NUTRITION AND ANIMAL DIGESTIVE SYSTEMS

NUTRITION AND ANIMAL DIGESTIVE SYSTEMS

Dr. Szuroczki Chapter 46.1 – 46.7

Introduction to Nutrition

• Nutrient:

  • Any substance taken in by an organism that is vital for survival, growth, development, tissue repair, or reproduction.

• Nutrition:

  • The process of consuming and utilizing food and nutrients.

• Animals obtain nutrients through food consumption.

Food Processing in Animals

Stages of Food Processing

1. Ingestion:

   • The act of consuming food.

2. Digestion:

   • The breakdown of food into smaller molecular components in the alimentary canal.

3. Absorption:

   • The process where ions, water, and small molecules diffuse or are actively transported from the alimentary canal to body fluids.

4. Egestion:

   • The excretion of undigested waste products.

Dietary Categories

• Organ system functions lead to similarities in nutritional requirements across species.

• Various animal physiologies exhibit differing nutritional needs.

• Existing categories may not sufficiently capture the vast diversity among animals.

Organic Nutrients

• All animals share essential fundamental organic molecules for two key purposes:

  • Energy provision:

  • Essential for ATP synthesis.

  • Molecule synthesis:

  • To construct new cellular components.

Major Organic Nutrients in Animals

Essential Nutrients

• Specific compounds that cannot be synthesized from any precursor molecule must be included in the diet, termed essential nutrients.

Four Groups of Essential Nutrients:

1. Essential Amino Acids

2. Essential Fatty Acids

3. Vitamins

4. Minerals

Essential Amino Acids

• Nine specific amino acids required by numerous animals, including humans.

• These cannot be synthesized by animal cells and are not stored.

• All twenty amino acids can be derived from meat; most plants do not provide every essential amino acid in adequate quantities.

Essential Fatty Acids

• Certain unsaturated fatty acids that cannot be constructed by animal cells.

• Generally, unsaturated fatty acids are predominantly derived from plant sources.

• Strict carnivores derive these from fish or adipose tissues of mammals and birds.

Health Benefits of Essential Fatty Acids

• Omega-3 Fatty Acids:

  • Reduce the risk of sudden heart attack fatalities, lower blood pressure, and impede blood clotting.

  • They also diminish inflammation and may lower the risk of certain cancers in animals.

General Principles of Digestion and Absorption of Nutrients

Intracellular Digestion

• Occurs in some simple invertebrates (sponges and certain unicellular organisms).

• Utilizes phagocytosis to directly incorporate food particles into cells.

• Not sufficient to meet the metabolic needs of more active organisms over extended periods.

Extracellular Digestion

• Common in most animals, occurring within a cavity which shields interior cells from hydrolytic enzymes.

• The simplest form features a single opening that serves both as the entrance and exit.

• Facilitates the consumption of larger food items, allowing for slow digestion and nutrient absorption over duration.

Structure of the Digestive System

• In vertebrates, comprising the alimentary canal coupled with several accessory structures:

  • Alimentary Canal (Gastrointestinal, GI tract):

  • An elongated tube with openings at each end, where food transits through the hollow cavity (lumen).

  • In humans, includes the mouth, pharynx, esophagus, stomach, intestines, and anus.

  • Accessory Structures:

  • May encompass the tongue, teeth, salivary glands, liver, gallbladder, and pancreas, not all are present in every vertebrate.

Absorption Mechanisms

• Nutrient absorption by epithelial cells of the alimentary canal occurs via:

  • Simple Diffusion

  • Facilitated Diffusion

  • Active Transport

• While some nutrients are utilized by canal cells, the majority is transported into the bloodstream for systemic distribution.

The Human Digestive System

Primary Components and Functions

1. Mouth:

   • Initiates digestion and breaks down food.

2. Esophagus: (highway)

   • Transports food to the stomach.

3. Liver:

   • Produces bile to facilitate fat digestion.

4. Gallbladder:

   • Stores bile until required and secretes it into the small intestine.

5. Large Intestine: (dump)

   • Absorbs water and minerals; prepares wastes for defecation.

6. Salivary Glands:

   • Secrete saliva.

7. Pharynx: (gate)

   • Acts as passageway to the esophagus.

8. Stomach: (starge unit)

   • Functions in temporary food storage and initial digestion of proteins.

9. Pancreas:

   • Secretes digestive enzymes and bicarbonate (HCO³⁻) into the small intestine.

10. Small Intestine:

    • Site where most digestion and absorption occurs.

11. Rectum:

    • Stores waste (feces).

12. Anus:

    • Eliminates waste through defecation.

Alimentary Canal Regions

• Anterior End:

  • Primarily involved in ingestion (mouth, pharynx, esophagus).

• Middle Portion:

  • Involves food storage and initial digestion (crop, gizzard, stomach, upper part of small intestine, and associated organs).

• Posterior Part:

  • Responsible for final digestion, absorption, and waste elimination (lower small intestine, large intestine).

• Undigested materials are expelled through the anus or cloaca.

Structure of the GI Tract

• The GI tract is lined with epithelial cells that perform several functions:

  • Secretion of digestive enzymes and protective mucus.

  • Release hormones into the bloodstream.

  • Transport digested materials from the canal.

• Secretory gland ducts cut through the epithelial layer to release acids, enzymes, water, and ions into the lumen.

• Tight junctions along the apical surfaces prevent substances from diffusing between epithelial cells, maintaining the integrity of the gastrointestinal barrier.

• The epithelial layer is bordered by layers of smooth muscle, neurons, connective tissue, and blood vessels, with neuronal activation stimulated by food sight, smell, or presence in the tract.

Salivary Functions

• During the tasting of food, salivary glands produce saliva, a watery fluid containing proteins, mucus, and antibacterial agents.

Functions of Saliva

• Moisten and lubricate food for easier swallowing.

• Dissolve food particles to enhance taste perception.

• Kill bacteria ingested with food.

• Initiate carbohydrate digestion via salivary amylase.

Pharynx and Esophagus

• Pharynx:

  • Facilitates food movement to storage organs; does not contribute to digestion or absorption.

  • Swallowing begins as a voluntary act in the pharynx and continues involuntarily via peristalsis in the esophagus, which propels food through rhythmic contractions.

The Crop

• In certain animals, food travels from the esophagus to the crop, which is a storage organ that expands the lower esophagus.

• The crop holds and softens food with little or no digestion, and is chiefly found in birds.

• Its size often depends on dietary habits, and regurgitated material for offspring originates from the crop.

The Stomach

• A C-shaped organ situated on the left side within the abdominal cavity.

• Entry and Exit:

  • Food enters via the cardioesophageal sphincter from the esophagus and exits into the small intestine through the pyloric sphincter.

Functions of the Stomach

• Acts as a temporary storage reservoir for food.

• Site for both chemical and mechanical digestion, particularly for proteins via enzymes and acids.

• Produces chyme, a processed and liquefied mixture of food and gastric juices, for delivery to the duodenum of the small intestine.

• Note:

  • Only alcohol and aspirin are absorbed directly in the stomach.

Gastric Glands in the Stomach

• Located within gastric pits of the sub-mucosa, these glands consist of different cell types:

  • Mucous Cells:

  • Produce alkaline mucus rich in bicarbonate to balance pH levels.

  • Chief Cells:

  • Synthesize (enzyme ) pepsinogen, a precursor to the protein-digesting enzyme pepsin.

  • Parietal Cells:(acid)

  • Produce hydrochloric acid (HCl), essential for the acidic environment necessary for pepsin activity.

Stomach Structure in Birds

• Composed of two segments:

  • Proventriculus:

  • Glandular segment secreting acid and pepsinogen.

  • Gizzard:

  • A muscular chamber with a rough inner lining that grinds food with the assistance of ingested small stones or sand.

  • • Also found in some reptiles and similar structure in some fishes

Stomach Specialization in Ruminants

• Cellulose:

  • The primary macromolecule in plant cell walls that cannot be digested by humans.

  • Herbivores called ruminants can decompose cellulose with assistance from microorganisms in their digestive tracts, converting it into absorbable monosaccharides.

Glycosidic Bonds

• Refer to the linkage of monosaccharides (the fundamental unit of carbohydrates) to each other or to alcohols/amines through glycosidic bonds.

• Formed via dehydration reactions.

Ruminant Digestive System Complexity

• Ruminants possess a sophisticated esophagus and stomach with multiple compartments:

  1. Rumen and Reticulum:

  • Houses microbes essential for cellulose digestion.

  1. Omasum:

  • Absorbs some of the water and ions from the chewed cud.

    • Cud occasionally regurgitated, rechewed and swallowed

  1. Abomasum:

  • Functions as the true stomach containing acid and proteolytic enzymes typical of vertebrate digestive systems.

The Small Intestine

• Primary Function:

  • Almost all digestion and nutrient absorption occurs here.

• Hydrolytic enzymes disassemble organic nutrients into fragments or monomers.

• Enzymes are either located on the apical membranes of intestinal epithelial cells or secreted by the pancreas into the lumen of the small intestine.

• Digestion products are absorbed across the epithelial cells into the bloodstream.

• Water, vitamins, and minerals are also absorbed in this region.

Surface Area Adaptations in the Small Intestine

• Mucosal Infoldings:

• Each villus (finger-like projections) extends into the intestine's lumen and is lined with epithelial cells that showcase microvilli (extensions of plasma membranes), enhancing surface area for absorption, collectively forming a brush border.

Villi Structure and Function

• Each villus houses capillaries and a lacteal (a lymphatic vessel).

• Due to size, most fat particles remain too large for capillaries and are absorbed through the lacteals into the circulatory system.

Accessory Organs in Digestion

• Pancreas:

  • Secretes digestive enzymes and a bicarbonate-rich fluid (HCO₃⁻).

  • Bicarbonate neutralizes the acidity of chyme entering the small intestine.

• Liver:

  • Produces bile, essential for fat digestion, containing HCO₃⁻, cholesterol, phospholipids, organic wastes, and bile salts.

• Gallbladder:

  • Stores bile and releases it during meals.

The Large Intestine

• Shows substantial variability in size among vertebrates.

• Consists of the cecum, colon, rectum, and anus.

• Appendix:

  • Projects from the cecum.

• Function:

  • Primarily for fecal matter storage and concentration, while absorbing remaining ions and water.

  • Hosts significant bacterial populations which contribute vitamins and gases.

Digestion of Carbohydrates

• In typical omnivores, primary carbohydrate consumption involves polysaccharides like starch (from plants) and glycogen (from animals).

• Some monosaccharides and disaccharides are also ingested directly.

• Polysaccharide digestion commences in the mouth but mainly occurs in the small intestine via pancreatic amylase, producing disaccharide maltose that is further broken down to monosaccharides by enzymes on the brush border of the small intestine.

Digestion Steps

1. Polysaccharides are broken down into disaccharides by pancreatic amylase.

2. Disaccharides are further digested into monosaccharides by enzymes on the epithelial cell surfaces in the small intestine.

3. Monosaccharides enter the epithelial cells by facilitated diffusion (fructose) or secondary active transport coupled with sodium (glucose and galactose).

4. Monosaccharides exit epithelial cells through facilitated diffusion to enter the bloodstream and distribute throughout the body.

Digestion of Lactose

• Lactose:

  • A disaccharide found in milk, digested by the enzyme lactase.

• In mammals, lactase expression is prominent during infancy but diminishes post-weaning.

• Approximately 90% of the global human population loses the ability to digest lactose after early childhood.

• In certain human populations domesticated cattle, a mutation for lactase persistence provided selective advantages.

Digestion of Proteins

• Proteins are initially broken down into polypeptide fragments by pepsin in the stomach and by proteases like trypsin in the small intestine.

• Proteins are secreted from the pancreas in inactive forms and activated in the small intestine (e.g., trypsinogen is activated by enteropeptidase to form trypsin).

• Brush border enzymes on the small intestine complete the breakdown to amino acids, which are then transported into the bloodstream.

Digestion Steps

1. Trypsinogen from the pancreas is activated into trypsin by enteropeptidase.

2. Proteins are cleaved into smaller polypeptides by trypsin and other enzymes.

3. Brush border proteases further cleave polypeptides into amino acids.

4. Amino acids are absorbed via secondary active transport and facilitated diffusion.

Digestion of Lipids

• The primary form of lipids in the diet is as triglycerides (fats).

• Lipid digestion predominantly occurs in the small intestine.

• Lipase:

  • The major enzyme secreted by the pancreas that hydrolyzes triglycerides into two free fatty acids and one monoglyceride.

• Fats are poorly soluble, aggregating into large droplets requiring emulsification to increase surface area for lipase action.

Formation of Micelles & Chylomicrons

• Bile salts and phospholipids, which are amphipathic, cluster with fatty acids and monoglycerides to form micelles.

• With enhanced surface area, micelles facilitate lipid digestion in soluble, small aggregates that can gradually release lipids for intestinal epithelium absorption.

• Once inside the intestinal cell, fatty acids and monoglycerides reform into triglycerides within the smooth endoplasmic reticulum.

• These aggregate into lipid droplets surrounded by phospholipids, cholesterol, and proteins, forming chylomicrons.

• Chylomicrons are released via exocytosis into lacteals, eventually draining into systemic circulation.

Vitamins, Minerals, and Water

• Vitamins typically do not undergo digestion and can be absorbed intact.

• Water-soluble Vitamins:

  • Absorbed by diffusion or active transport within the small intestine.

• Fat-soluble Vitamins (A, D, E, K):

  • Follow the absorption route of lipids.

• While small quantities of water may be absorbed in the stomach, the majority is absorbed in the small intestine.