Digestive System and Metabolism: Test 1 Review Study Guide

Digestive System Overview

Major Processes of Digestion

• Mastication: The process of chewing food to break it down into smaller pieces, increasing surface area for enzymatic action.

• Deglutition (Swallowing): The act of moving food from the mouth to the stomach, involving voluntary and involuntary muscle contractions. Phases: oral, pharyngeal, esophageal.

• Organs and Nerves: Includes the mouth, pharynx, esophagus, stomach, small and large intestines, and accessory organs (salivary glands, liver, pancreas). The enteric nervous system and autonomic nerves regulate motility and secretion.

GI Tract Structure and Function

• General Structure: The GI tract is a continuous tube from mouth to anus, with specialized regions for digestion and absorption.

• Accessory Organs: Salivary glands, liver, gallbladder, and pancreas secrete enzymes and other substances aiding digestion.

• Histological Layers:

  1. Mucosa

  2. Submucosa

  3. Muscularis externa

  4. Serosa (or adventitia)

Types of Tissues in the GI Tract

• Epithelial Tissue: Lines the lumen, specialized for secretion and absorption.

• Muscle Tissue: Smooth muscle in muscularis externa for peristalsis and segmentation.

• Nervous Tissue: Enteric plexuses coordinate local reflexes.

GI Tract Regions and Functions

• Oral Cavity: Mechanical and chemical digestion begins here.

• Pharynx and Esophagus: Conduct food to the stomach.

• Stomach: Mixes food with gastric juice, initiates protein digestion.

• Small Intestine: Main site for digestion and absorption.

• Large Intestine: Absorbs water, forms feces.

Digestive Glands and Secretions

Major Salivary Glands

• Parotid, submandibular, and sublingual glands secrete saliva containing amylase and mucus.

Gastric Glands and Secretions

• Parietal Cells: Secrete HCl and intrinsic factor.

• Chief Cells: Secrete pepsinogen (activated to pepsin by HCl).

• G Cells: Secrete gastrin, a hormone stimulating gastric activity.

Phases of Gastric Secretion

• Cephalic Phase: Triggered by sight, smell, taste, or thought of food.

• Gastric Phase: Initiated by food in the stomach.

• Intestinal Phase: Begins when chyme enters the small intestine.

Hormonal Regulation

• Gastrin: Stimulates gastric secretion and motility.

• Secretin: Stimulates bicarbonate secretion from pancreas.

• Cholecystokinin (CCK): Stimulates gallbladder contraction and pancreatic enzyme secretion.

Accessory Digestive Organs

Liver and Gallbladder

• Liver: Produces bile, processes nutrients, detoxifies substances.

• Gallbladder: Stores and concentrates bile, releases it into the duodenum.

• Bile Ducts: Common hepatic duct, cystic duct, common bile duct.

Pancreas

• Secretes digestive enzymes (amylase, lipase, proteases) and bicarbonate into the duodenum.

Digestion and Absorption

Enzymatic Breakdown

• Carbohydrates: Broken down by amylases and brush border enzymes to monosaccharides.

• Proteins: Digested by pepsin, trypsin, chymotrypsin, and peptidases to amino acids.

• Lipids: Emulsified by bile, digested by lipases to fatty acids and monoglycerides.

Absorption Mechanisms

• Monosaccharides and Amino Acids: Absorbed via active transport and facilitated diffusion in the small intestine.

• Lipids: Absorbed as micelles, reassembled into chylomicrons, and transported via lymphatics.

Large Intestine and Fecal Formation

Functions

• Absorbs water and electrolytes, forms and stores feces, houses beneficial bacteria.

Movements

Movement	Description
Peristalsis	Propulsive movement of contents
Segmentation	Mixing movements
Mass peristalsis	Strong contractions moving feces
Haustral churning	Mixing in haustra (pouches)

Metabolism

Carbohydrate Metabolism

• Glycolysis: Breakdown of glucose to pyruvate, producing ATP and NADH.

• Gluconeogenesis: Formation of glucose from non-carbohydrate sources.

• Glycogenolysis: Breakdown of glycogen to glucose.

• Glycogenesis: Formation of glycogen from glucose.

Cellular Respiration

• Anaerobic: Glucose → Lactic acid + ATP

• Aerobic: Glucose + O2 → CO2 + H2O + ATP

Overall equation for aerobic respiration:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + 36-38 ATP$

Major Steps of Cellular Respiration

1. Glycolysis

2. Pyruvate oxidation (formation of acetyl-CoA)

3. Krebs (Citric Acid) Cycle

4. Electron Transport Chain

ATP Yield

• Glycolysis: 2 ATP, 2 NADH

• Krebs Cycle: 2 ATP, 6 NADH, 2 FADH2 (per glucose)

• Electron Transport Chain: Most ATP generated here

Lipid and Protein Metabolism

• Lipolysis: Breakdown of triglycerides to fatty acids and glycerol.

• Beta-oxidation: Fatty acids broken down to acetyl-CoA.

• Ketogenesis: Formation of ketone bodies from acetyl-CoA.

• Deamination: Removal of amino group from amino acids for energy production.

Enzymes and Regulation

Enzyme Function

• Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy.

• Most enzymes are proteins; their activity is affected by temperature, pH, and substrate concentration.

ATP and Energy Transfer

• ATP is the main energy currency of the cell, produced by substrate-level and oxidative phosphorylation.

• ATP is formed from ADP and inorganic phosphate ($ADP + P_i \rightarrow ATP$).

Thermoregulation

• Basal Metabolic Rate (BMR): The energy expended at rest to maintain vital body functions.

• Heat is lost from the body by radiation, conduction, convection, and evaporation.

• Vasodilation increases heat loss; vasoconstriction conserves heat.

Summary Table: Key Digestive Enzymes

Enzyme	Source	Substrate	Product
Amylase	Salivary glands, pancreas	Starch	Maltose
Pepsin	Stomach (chief cells)	Proteins	Peptides
Trypsin	Pancreas	Proteins	Peptides
Lipase	Pancreas	Triglycerides	Fatty acids, monoglycerides

Additional info: This guide is based on a comprehensive review of the digestive system and metabolism, including structure, function, regulation, and clinical relevance, as outlined in the provided test review questions.