Digestive

Digestive Systems

Theme: Consumption and Breakdown of Nutrients

Goals:

  • Understand the physical features of the system.

  • Analyze metabolic rates.

  • Examine chemical breakdown and control.

1. Nutrition

a. Nutrients

  • Substances necessary for normal body function.

b. Nutrient Types

  1. Lipids

  2. Carbohydrates

  3. Proteins

  4. Minerals

  5. Vitamins

c. Uses of Nutrients

  • Fuel for cellular metabolism and activities.

  • Organic raw materials for tissue formation.

  • Provide essential nutrients not synthesized by the body.

2. Nutrients that are Fuel

I. Three Basic Units of Fuel

  1. Carbohydrates

  2. Proteins

  3. Lipids

II. Calorie Measurements

  • Defined as the energy required to raise 1 g of water from 14.5 to 15.5 degrees Celsius.

III. Kilocalorie Definition

  • 1 kilocalorie (Kcal) is equal to 1000 calories.

3. Energy Content of Nutrients

  • Lipids: 9.5 Kcal per gram

  • Proteins: 4.25 Kcal per gram

  • Carbohydrates: 4.25 Kcal per gram

4. Basal Metabolic Rate (BMR) and Standard Metabolic Rate (SMR)

a. BMR

  • The minimal energy necessary to sustain life functions at rest.

b. Variations in Metabolic Rate

  1. Body Temperature

    • Ectotherms: Lower metabolic rates due to external temperature reliance.

    • Endotherms: Higher metabolic rates due to internal temperature maintenance.

  2. Body Size

    • Smaller animals generally exhibit a higher BMR.

5. Other Uses of Energy

Feeding and Digestion

  1. Endotherms: Metabolic increase by 25% during digestion.

  2. Ectotherms: Metabolic increase by 40 times during digestion.

Fabrication of New Molecules

  • Energy is used to synthesize new molecules necessary for life.

Essential Nutrients

  • Nutrients that cannot be synthesized by the organism and must be acquired from the diet.

6. Digestive Systems and Strategies

The Digestive Systems Overview

There are various feeding systems linked closely to the evolution of digestive structures:

  1. Suspension Feeders

  2. Manipulative Mechanisms

  3. Suction Feeding

  4. Surface Absorption

  • Example: Baleen mechanisms found in blue whales for filtering krill.

  • Example: The serrated bill of the hooded duck for consuming slippery fish.

Classification of Digestive Strategies

  1. Herbivores

  2. Carnivores

  3. Omnivores

7. Phases of Digestion

  1. Ingestion: Introduction of food into the system.

  2. Mechanical Breakdown: Physical processing, such as chewing.

  3. Chemical Breakdown: Enzymatic degradation of food substances.

  4. Absorption: Nutrient uptake into the bloodstream.

  5. Elimination: Removal of undigested materials from the body.

8. Evolution of Digestive Methods

A. Intracellular Digestion

  1. Food Vacuole: Membrane-bound compartments.

  2. Lysosome: Organelles containing enzymes for digestion.

B. Extracellular Digestion

  1. Gastrovascular Cavity: A sac-like digestive organ.

  2. Alimentary Canal: Complete digestive tract with specialized regions.

9. Structure and Function of the Digestive Tract

Evolutionary Advantages

  1. Ability to feed on larger food items.

  2. Cellular specialization for different functions.

  3. Separation of digestive processes for efficiency.

10. Structure of the Vertebrate Digestive System

Components

  1. Gastrointestinal Tract: Includes the organs involved in digestion.

  2. Accessory Glands: Assist in digestion, such as salivary glands and pancreas.

Layers of the Tract

  1. Mucosa: Lining responsible for nutrient absorption.

  2. Connective Tissue: Provides structural support.

  3. Smooth Muscle: Facilitates peristalsis.

  4. Connective Tissue (Serosa): Outermost layer providing protection.

11. Smooth Muscle Contraction

Characteristics

  • Contains actin and myosin proteins.

  • Myosin Filament Characteristics:

    • Long structures facilitating contraction movements.

Intracellular Ca+2 Dynamics for Contraction

  1. Contraction strength increases with intracellular calcium levels.

  2. Calcium enters through voltage-sensitive Ca channels.

The Process of Muscle Contraction

  1. Depolarization: Triggering action potentials that lead to contraction.

  2. Calcium release from the endoplasmic reticulum and extracellular space.

  3. Calcium binds to calmodulin, activating myosin light-chain kinase.

  4. Phosphorylation of myosin light chains exposing active sites.

  5. ATP is cleaved, allowing the myosin head to bind actin and perform power strokes.

  6. Myosin head must undergo dephosphorylation to end contraction cycle.

12. Activity of the GI Tract

Key Functional Processes

  1. Peristalsis: Wave-like muscle contractions that move food through the GI tract.

  2. Sphincters: Circular muscles that close off sections of the GI tract, regulating passage.

  3. Muscle Layers of the GI Tract:

    • Longitudinal layer for lengthwise movement.

    • Circular layer for segmentation and mixing of food.

13. Mammalian Digestive System

A. Oral Cavity

  • Evolved teeth and jaw structures adapted for different dietary needs:

    • Herbivores: Flat teeth for grinding and a jaw set at a 90-degree angle.

    • Carnivores: Sharp teeth for tearing and a jaw set at a greater angle.

    • Omnivores: Combination features of both.

B. Salivary Glands

  • Produce saliva containing:

    • Glycoproteins.

    • Amylase: Initiates starch breakdown into sugars.

    • Antibacterial agents.

    • Growth factors for tissue repair.

    • Lingual Lipase: Begins fat digestion.

C. Pharynx and Esophagus

  • Pharynx: Connects the mouth to the esophagus.

  • Esophagus: Conducts food to the stomach, with different muscle types:

    • Striated muscle in the upper third.

    • Smooth muscle in the lower two-thirds.

D. Stomach

  • Secretes gastric juice composed of:

    • Hydrochloric Acid: Produced by parietal cells.

    • Pepsin: Activated from pepsinogen secreted by chief cells.

  • The stomach lining secretes mucus for protection against acids.

  • Resulting mixture after digestion is called chyme.

E. Gastric Glands and Pits

  • Gastric Glands: Responsible for secretion of gastric juice containing HCl and pepsin.

  • Gastric pits on the stomach's interior surface accommodate cellular structures involved in secretion.

14. Small Intestines and Digestion

Structure and Function of the Small Intestine

  • Divided into sections:

  1. Duodenum: Primary site for digestion.

    • Receives secretions from the pancreas:

    1. Bicarbonate: Increases pH of chyme.

    2. Amylase: Breaks down carbohydrates.

    3. Lipase: Breaks down fats.

    4. Trypsinogen: Activated to trypsin for protein digestion.

    • Receives bile from the liver for lipid emulsification.

  2. Jejunum: Middle section, involved in nutrient absorption.

  3. Ileum: Final section, continuing absorption of nutrients and bile salts.

II. Absorption Mechanisms

  1. Proteins and carbohydrates: Actively transported to the liver.

  2. Fats are hydrolyzed into glycerol and fatty acids, forming chylomicrons.

III. Villi and Microvilli

  • Villi: Finger-like projections that increase surface area for absorption.

  • Brush Border: Microvilli add more surface area and contain enzymes for nutrient breakdown.

  • Lacteals: Lymphatic vessels that transport fat-soluble nutrients from the intestine.

15. Large Intestine

  • Shorter section of the GI tract that absorbs water and electrolytes; not primarily involved in digestion.

16. Evolutionary Trends in Digestion

Trends Observed in Various Species

  1. Dentition Variations: Different tooth structures based on diets.

  2. Chamber Development:

    • Birds possess specialized structures like crops and gizzards.

    • Ruminants have complex stomachs for digesting plant material.

  3. GI Tract Length: Overall length varies across species based on dietary needs.

17. Accessory Organs Functions

A. Pancreas

  • Produces important digestive enzymes that are activated in the small intestines.

  • Secretes bicarbonate to neutralize stomach acid.

B. Liver and Gallbladder

  • The liver produces bile, concentrated in the gallbladder and released into the small intestine to emulsify fats.

18. Regulation of Digestive Processes

Components of Digestion Regulation

  1. Hormonal Regulation: Hormones control digestive enzyme secretion and bile release.

  2. Neural Regulation: Sights, smells, or tastes of food can stimulate digestive secretions.

Key Hormones in Digestion

  1. Gastrin: Released when proteins are present in the stomach; stimulates secretion of pepsinogen and HCl.

    • Regulation: pH decrease inhibits further gastrin release.

  2. Cholecystokinin (CCK): Released in response to lipids in chyme; causes gallbladder contraction and bile release.

  3. Secretin: Released when acidic chyme enters the small intestine; stimulates bicarbonate release from the pancreas to neutralize acidity.

19. Summary of Digestive Process Regulation

  1. Gastrin: Stimulates gastric juices; inhibited by low pH.

  2. CCK: Stimulates bile release; involved in lipids' digestion.

  3. Secretin: Stimulates bicarbonate release to neutralize chyme.