The Digestive System and Body Metabolism — Comprehensive Study Notes

1. Chapter 14: The Digestive System and Body Metabolism

Movement in the Digestive Tract

The digestive tract moves food using specialized smooth muscle contractions. Understanding these movements is essential for grasping how digestion and absorption occur.

• Peristalsis: Coordinated waves of muscle contraction and relaxation that propel food forward through the digestive tract.

• Segmentation: Localized, back-and-forth contractions that mix contents without moving them forward.

• Mastication: Mechanical breakdown of food by chewing in the mouth.

• Emulsification: Breakdown of large fat globules into smaller droplets by bile, increasing surface area for enzyme action (not a muscular movement).

The Stomach

Gastric Juice Composition

Gastric juice is secreted by glands in the stomach lining and contains several important components:

• Hydrochloric acid (HCl): Produced by parietal cells; creates an acidic environment.

• Pepsinogen/Pepsin: Produced by chief cells; pepsinogen is activated to pepsin by HCl for protein digestion.

• Intrinsic factor: Produced by parietal cells; essential for Vitamin B12 absorption.

• Bile: Not a component of gastric juice; produced by the liver and delivered to the duodenum.

Functions of Hydrochloric Acid (HCl)

• Denatures proteins, making them accessible to digestive enzymes.

• Activates pepsinogen to pepsin.

• Destroys most ingested pathogens by creating a hostile acidic environment.

• Note: HCl does not facilitate Vitamin B12 absorption; intrinsic factor does.

Gastric Cell Types

Cell Type	Function
Chief cells	Produce and secrete pepsinogen (inactive precursor to pepsin)
Parietal cells	Secrete HCl and intrinsic factor
Enteroendocrine cells	Secrete hormones such as gastrin
Mucous cells	Secrete protective mucus layer

Stomach Adaptations to Acidity

• Tight junctions between epithelial cells prevent acid penetration.

• Alkaline mucus coating (with bicarbonate) neutralizes acid at the surface.

• Note: Microvilli and circular folds are adaptations of the small intestine, not the stomach.

Nutrient Absorption in the Stomach

• The stomach does not absorb nutrients such as glucose, amino acids, or fatty acids.

• Most nutrient absorption occurs in the small intestine.

The Small Intestine

Regions (from stomach to large intestine)

• Duodenum: First and shortest segment; receives chyme from the stomach.

• Jejunum: Middle segment; main site for chemical digestion and nutrient absorption.

• Ileum: Final segment; connects to the large intestine and absorbs Vitamin B12 and bile salts.

Where Digestion and Absorption Occur

• Bile and pancreatic enzymes enter the duodenum via the common bile duct.

• Most chemical digestion and nutrient absorption occur in the jejunum.

• The ileum absorbs Vitamin B12 and bile salts.

Surface Area Adaptations (Largest to Smallest)

Structure	Description
Circular folds (plicae circulares)	Large, permanent folds of mucosa and submucosa
Villi	Finger-like projections containing capillaries and a lacteal
Microvilli (brush border)	Tiny projections on villus epithelial cells; site of brush border enzymes

Brush Border Enzymes

• Located in the microvilli of the small intestine.

• Responsible for the final breakdown of carbohydrates and proteins before absorption.

• Lipids and cholesterol are absorbed differently and do not require brush border enzymes.

The Large Intestine

The large intestine primarily absorbs water and vitamins, and compacts undigested material into feces.

• Absorbs water from undigested material.

• Absorbs vitamins (especially Vitamin K) produced by resident bacteria.

• Compacts undigested material into feces.

• No digestive enzymes are active here; bacteria in the cecum produce Vitamin K.

Liver and Gallbladder

Functions of the Liver

• Produces bile (stored in the gallbladder).

• Manufactures and stores glycogen.

• Performs gluconeogenesis (synthesizes glucose from fatty acids and amino acids).

• Performs glycogenolysis (breaks down glycogen to release glucose).

• Detoxifies drugs and alcohol.

• Produces blood proteins (including clotting factors) and cholesterol.

• Degrades hormones and converts ammonia to urea.

• Does not secrete digestive enzymes (that is the role of the pancreas, salivary glands, and stomach).

Bile and Emulsification

• Bile is produced by the liver and stored in the gallbladder.

• Bile emulsifies lipids, increasing surface area for lipase activity.

• Bile is not an enzyme; it does not chemically digest fats.

• Bile is released into the duodenum.

Digestive Enzymes and Hormones

Sources of Digestive Enzymes

Organ	Enzymes / Secretions
Salivary glands	Salivary amylase (starch digestion)
Stomach	Pepsinogen/pepsin (protein digestion); HCl
Pancreas	Pancreatic amylase, lipase, proteases, nucleases, bicarbonate ions
Small intestine (brush border)	Peptidases, disaccharidases
Liver	Bile (not an enzyme)
Esophagus / Colon	No digestive enzymes produced

Key Enzymes — Classification

Enzyme	Type / Function
Salivary amylase	Carbohydrase — begins starch breakdown in the mouth
Pancreatic amylase	Carbohydrase — continues carbohydrate digestion in the small intestine
Pepsin	Protease — digests proteins in the stomach
Lipase (pancreatic)	Lipase — digests triglycerides in the small intestine
Brush border enzymes	Peptidases and disaccharidases — final digestion of peptides and carbohydrates

Pancreatic Secretions

• The pancreas secretes lipase, amylase, proteases, nucleases, and bicarbonate ions.

• It does not secrete bile (bile is a liver product).

Gastrin — Hormone vs. Enzyme

• Gastrin is a hormone, not an enzyme.

• Secreted by enteroendocrine cells in the stomach.

• Stimulates increased secretion of gastric juice and enhances muscular churning.

• Not involved in lipid digestion.

Chemical Digestion by Nutrient Type

Nutrient	Sites of Chemical Digestion
Carbohydrates	Mouth (salivary amylase), Small intestine (pancreatic amylase + brush border enzymes)
Proteins and peptides	Stomach (pepsin), Small intestine (pancreatic proteases + brush border enzymes)
Lipids	Small intestine (bile emulsification + pancreatic lipase)
Large intestine	No enzymatic digestion; only bacterial fermentation and water absorption

Metabolic Processes (Liver Focus)

Process	Definition
Glycogenesis	Synthesis of glycogen from glucose
Glycogenolysis	Breakdown of glycogen to release glucose
Gluconeogenesis	Synthesis of new glucose from non-carbohydrate sources
Glycolysis	Breakdown of glucose to pyruvate (in cytoplasm of all cells)

Fat Metabolism

• Most lipid metabolism occurs in the smooth endoplasmic reticulum.

• Excessive fat breakdown produces ketones, which can cause ketoacidosis (dangerous lowering of blood pH).

Protein Catabolism

• Breakdown of amino acids produces toxic ammonia.

• The liver converts ammonia into urea, which is excreted by the kidneys.

Anabolic vs. Catabolic Reactions

• Anabolic reactions: Build larger molecules from smaller ones (require energy); e.g., dehydration synthesis.

• Catabolic reactions: Break larger molecules into smaller ones (release energy); e.g., hydrolysis.

• Dehydration synthesis (joining monomers with loss of water) is an anabolic reaction.

Cellular Respiration

Aerobic Respiration — Products from 1 Glucose

• Requires oxygen; occurs in cytoplasm (glycolysis) and mitochondria (Krebs cycle, electron transport chain).

Product	Quantity per glucose
ATP (energy)	~36–38 ATP
CO2 (carbon dioxide)	6 molecules
H2O (water)	6 molecules

Anaerobic Respiration (Fermentation) — Products from 1 Glucose

• Occurs without oxygen; takes place entirely in the cytoplasm.

• Produces much less ATP than aerobic respiration.

• Lactic acid buildup causes muscle fatigue.

Product	Quantity per glucose
ATP	2 ATP (net)
Pyruvate → Lactic acid	2 molecules
NAD+	Regenerated to allow glycolysis to continue

Quick-Reference Summary

Question	Answer
Where does bile go?	Duodenum (from gallbladder via bile duct)
Where does most absorption occur?	Jejunum
Where are brush border enzymes located?	Small intestine (microvilli)
Where is Vitamin K produced?	By bacteria in the cecum (large intestine)
Where does lipid metabolism occur?	Smooth endoplasmic reticulum
Where does anaerobic respiration occur?	Cytoplasm (cytosol)
What does the gallbladder store?	Bile (produced by the liver)
What organ stores glycogen?	Liver

Trap Statement	Correct Answer
The liver secretes digestive enzymes	FALSE — the liver makes bile, not enzymes
HCl facilitates Vitamin B12 absorption	FALSE — intrinsic factor (from parietal cells) does this
Bile is an enzyme	FALSE — bile is a detergent-like emulsifier, not an enzyme
The pancreas secretes bile	FALSE — bile comes from the liver
Gastrin is an enzyme	FALSE — gastrin is a hormone
Nutrients are absorbed in the stomach	FALSE — no nutrient absorption in stomach
Enzymes are active in the large intestine	FALSE — no digestive enzymes act there