Animal Nutrition and Digestion

• All animals are heterotrophs, meaning they must consume food for energy.

4 Stages of Food Processing

• Ingestion: The act of eating food.
• Digestion: The breakdown of food into smaller, absorbable pieces.
• Mechanical digestion: Physical breakdown, like chewing, to increase surface area.
• Chemical digestion: Using enzymes to break down food molecules through hydrolysis.
• Absorption: Nutrients entering the bloodstream and cells.
• Elimination: Removal of undigested waste (feces).

How the Body Avoids Self-Digestion

• Digestion occurs within specialized compartments like the stomach and intestines.

Types of Digestion

• Single-celled organisms: Utilize food vacuoles and lysosomes for digestion.
• Multicellular organisms: Employ extracellular digestion (outside cells).
• Small animals: Use a gastrovascular cavity (single opening for entry/exit).
• Large animals: Use an alimentary canal (two openings – mouth & anus).

Alimentary Canal (Human Digestive Tract)

• Accessory organs:
• Three pairs of salivary glands.
• Pancreas.
• Liver and gallbladder.
• Key movements:
• Peristalsis: Wave-like muscle contractions that propel food along the tract.
• Sphincters: Muscular rings that control the passage of food between compartments.

Steps in Digestion

• Oral cavity (mouth):
• Mechanical digestion: Teeth chew food, increasing surface area.
• Tongue: Forms a bolus (ball of food) for swallowing.
• Saliva:
• Amylase: Initiates starch breakdown.
• Mucus: Lubricates food for easier swallowing.
• The bolus goes to pharynx (throat) and then to the esophagus during swallowing.
• The epiglottis prevents food from entering the trachea (windpipe).
• Esophagus:
• Transports food to the stomach via peristalsis.
• Stomach:
• Highly expandable (can hold up to 2 liters).
• Primarily digests proteins.
• Gastric glands in the stomach lining contain:
• Mucus cells: Protect the stomach lining from acid.
• Parietal cells: Produce hydrochloric acid (HCl).
• Chief cells: Secrete pepsinogen (inactive form of pepsin).
• HCl activates pepsinogen into pepsin, which breaks down proteins into smaller polypeptides.
• Food mixes with HCl and pepsin, creating chyme.
• Chyme exits the stomach through the pyloric sphincter and enters the small intestine.
• Small intestine (SI):
• Approximately 6 meters long.
• Main site for nutrient absorption.
• Villi and microvilli line the small intestine to increase surface area for absorption.
• Receives secretions from:
• Pancreas: Delivers pancreatic juice (enzymes and bicarbonate).
• Liver: Provides bile (stored in the gallbladder) via the bile duct.
• Sections of the small intestine include:
• Duodenum: Primarily responsible for digestion.
• Jejunum and Ileum: Primarily responsible for absorption.

Nutrient Digestion Breakdown

• Carbohydrates:
• Digestion begins in the mouth with amylase and finishes in the small intestine.
• Proteins:
• Digestion starts in the stomach with pepsin and continues in the small intestine with trypsin and chymotrypsin.
• Nucleic acids (DNA/RNA):
• Digestion occurs only in the small intestine, using enzymes from the pancreas and intestine.
• Fats (lipids):
• Bile salts emulsify fats into smaller droplets.
• Pancreatic lipase digests fats into glycerol and fatty acids.

Absorption

• Takes place in the small intestine.
• Nutrients move into the blood (capillaries) or lymph (lacteal).
• Transport mechanisms include:
• Simple diffusion for water.
• Facilitated diffusion for fructose.
• Active transport for ions.
• Fats:
• After absorption, fats are reassembled, packaged into chylomicrons, and transported through the lymph system before entering the bloodstream.

Large Intestine (Colon)

• Chyme enters from the small intestine via the ileocecal valve.
• Primary function: Water absorption.
• Four sections: Ascending, transverse, descending, and sigmoid.
• Contains beneficial bacteria.
• Produces vitamins: K, B12, thiamine, and riboflavin.
• Waste becomes feces (about 75% water) and is eliminated through the anus.

Liver Functions

• Processes nutrients and synthesizes new compounds.
• Detoxifies drugs and alcohol.
• Stores excess glucose as glycogen.
• The hepatic portal vein transports absorbed nutrients directly to the liver.

What is Nutrition?

• Food provides:
• Energy (ATP).
• Building blocks for the body.
• Essential nutrients (those the body cannot synthesize).

Feeding Types

• Herbivores: Consume plants.
• Carnivores: Consume animals.
• Omnivores: Consume both plants and animals.
• Opportunistic feeders: Consume whatever is available.

Energy Needs

• Energy is measured in Calories (kcal), where 1 kcal = 1000 calories.
• Energy content per gram:
• Carbohydrates: 4 Calories.
• Protein: 4 Calories.
• Fat: 9 Calories.

Essential Nutrients

• Essential amino acids:
• 20 total, humans must obtain 8 from their diet (infants require 9).
• Essential fatty acids:
• Examples include linoleic and linolenic acid (found in seeds and vegetables).
• Essential vitamins:
• Act as coenzymes to aid enzyme function.

Vitamins Overview

• Water-soluble (not stored):
• B Vitamins (energy metabolism):
• B3 (niacin) → NAD/NADP.
• B9 (folic acid) → essential during pregnancy for neural tube development.
• B12 → needed for red blood cell production (found only in animal products).
• Vitamin C:
• Involved in collagen synthesis.
• Acts as an antioxidant.
• Enhances iron absorption.
• Deficiency leads to scurvy.
• Fat-soluble (stored in fat):
• Vitamin A: Critical for vision (retinal synthesis).
• Vitamin D: Facilitates calcium absorption.
• Deficiency can cause rickets in children and osteomalacia in adults.