chemical digestion
Summary
Chemical digestion is the enzyme-catalyzed hydrolysis of food macromolecules into small, absorbable units. Most of it occurs in the small intestine. The testable point is matching each enzyme to its site and pH, such as pepsin in the acidic stomach and pancreatic enzymes at higher pH.
Detail
Meaning: Chemical digestion is the enzymatic hydrolysis of food macromolecules into units small enough to absorb: starch and disaccharides into monosaccharides, proteins into amino acids and short peptides, and triglycerides into fatty acids and monoglycerides. Each enzyme works in a region that suits its pH: salivary amylase in the mouth, pepsin in the acidic stomach, and pancreatic and brush-border enzymes in the small intestine, where bicarbonate raises the pH.
Equation: polymer (n units) + (n − 1) H₂O → n monomers, where n is the number of monomer units (unitless) and each water molecule (H₂O) is consumed at one cleaved bond. A hydrolase enzyme catalyzes each cleavage.
On the MCAT: Passages ask which enzyme acts where and what happens when a condition changes. Raising stomach pH lowers pepsin activity, blocking pancreatic lipase leaves fat undigested, and lactase deficiency leaves lactose for gut bacteria. Know zymogen activation: HCl converts pepsinogen to pepsin, and enteropeptidase converts trypsinogen to trypsin.
Common confusion: Bile is not an enzyme. Bile salts emulsify fat, which increases surface area for lipase but breaks no covalent bonds, so it counts as physical, not chemical, digestion. Chewing and stomach churning are physical too. Salivary amylase is inactivated by stomach acid, so most starch digestion occurs in the small intestine.
Sources
- Campbell Biology
- Human Physiology: An Integrated Approach
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.