Digestive System: Digestion and Enzymes

• The digestive system's function is to break down food into molecules the body can use via digestion, absorb these molecules into the bloodstream through the intestinal walls, and eliminate unabsorbed waste through the anus.

Digestion

• Digestion is the process of breaking down food into smaller nutrient molecules.
• Mechanical digestion physically reduces the size of food, such as chewing in the mouth.
• Chemical digestion uses enzymes to simplify food substances.
• An example is sucrase digesting sucrose into glucose and fructose.
• Food must be digested into the simplest and smallest form to pass through intestinal cell membranes.

Mouth

• The mouth uses teeth for mechanical digestion: incisors cut, canines tear, and premolars & molars grind food.
• Saliva in the mouth contains salivary amylase for chemical digestion.
• Amylase digests starch into maltose.
• Food becomes a semi-solid round ball called a bolus in the mouth.

Enzyme Function

• Enzymes are specific to molecules, like a lock and key, to cause a chemical reaction to break down or build up substances.
• An enzyme is a substance that speeds up the breakdown or buildup of another substance.
• A substrate is the substance that an enzyme acts upon.
• The active site on an enzyme allows a molecule to fit and facilitates a chemical reaction.

Esophagus

• The epiglottis, located at the beginning of the esophagus, prevents food from entering the windpipe.
• The esophagus is a muscular tube lined with mucus for easier food passage.
• Smooth muscles push food through the esophagus towards the stomach via peristalsis.
• Peristalsis is a wave of muscle movement.

Stomach

• The stomach is a muscular pouch that expands to hold swallowed food.
• Mechanical digestion in the stomach involves three layers of smooth muscles contracting and relaxing to churn food and mix it with stomach fluids.
• Churning reduces particle size, increasing enzyme action.
• Chemical digestion in the stomach uses pepsin, an enzyme that digests large protein chains into smaller amino acid chains.
• Pepsin requires an acidic environment to function.
• Hydrochloric acid in the stomach kills bacteria in food and provides the ideal environment for pepsin.
• The stomach lining produces a thick mucus layer to protect itself, and stomach cells are quickly replaced.
• Food stays in the stomach for about 2 hours.
• Food is converted into a liquid form called chyme.
• Proteins are chemically digested into shorter amino acid chains.
• Chyme is released into the small intestine for complete digestion.

Discussion point

• Gastric bands can limit food intake.