Animal Nutrition and Digestive System: Chapter 41 Study Notes

Animal Nutrition

Introduction to Nutrition

Nutrition is the process by which organisms acquire and assimilate nutrients necessary for growth, maintenance, and energy. Digestion breaks down food into absorbable units, allowing cells to utilize nutrients.

• Definition: Nutrition involves acquiring food and breaking it down into usable molecules.

• Digestion: Food must be digested because cells cannot absorb nutrients in their original polymeric form.

• Hydrolysis: The process that breaks polymers into monomers using water and enzymes.

Macromolecules and Their Monomers

• Carbohydrates: Monosaccharides (e.g., glucose)

• Proteins: Amino acids

• Lipids: Fatty acids and glycerol

• Nucleic acids: Nucleotides

Essential Nutrients

Types of Essential Nutrients

Essential nutrients are those that cannot be synthesized by the body and must be obtained from the diet.

• Proteins/Amino Acids: Nine amino acids are essential for humans (e.g., lysine, methionine, phenylalanine).

• Lipids: Some fatty acids (e.g., omega-3 and omega-6) are essential.

• Vitamins: Organic molecules required in small amounts (e.g., vitamins A, B, C, D, E, K).

• Minerals: Inorganic elements (e.g., iron, calcium, sodium, potassium).

• Carbohydrates: Major source of energy, especially for the brain.

Variety of Digestive Systems

Digestive System Diversity

Animals have evolved a variety of digestive systems to suit their diets and environments.

• Simple systems: Hydra uses gastrodermal cells to digest food externally and internally.

• Complex systems: Earthworms, grasshoppers, and birds have specialized organs (crop, gizzard, intestine) for digestion.

Chemical Digestion

Types of Digestion

Digestion is required to break down food into absorbable molecules. There are two main types:

• Mechanical digestion: Physical breakdown (chewing, churning).

• Chemical digestion: Enzymatic breakdown of macromolecules.

Digestive Enzymes Table

Macromolecule	Main Enzyme(s)
Carbohydrates	Amylase
Proteins	Protease
Lipids	Lipase (after emulsification by bile)
Nucleic Acids	Nuclease (DNase, RNase)

Organs of the Alimentary Canal

Overview of Digestive Tract

The alimentary canal is a continuous tube from mouth to anus, about 30 feet (9 meters) long.

• Oral Cavity

• Pharynx

• Esophagus

• Stomach

• Small Intestine

• Large Intestine

• Anus

Oral Cavity

Teeth

The mouth is lined with stratified epithelium and contains teeth for mechanical digestion.

• Children: 20 deciduous teeth

• Adults: 32 teeth

• Types: Incisors, canines, premolars, molars

Salivary Glands

Saliva contains water, mucin, enzymes (amylase), and ions. It moistens food and begins carbohydrate digestion.

• Parotid: In front of ears, secretes serous fluid

• Submandibular: Under jaw, secretes mucin and amylase

• Sub-lingual: Under tongue, secretes mucin

Tongue

The tongue is a skeletal muscle with papillae containing taste buds.

• Manipulates food for chewing

• Helps in swallowing

• Contains taste buds for sensory function

Pharynx and Swallowing Mechanism

Pharynx

Lined with stratified epithelium, the pharynx divides into respiratory (trachea) and digestive (esophagus) systems.

Mechanism of Swallowing

• Food is pushed to the back of the mouth

• Epiglottis closes off the trachea

• Bolus enters the esophagus

Anatomical Planes

• Ventral: Trachea is more ventral than the esophagus

Esophagus

Structure and Function

The esophagus is a muscular tube lined with stratified epithelium. Peristalsis begins here, moving food toward the stomach.

• About 10 inches long

• Leads to the stomach

Stomach

Structure and Function

The stomach is lined with simple columnar epithelium and acts as a muscular sac for food storage and digestion.

• Cardiac sphincter: Prevents acid reflux

• Mechanical digestion: Rugae (folds)

• Chemical digestion: Gastric glands secrete acid and enzymes

• Pyloric sphincter: Controls food passage to small intestine

Small Intestine

Structure and Function

The small intestine is the site of final enzymatic digestion and nutrient absorption. It is lined with simple columnar epithelium and contains folds (plicae, villi, microvilli) to increase surface area.

• Plica: Large folds

• Villi: Finger-like projections

• Microvilli: Microscopic extensions

• Lacteal: Central lymph vessel for fat absorption

Three Regions of the Small Intestine

• Duodenum: First part, involved in enzymatic digestion

• Jejunum: Middle part, major site of nutrient absorption

• Ileum: Last part, final absorption

Digestive Feeders

Types of Digestive Feeders

• Carnivore: Eats mainly protein, simple digestive tract (e.g., shark)

• Herbivore: Eats mainly plants, complex tract with cellulose-digesting microbes (e.g., sheep, rabbit)

• Omnivore: Eats both, moderately complex tract

Large Intestine (Colon)

Structure and Function

The large intestine stores and eliminates waste, absorbs water, vitamins, and minerals, and houses beneficial microbes.

• Microbiome: Bacteria aid in digestion and immune function

• Six parts: Cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum

• Anal sphincters: Internal (involuntary), external (voluntary)

Gut Histology

Tissue Layers of the Digestive Tract

• Mucosa: Lining with digestive glands

• Submucosa: Connective tissue, blood vessels

• Muscularis: Muscle layers for peristalsis (longitudinal, circular, oblique)

• Serosa: Outer layer, secretes fluid

Auxiliary Organs in Digestion

Liver

• Storage of glycogen

• Nutrient processing

• Detoxification

• Steroid synthesis (bile)

• Bile: Emulsifies fats, derived from cholesterol

• Cirrhosis: Chronic inflammation, scar tissue formation

Gallbladder

• Stores and concentrates bile

• CCK (cholecystokinin): Hormone that stimulates bile release

• Gallstones: Crystallized cholesterol, can block ducts

Pancreas

• Endocrine: Regulates blood sugar (insulin, glucagon)

• Exocrine: Secretes digestive enzymes (amylase, lipase, proteases, nuclease)

• CCK: Stimulates pancreatic juice secretion

Pancreatic and Small Intestine Proteases

Protein Digestion

• Enteropeptidase: Activates trypsinogen to trypsin

• Trypsin and chymotrypsin: Break peptides into smaller fragments

• Carboxypeptidase: Cleaves amino acids from C-terminus

• Aminopeptidase: Cleaves amino acids from N-terminus

Hormones Involved in Digestion

Hormonal Regulation

• Gastrin: Stimulates gastric juice secretion

• Secretin: Stimulates bicarbonate secretion from pancreas

• CCK: Stimulates bile and pancreatic enzyme release

• GIP: Gastric inhibitory peptide, slows gastric activity

Small Intestine in Detail

Enzymatic Digestion and Absorption

• Dimerases: Break down dimers (e.g., dipeptidase, pancreatic lipase, nuclease)

• Disaccharidases: Break down disaccharides (e.g., maltase, sucrase, lactase)

• Monomers are absorbed into capillaries or lacteals

Lipid Digestion

Process and Absorption

• Lipases: Break down triglycerides into monoglycerides and fatty acids

• Chylomicrons: Transport lipids via lymphatic system to the heart

Summary Table: Digestive Enzymes and Their Substrates

Enzyme	Substrate	Product
Amylase	Starch	Maltose
Protease	Proteins	Peptides/Amino acids
Lipase	Triglycerides	Monoglycerides/Fatty acids
Nuclease	Nucleic acids	Nucleotides

Key Equations

• Hydrolysis of Polymers: $\text{Polymer} + \text{H}_2\text{O} \xrightarrow{\text{Enzyme}} \text{Monomer}$

Additional info:

• Some slides mention clinical conditions (ulcers, gallstones, cirrhosis) for context but are not required for exams.

• Microbiome in the colon is increasingly recognized for its role in health and disease.