Because humans are heterotrophs, they rely on the food they eat to provide their bodies with the energy needed to perform 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 repeating basic molecular units called monomers, which are too large to be absorbed directly by cells. The digestive system is responsible for breaking down these polymers into monomers so that 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 of all macronutrients begins in the mouth and involves the physical breakdown of food through tearing, chewing, mixing, and churning (Bowers et al., 2002). Mechanical digestion prepares food for chemical digestion by increasing the surface area on which digestive enzymes can act (Bowers et al., 2002). In chemical digestion, polymers are chemically broken down through the process of hydrolysis with the help of digestive enzymes that speed up the process (Collin County Community College, 2014): Proteins + amino acids water proteases Complex sugar + water carbohydrates simple sugars Fats + water lipase glycerol + fatty acids Polymers are formed during dehydration synthesis reactions, as a covalent bond is formed 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 the center of the paper… enzymes to digest them. In chemical digestion, hydrolysis breaks the bonds between monomers with the help of enzymes, secreted throughout the digestive tract and from accessory organs, to catalyze the process. These enzymes function optimally in certain areas of the alimentary canal based on the pH of the surrounding environment. The carbohydrate and protein monomers are then absorbed into the mucosal cells of the microvilli lining the small intestine and diffuse into the capillaries surrounding the villi where they are transported in the bloodstream to the cells of the body. Most nutrient absorption occurs in the duodenum and jejunum (University of Illinois Chicago, 2004). The lipids are too large to diffuse into the capillaries and are reassembled into triglycerides and enter the lymphatic system. All undigested material is eliminated from the digestive system in the form of feces.