Chemical Digestion and Absorption: A Closer Look
Learning Objectives
By the end of this section, you will be able to:
- Identify the locations and primary secretions involved in the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids
- Compare and contrast absorption of the hydrophilic and hydrophobic nutrients
As you have learned, the process of mechanical digestion is relatively simple. It involves the physical breakdown of food but does not alter its chemical makeup. Chemical digestion, on the other hand, is a complex process that reduces food into its chemical building blocks, which are then absorbed to nourish the cells of the body (Figure 23.28). In this section, you will look more closely at the processes of chemical digestion and absorption.
Figure 23.28 Digestion and Absorption Digestion begins in the mouth and continues as food travels through the small intestine. Most absorption occurs in the small intestine.
Chemical Digestion
Large food molecules (for example, proteins, lipids, nucleic acids, and starches) must be broken down into subunits that are small enough to be absorbed by the lining of the alimentary canal. This is accomplished by enzymes through hydrolysis. The many enzymes involved in chemical digestion are summarized in Table 23.8.
The Digestive Enzymes
Enzyme Category |
Enzyme Name |
Source |
Substrate |
Product |
Salivary Enzymes |
Lingual lipase |
Lingual glands |
Triglycerides |
Free fatty acids, and mono- and diglycerides |
Salivary Enzymes |
Salivary amylase |
Salivary glands |
Polysaccharides |
Disaccharides and trisaccharides |
Gastric enzymes |
Gastric lipase |
Chief cells |
Triglycerides |
Fatty acids and monoacylglycerides |
Gastric enzymes |
Pepsin* |
Chief cells |
Proteins |
Peptides |
Brush border enzymes |
α-Dextrinase |
Small intestine |
α-Dextrins |
Glucose |
Brush border enzymes |
Enteropeptidase |
Small intestine |
Trypsinogen |
Trypsin |
Brush border enzymes |
Lactase |
Small intestine |
Lactose |
Glucose and galactose |
Brush border enzymes |
Maltase |
Small intestine |
Maltose |
Glucose |
Brush border enzymes |
Nucleosidases and phosphatases |
Small intestine |
Nucleotides |
Phosphates, nitrogenous bases, and pentoses |
Brush border enzymes |
Peptidases |
Small intestine |
Aminopeptidase: amino acids at the amino end of peptidesDipeptidase: dipeptides |
Aminopeptidase: amino acids and peptidesDipeptidase: amino acids |
Brush border enzymes |
Sucrase |
Small intestine |
Sucrose |
Glucose and fructose |
Pancreatic enzymes |
Carboxy-peptidase* |
Pancreatic acinar cells |
Amino acids at the carboxyl end of peptides |
Amino acids and peptides |
Pancreatic enzymes |
Chymotrypsin* |
Pancreatic acinar cells |
Proteins |
Peptides |
Pancreatic enzymes |
Elastase* |
Pancreatic acinar cells |
Proteins |
Peptides |
Pancreatic enzymes |
Nucleases |
Pancreatic acinar cells |
Ribonuclease: ribonucleic acidsDeoxyribonuclease: deoxyribonucleic acids |
Nucleotides |
Pancreatic enzymes |
Pancreatic am