Digestive System: Key Concepts and Enzymes for Biology Students

**Purpose of the Digestive System

Breaks food into monomers the body can absorb**

**Main Nutrients Digested

Carbohydrates, proteins, lipids**

**GI Tract Pathway

Mouth then esophagus then stomach then small intestine then large intestine then anus**

**Ingestion

Taking food into the mouth**

**Propulsion

Movement of food by swallowing and peristalsis**

**Mechanical Digestion

Physical breakdown of food by chewing and churning**

**Chemical Digestion

Enzymatic breakdown of polymers into monomers**

**Absorption

Movement of nutrients into blood or lymph (mostly in small intestine)**

**Defecation

Elimination of indigestible waste**

**Small Intestine

Main site of digestion and absorption**

**Start and End of Small Intestine

From pyloric sphincter to large intestine**

**Duodenum

Shortest region; receives bile and pancreatic enzymes**

**Jejunum

Region responsible for most nutrient absorption**

**Ileum

Longest region; ends at ileocecal valve**

**Circular Folds

Spiral chyme and increase absorption surface area**

**Villi

Fingerlike projections containing capillaries and a lacteal**

**Microvilli (Brush Border)

Microscopic projections containing brush‑border enzymes**

**Carbohydrate Brush‑Border Enzymes

Sucrase, maltase, lactase, alpha‑dextrinase**

**Protein Brush‑Border Enzymes

Peptidases**

**Nucleic Acid Brush‑Border Enzymes

Nucleosidases and phosphatases**

**Segmentations

Mix chyme and enhance absorption without moving chyme forward**

**Peristalsis (Migrating Motor Complex)

Moves chyme forward; one wave takes 90-120 minutes**

**Chyme Duration in Small Intestine

Chyme remains 3-5 hours**

**Carbohydrate Digestion Location

Mouth (salivary amylase) and small intestine (pancreatic amylase + brush‑border enzymes)**

**Carbohydrate End Product

Monosaccharides**

**Protein Digestion Location

Stomach (pepsin) and small intestine (trypsin, chymotrypsin, elastase, carboxypeptidase)**

**Protein End Products

Amino acids, dipeptides, tripeptides**

**Nucleic Acid Digestion

Pancreatic ribonuclease and deoxyribonuclease; brush‑border nucleosidases and phosphatases**

**Lipid Digestion Location

Stomach (lingual and gastric lipase) and small intestine (pancreatic lipase)**

**Role of Bile

Emulsifies fats to increase surface area for lipase**

**Lipid End Products

Fatty acids and monoglycerides**

**Carbohydrate Absorption into Cell

Glucose and galactose via Na⁺ symporter; fructose via facilitated diffusion**

**Carbohydrate Absorption into Blood

Facilitated diffusion**

**Protein Absorption into Cell

Active transport with Na⁺ or H⁺**

**Protein Absorption into Blood

Diffusion**

**Short‑Chain Fatty Acid Absorption

Simple diffusion into blood**

**Long‑Chain Fatty Acid Absorption Step 1

Form micelles**

**Long‑Chain Fatty Acid Absorption Step 2

Enter cells and are rebuilt into triglycerides**

**Long‑Chain Fatty Acid Absorption Step 3

Packaged into chylomicrons**

**Long‑Chain Fatty Acid Absorption Step 4

Enter lacteals → lymph → blood**

**Long‑Chain Fatty Acid Delivery

Transported to liver and adipose tissue**

**Bile Function

Emulsifies fats but does not digest them**

**Pancreatic Enzymes Location

Work in the small intestine**

**Carbohydrate Digestion in Stomach

Does not occur**

**Protein Digestion Begins

In the stomach**

**Enzymes

Proteins that speed up chemical reactions**

**Excess Glucose Storage

Stored as glycogen**

**Goal of Carbohydrate Digestion

Get glucose into the bloodstream**

**Hepatic Portal Vein Function

Delivers absorbed nutrients to liver, giving liver first access**