mechanical digestion

Meaning: Mechanical digestion is the physical breakdown of food into smaller pieces without breaking covalent bonds in the food molecules. Examples are chewing in the mouth, churning of the stomach wall that mixes food with gastric juice to form chyme, and segmentation in the small intestine, which mixes chyme with enzymes and bile. Smaller pieces have more surface area per volume, so enzymes can reach their substrates and catalyze hydrolysis faster.

On the MCAT: Questions ask you to sort a process as mechanical or chemical, or to predict what happens when one is impaired. For example, poor chewing leaves larger particles and slows chemical digestion. Smooth muscle motility in the stomach and intestine is regulated by the autonomic nervous system, with parasympathetic input generally increasing it.

Common confusion: Students label bile as chemical digestion. Bile salts are not enzymes and do not hydrolyze bonds. They emulsify fat, breaking large droplets into smaller ones, so this is usually classed as mechanical (physical) digestion. Amylase, pepsin and lipase, by contrast, are chemical digestion because they hydrolyze bonds. Peristalsis mainly moves food along the tract; some texts count it as mechanical, others treat it separately as motility.