Animal Digestion

BL1004-Animal Digestion

Instructor Information

Dietary Categories

  • Herbivores: Animals that primarily consume plants.

  • Carnivores: Animals that primarily consume other animals.

  • Omnivores: Animals that consume both plants and animals.

Nutritional Needs

  • Fuel Requirements: Energy for normal bodily functions.

  • Biosynthesis: The process of creating new molecules necessary for the body.

  • Essential Nutrients: Nutrients that cannot be synthesized by the animal and must be obtained through diet.

    • Includes 10 amino acids, some fatty acids, vitamins, and minerals.

Digestion Overview

  • Overall Process:

    1. Ingestion: Intake of food.

    2. Digestion: Breakdown of food into smaller molecules.

    3. Absorption: Nutrient molecules enter body cells.

    4. Elimination: Removal of undigested material.

Types of Digestion

  • Intracellular Digestion:

    • Occurs within a cell.

    • Example: Food vacuoles that contain enzymes for digestion in lysosomes.

  • Extracellular Digestion:

    • Occurs in a specific compartment outside the cell.

    • Example: Cnidarians like hydra have a gastrovascular cavity to digest food externally.

  • Self-digestion Prevention:

    • Animals must avoid self-digestion by processing food in specialized compartments to separate digestive processes from bodily tissues.

Gastrovascular Cavity (Cnidarians)

  • Definition: Structure that digests food and distributes nutrients.

  • Composition:

    • Epidermis: Outer layer.

    • Mesenchyme: Gel-like matrix.

    • Gastrodermis: Inner layer facilitating digestion.

    • Mouth: Entry point for food and excretory outlet for waste.

    • Nutritive Muscular Cells: Involved in intracellular digestion after food is engulfed.

  • Function: Digestion begins in the gastrovascular cavity where enzymes are secreted and is completed intracellularly by nutritive muscular cells.

  • Common Species: Animals in Phylum Cnidaria, such as jellyfish and corals, possess this system.

Comparison of Digestive Systems

  • Simple Digestive Systems:

    • Represented by Porifera (sponges) and Cnidaria (jellyfish) which have one opening functioning as both mouth and anus.

  • Complex Digestive Systems:

    • Animals with two openings (mouth and anus) enable a complete digestive tract or alimentary canal; facilitates extracellular digestion.

    • Allows for specialized compartments for various digestive processes.

Human Digestive System

o Alimentary Canal:
- Structure characterized by its complete digestive tract with two openings, allowing food to move in one direction.
- Components:
- Mouth -> Pharynx -> Esophagus -> Stomach -> Small Intestine -> Large Intestine -> Anus
- Schematic Components:
- Accessory Organs: Salivary glands, pancreas, liver, and gallbladder aid in digestion.

Digestive Phases and Timeframes

  • Mouth to Stomach:

    • Esophageal Phase:

      • Peristalsis (involuntary contractions) helps move food down the esophagus.

      • Initial movement takes approximately 5-6 seconds.

  • Stomach:

    • Retains food for 2-6 hours.

  • Small Intestine:

    • Digestive time between 5-6 hours for further breakdown.

  • Large Intestine:

    • Food may spend 12-24 hours processing here.

Enzymatic Digestion Summary

  • Salivary Glands:

    • Produce salivary amylase to hydrolyze starch and glycogen into smaller polysaccharides and maltose.

  • Stomach:

    • Pepsin initiates protein digestion turning proteins into polypeptides.

    • Acids neutralize and activate enzymes in the digestive process.

  • Small Intestine:

    • Pancreatic Amylases convert polysaccharides into disaccharides.

    • Trypsin & Chymotrypsin break down proteins to smaller polypeptides.

    • Pancreatic Nucleases digest DNA and RNA into nucleotides.

    • Pancreatic Lipase converts fats into glycerol, fatty acids, and glycerides with help of bile salts.

  • Digestive Enzymes Breakdown Process**:

    • Polysaccharides → (Salivary Amylase) → Smaller Polysaccharides → (Pancreatic Amylases) → Disaccharides → (Disaccharidases) → Monosaccharides.

    • Proteins → (Pepsin) → Small Polypeptides → (Pancreatic Trypsin) → Amino Acids.

    • Nucleic Acids → (Pancreatic Nucleases) → Nucleotides → (Nucleotidases) → Nitrogenous Bases, Sugars, Phosphates.

    • Fats → (Bile salts) → Fat globules → (Pancreatic Lipase) → Glycerol, Fatty Acids.

Specific Structures and Functions

  • Stomach Structure:

    • Lined with highly folded epithelium containing gastric glands.

    • Cell Types within Gastric Glands:

      • Mucus Cells: Secrete mucus for lubrication and protection.

      • Chief Cells: Secrete inactive pepsinogen.

      • Parietal Cells: Secrete hydrochloric acid (HCl).

    • Pepsin Activation: HCl converts pepsinogen to active pepsin, which further activates more pepsinogen, promoting protein digestion.

  • Chyme: Mixture of gastric juices and partly digested food, regulated by the pyloric sphincter to control passage to the small intestine.

Hormonal Control of Digestion

  • Cholecystokinin (CCK):

    • Stimulated by amino acids/fatty acids in the duodenum; prompts release of digestive enzymes from pancreas and bile from gallbladder.

  • Secretin:

    • Released by the duodenum; stimulates pancreas to release bicarbonate to neutralize acidic chyme.

  • Enterogastrone:

    • Inhibitory hormone secreted by the duodenum to slow gastric emptying.

Absorption of Nutrients in Small Intestine

  • Structure: Composed of villi, microvilli (brush border), and epithelial cells to maximize absorption.

  • Blood Supply: Nutrient-rich blood transported via the hepatic portal vein.

  • Lymphatic System: Lacteals absorb dietary fats for transport.

The Large Intestine (Colon)

  • Length: 1.5 m long.

  • Appendix: Plays a role in defense.

  • Microflora: Rich in bacteria like E. coli that produce vitamins absorbed by the colon.

  • Functions:

    • Water reabsorption, production of feces.

Cecum and Its Variations

  • Cecum:

    • Larger in mammalian herbivores for accommodating bacteria aiding cellulose breakdown.

    • Reduced in obligate carnivores whose diets consist of minimal to no plant material.

Microbiome and Its Impact

  • Composition: 500-1,000 microbial species reside in human gut, influencing health.

  • Functions:

    • Commensal bacteria convert dietary fibers to short-chain fatty acids and help absorb Vitamin K.

  • Research Areas:

    • Gut-brain axis and effects of microbiome composition on anxiety, obesity, and memory.

  • Factors Affecting Microflora:

    • Antibiotics, fermented foods (e.g., sauerkraut, yogurt), and fecal transplants.

Revision Questions

  • Where does absorption of nutrients occur?

  • Where does protein digestion commence in humans?

  • What food group does amylase break down?

  • What is the inactive form of pepsin?

  • What are three functions of the colon?

Acknowledgements

  • Majority of text and PowerPoint slides derived from Campbell's Biology.

Additional Revision Questions

  • Define intracellular digestion.

  • Define extracellular digestion.

  • List two functions of a gastrovascular cavity.

  • What is the difference between complete and incomplete digestive systems?