Since humans are heterotrophs, they rely on the food they eat to provide their bodies with the energy needed to carry out vital cellular functions. Humans require six essential nutrients for survival: water, vitamins, minerals and three macronutrients; carbohydrates, lipids and proteins (Bowers et al, 2002). These macronutrients are often ingested as polymers, long chains of repeated basic molecular units called monomers, which are too large to be absorbed by cells directly. The digestive system is responsible for breaking these polymers down into monomers so that the nutrients can be absorbed into the bloodstream and transported throughout the body. This breakdown of nutrients is achieved through mechanical and chemical digestion. Mechanical digestion for all macronutrients begins in the mouth and it involves the physical breakdown of food through tearing, chewing, mixing and churning (Bowers et al., 2002). Mechanical digestion prepares the food for chemical digestion by increasing the surface area upon which the digestive enzymes can act (Bowers et al., 2002). In chemical digestion, polymers are broken down chemically through the process of hydrolysis with the aid of digestive enzymes which accelerate the process (Collin County Community College, 2014):
Protein + water protease amino acids
Complex sugar + water carbohydrase simple sugars
Fats + water lipase glycerol + fatty acids
Polymers are formed during dehydration synthesis reactions, as a covalent bond forms between two monomers when a water molecule is lost (Collin County Community College, 2014). In hydrolysis, the covalent bond between monomers in a polymer is broken by the addition of a water molecule as the hydrogen in the water molecule attaches to one monomer while the hydroxyl group (OH) attaches to the other monomer (Collin County Community College,2014).
Carbohydrates are the body’s main source of energy, providing four calories of energy per gram, and help form the structural components of tissues and cells (Bowers et al., 2002). Mechanical digestion begins in the mouth where food is broken down into bite size pieces by the teeth and manipulated by the tongue to form a moist bolus that moves down the esophagus by peristaltic contractions (Bowers et al., 2002). Chemical digestion of carbohydrates beings in the mouth where salivary amylase, secreted by the salivary glands, breaks down polymers of starch into smaller disaccharide molecules (maltose) (Tortora and Derrickson, 2009). The carbohydrates enter the stomach where mechanical digestion continues as they are mixed with gastric juices. However, the pH of the stomach destroys salivary amylase so the majority of carbohydrate digestion occurs in the small intestine (Tortora and Derrickson, 2009). When chyme enters the duodenum, pancreatic amylase is released from the pancreas and splits starch and glycogen the disaccharide maltose (University of Illinois Chicago, 2004). The pancreas also releases...