Complete Digestive System - BIOL 221

Main function of the digestive system

Mechanically and chemically breaks down food so nutrients can be absorbed.

Why do we need digestion?

Many nutrient molecules are too large to cross cell membranes.

What must the digestive system do?

Break food into smaller molecules and absorb them into blood or lymph.

Major nutrients found in food

Carbohydrates, proteins, lipids, vitamins, minerals, and water.

Ingestion

Food enters through the mouth.

Digestion

Food is broken into smaller pieces and molecules.

Types of digestion

Mechanical digestion and chemical digestion.

Absorption

Nutrients move from the digestive tract into blood or lymph.

Defecation

Elimination of non-digestible material as feces.

Mechanical digestion

Physically breaks food into smaller pieces.

Examples of mechanical digestion

Chewing, stomach churning, and intestinal segmentation.

Why is mechanical digestion important?

Smaller food particles have more surface area exposed to digestive enzymes.

Chemical digestion

Digestive enzymes break large nutrient molecules into smaller absorbable molecules by hydrolysis.

Carbohydrate digestion

Starch (amylose) → sugars.

Protein digestion

Proteins → amino acids.

Lipid digestion

Lipids → fatty acids and glycerol.

Amylase

Breaks starch (amylose) into sugars.

Where is amylase produced?

Salivary glands and pancreas.

Where is amylase found?

Saliva and pancreatic juice.

Pepsin

Breaks proteins into amino acids.

Where is pepsin produced?

Stomach.

What is pepsin secreted as?

Pepsinogen (inactive form).

How is pepsin activated?

By acid in the stomach lumen.

Lipase

Breaks lipids into fatty acids and glycerol.

Where is lipase produced?

Pancreas.

Where is lipase active?

Small intestine.

When does lipase break down fats?

After bile emulsifies them.

Alimentary canal (GI tract)

Continuous tube that food passes through.

Pathway of food through the GI tract

Mouth → Pharynx → Esophagus → Stomach → Small intestine → Large intestine → Rectum → Anus.

Accessory organs of digestion

Salivary glands, liver, gallbladder, and pancreas.

Role of accessory organs

Produce secretions that help digestion.

Difference between alimentary canal and accessory organs

Food passes through the alimentary canal, while accessory organs assist digestion from the outside.

Four layers of the GI tract wall

Mucosa, submucosa, muscularis, and serosa.

Mucosa

Inner layer that secretes mucus and enzymes, absorbs nutrients, and protects underlying tissues.

Submucosa

Contains blood vessels, lymphatic vessels, nerves, and glands.

Muscularis

Smooth muscle layers that move food.

Circular muscle layer

Narrows the digestive tube.

Longitudinal muscle layer

Shortens the digestive tube.

Serosa

Outer protective covering that produces fluid to reduce friction between digestive organs.

Why does the digestive tract have many folds?

To increase surface area.

Why is increased surface area important?

Allows more digestion, more absorption, and greater contact with nutrients.

Examples of structures that increase surface area

Gastric rugae, villi, and microvilli.

GI motility

Smooth muscle contractions that mix food with digestive secretions and move food through the tract.

Two major movement patterns of the GI tract

Segmentation and peristalsis.

Segmentation

Local mixing contractions that move contents back and forth.

Purpose of segmentation

Improves mixing with enzymes and absorption.

Peristalsis

Wave-like propulsion that moves food forward.

How does peristalsis move food?

Circular muscles contract behind the bolus and push material forward.

Bolus

Chewed food mixed with saliva, formed in the mouth.

Chyme

Partially digested food mixed with gastric juice, formed in the stomach.

Pathway of food processing

Food → Bolus → Chyme → Absorbable nutrients.

Functions of the mouth

Ingestion, mechanical digestion, beginning chemical digestion, and swallowing.

Mastication (chewing)

Breaks food into smaller pieces, increases surface area, and mixes food with saliva.

What does saliva begin digesting?

Carbohydrates and some lipids.

What does saliva contain?

Water, mucus, digestive enzymes, and antibacterial molecules.

Functions of saliva

Moistens food, forms a bolus, dissolves molecules for taste, and cleans mouth.

Salivary amylase

Begins carbohydrate digestion.

Salivary lipase

Begins limited lipid digestion.

Functions of the tongue

Manipulates food, mixes food with saliva, forms bolus, pushes food to pharynx.

Taste buds

Clusters of taste receptor cells housed in papillae on the tongue.

Sweet taste receptors

Detect sugars and energy-rich molecules.

Sour taste receptors

Respond mainly to hydrogen ions (H⁺).

Salty taste receptors

Respond mainly to sodium ions (Na⁺).

Bitter taste receptors

Detect many potentially toxic plant compounds and chemicals.

Umami taste receptors

Respond strongly to amino acids.

Incisors

Cut and bite food.

Canines

Grip and tear food.

Premolars and molars

Crush and grind food.

Why is chewing important?

Increases surface area available for digestive enzymes.

Effect of poor chewing

Reduces digestive efficiency.

Step 1 of swallowing (oral phase)

Tongue pushes food/liquid to back of mouth, triggering swallowing reflex.

Step 2 of swallowing (pharyngeal phase)

Soft palate blocks nasal cavity, epiglottis folds down, food enters esophagus.

Step 3 of swallowing (esophageal phase)

Peristalsis moves bolus through esophagus; lower esophageal sphincter relaxes.

Esophagus

Muscular tube that transports food from the pharynx to the stomach.

How does food move through the esophagus?

Peristalsis pushes the bolus toward the stomach.

Lower esophageal sphincter (LES)

Contracted to prevent reflux; relaxes during swallowing.

What happens if the LES relaxes inappropriately?

Acid enters the esophagus, causing irritation and heartburn.

Major functions of the stomach

Storage, mechanical churning, protein digestion start, controlled chyme release, defense.

Mechanical digestion in the stomach

Strong muscular contractions churn food into chyme.

Why does the stomach release food gradually?

To slowly deliver chyme into the small intestine.

Mucous cells

Produce mucus that protects the stomach lining.

Parietal cells

Produce hydrochloric acid (HCl).

Functions of hydrochloric acid (HCl)

Kills microbes, activates pepsin, and creates an acidic environment.

Chief cells

Produce pepsinogen.

What happens to pepsinogen?

HCl activates it into pepsin, which digests proteins.

Gastrin

Stomach hormone released when food enters, stimulating acid and enzyme production.

What does gastrin stimulate?

Increased HCl secretion, pepsinogen secretion, and stomach motility.

How does gastrin improve digestion?

Increases acid production, mixing, and protein digestion.

Why doesn't the stomach digest itself?

Protective mechanisms prevent self-digestion.

Protective mechanisms of the stomach

Mucus barrier, rapid cell replacement, and releasing pepsin as inactive pepsinogen.

Mucus barrier

Protects epithelial cells from acid.

Rapid cell replacement

Frequently replaces damaged epithelial cells.

Why is pepsin released as pepsinogen?

To prevent the stomach from digesting itself.

What causes ulcers?

Damage to the mucus layer allows acid injury.

Ghrelin ("hunger hormone")

Produced by stomach; stimulates hunger before meals and drops after eating.

Leptin ("satiety hormone")

Produced by adipose tissue; suppresses appetite and signals energy storage.

How are hunger and appetite regulated?

Digestive system and adipose tissue signal the brain to control feeding.

Main site of digestion and absorption

Small intestine.

Where are monosaccharides absorbed?

Into the blood.

Where are amino acids absorbed?

Into the blood.