Chapter 14: The Digestive System and Body Metabolism – Study Guide

Chapter 14: The Digestive System and Body Metabolism

Digestive System Functions

The digestive system is responsible for breaking down food, absorbing nutrients, and eliminating waste. It involves both mechanical and chemical processes to ensure efficient digestion and absorption.

• Ingestion: The process of taking food into the mouth.

• Propulsion: Movement of food through the digestive tract, primarily by peristalsis.

• Mechanical Digestion: Physical breakdown of food (e.g., chewing, churning in the stomach).

• Chemical Digestion: Enzymatic breakdown of food into smaller molecules.

• Absorption: Movement of nutrients from the digestive tract into the bloodstream.

• Defecation: Elimination of indigestible substances as feces.

Mechanical vs. Chemical Digestion

Digestion involves both mechanical and chemical processes, each with distinct roles:

• Mechanical Digestion: Physical breakdown of food into smaller pieces to increase surface area for enzymes. Examples include chewing (mastication) and stomach churning.

• Chemical Digestion: Breakdown of complex molecules into simpler ones by enzymes. For example, amylase breaks down starch into sugars.

Example: Chewing in the mouth is mechanical, while salivary amylase action is chemical.

Digestive Enzymes and Their Functions

Enzymes play a crucial role in chemical digestion by catalyzing the breakdown of macromolecules:

• Amylase: Breaks down starch into maltose.

• Protease (e.g., pepsin): Breaks down proteins into peptides.

• Lipase: Breaks down fats into fatty acids and glycerol.

Equation:

$\text{Starch} \xrightarrow{\text{amylase}} \text{Maltose}$

$\text{Protein} \xrightarrow{\text{pepsin}} \text{Peptides}$

$\text{Triglyceride} \xrightarrow{\text{lipase}} \text{Fatty acids} + \text{Glycerol}$

Phases of Swallowing

Swallowing is a complex process involving several phases:

• Buccal Phase: Voluntary; food is pushed into the pharynx by the tongue.

• Pharyngeal-Esophageal Phase: Involuntary; food moves through the pharynx and esophagus via peristalsis.

Stomach Functions and Enzymes

The stomach performs both mechanical and chemical digestion:

• Mechanical: Churning mixes food with gastric juices.

• Chemical: Pepsin begins protein digestion.

• Enzymes: Pepsin (protein digestion), gastric lipase (fat digestion).

• Absorption: Limited; mainly water, alcohol, and some drugs.

Example: Pepsin is activated by hydrochloric acid in the stomach.

Small Intestine Functions

The small intestine is the primary site for digestion and absorption:

• Enzymes: Pancreatic enzymes (amylase, lipase, proteases) and brush border enzymes.

• Bile: Emulsifies fats, aiding lipase action.

• Absorption: Nutrients are absorbed through villi into the bloodstream.

Enzyme	Source	Substrate	Product
Amylase	Pancreas	Starch	Maltose
Lipase	Pancreas	Triglycerides	Fatty acids, Glycerol
Peptidase	Intestinal wall	Peptides	Amino acids

Large Intestine Functions

The large intestine absorbs water and forms feces:

• Absorption: Water, electrolytes, and some vitamins.

• Formation of Feces: Indigestible material is compacted into feces.

• Defecation: Elimination of feces via the rectum and anus.

Defecation Reflex: Triggered by stretching of the rectal wall, leading to contraction of the rectal muscles and relaxation of the anal sphincters.

Summary Table: Digestive Processes

Organ	Mechanical Digestion	Chemical Digestion	Absorption
Mouth	Chewing	Salivary amylase	Minimal
Stomach	Churning	Pepsin	Water, alcohol
Small Intestine	Segmentation	Pancreatic enzymes, bile	Most nutrients
Large Intestine	Haustral contractions	Bacterial enzymes	Water, vitamins

Additional info: Academic context was added to expand on the brief questions and provide a self-contained study guide, including definitions, examples, and tables for clarity.