. Once food is broken down into smaller molecules that can be absorbed into the bloodstream, the nutrients can be distributed to all the cells in the body and used to fuel all the cells’ activities.
Digestive enzymes are proteins that break specific molecular bonds. The bonds release smaller molecules from the larger food particles in the digestive system. Many different digestive enzymes work in sequence to turn food into small molecules that can enter the blood stream.
There are enzymes specific for lipids (lipases), proteins (peptidases) and carbohydrates. Starches are polysaccharides, made up of many sugar molecules linked together, and are digested by amylases. There are specific enzymes that break apart specific pairs of sugar molecules after amylase has broken starches into disaccharides (2 sugar molecules linked together). Other digestive enzymes are specific for digesting nucleic acids (DNA and RNA molecules).
Digestion starts in the mouth. As the teeth grind the food into smaller bits, amylase begins to break starches down into sugars, and lipases begin to break down lipids. The stomach breaks food apart by a combination of acid, mixing and gastric enzymes (which work at the acid pH of the stomach). The pancreas makes amylase, lipase and a variety of enzymes to break apart proteins once the food is in the intestines. The intestines have a number of “brush border” enzymes, located on the intestine cells’ membranes, that digest disaccharides, small peptides and nucleotides into smaller molecules.
Once food is broken down into small molecules (single sugar molecules, amino acids, fatty acids and nucleic acid components) the nutrient molecules can enter the blood. Fatty acids cross intestinal cell membranes and enter the blood. Other nutrients bind specific proteins on the intestinal cell wall and are transported across the intestinal cells and released into the blood. The nutrients in the blood bind to receptors on cells in the body and are taken up by the cells to provide energy and building blocks for the molecules that the cells need to make to function properly.
- Principles of Anatomy and Physiology (10th ed.); Gerard J Tortora and Sandra Reynolds Grabowski; 2003