🔬General Biology I Unit 34 – Animal Nutrition and the Digestive System

Key Concepts and Terminology

• Nutrition involves the intake, digestion, absorption, and utilization of nutrients from food
• Nutrients include carbohydrates, proteins, lipids, vitamins, minerals, and water
• Digestion breaks down complex food molecules into simpler forms that can be absorbed by the body
  • Mechanical digestion physically breaks down food through chewing and churning
  • Chemical digestion uses enzymes to break chemical bonds in food molecules
• Absorption moves nutrients from the digestive tract into the bloodstream for distribution throughout the body
• Metabolism refers to all chemical reactions involved in maintaining the living state of cells and organisms
  • Catabolism breaks down complex molecules to release energy
  • Anabolism uses energy to construct components of cells (proteins, nucleic acids, carbohydrates, and lipids)
• Homeostasis maintains a stable internal environment despite changes in the external environment

Digestive System Anatomy

• The digestive system consists of the gastrointestinal (GI) tract and accessory organs
• The GI tract is a continuous tube extending from the mouth to the anus
  • Organs of the GI tract include the mouth, esophagus, stomach, small intestine, large intestine, and anus
• Accessory organs aid in digestion but are not part of the GI tract
  • Accessory organs include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas
• The mouth is responsible for mechanical digestion through chewing and chemical digestion with salivary enzymes
• The esophagus transports food from the mouth to the stomach via peristalsis
• The stomach stores food, mixes it with gastric juices, and begins protein digestion
• The small intestine is the primary site of digestion and absorption
  • The small intestine has three regions: duodenum, jejunum, and ileum
• The large intestine absorbs water and electrolytes, and eliminates undigested material as feces

Types of Digestive Systems

• Digestive systems can be classified as incomplete or complete
  • Incomplete digestive systems have a single opening for food intake and waste elimination (cnidarians and flatworms)
  • Complete digestive systems have separate openings for food intake (mouth) and waste elimination (anus)
• Complete digestive systems can be further classified as tubular or compartmentalized
  • Tubular digestive systems have a simple, linear tube from mouth to anus (annelids and nematodes)
  • Compartmentalized digestive systems have specialized regions for different digestive processes (mammals and birds)
• Ruminant digestive systems are a type of compartmentalized system found in animals that consume plant material
  • Ruminants (cattle, sheep, goats) have a four-chambered stomach for microbial fermentation of plant material
• Avian digestive systems are adapted for flight, with a crop for food storage and a gizzard for mechanical digestion

Digestive Processes and Enzymes

• Digestion involves the breakdown of complex food molecules into simpler forms that can be absorbed
• Carbohydrate digestion begins in the mouth with salivary amylase and continues in the small intestine
  • Enzymes involved in carbohydrate digestion include amylase, maltase, sucrase, and lactase
• Protein digestion begins in the stomach with pepsin and continues in the small intestine
  • Enzymes involved in protein digestion include pepsin, trypsin, chymotrypsin, and peptidases
• Lipid digestion occurs primarily in the small intestine with bile salts and pancreatic lipase
  • Bile salts emulsify fats, increasing surface area for enzymatic action
• Nucleic acid digestion occurs in the small intestine, breaking down DNA and RNA into nucleotides
• Absorption of nutrients occurs primarily in the small intestine
  • Carbohydrates are absorbed as monosaccharides (glucose, fructose, galactose)
  • Proteins are absorbed as amino acids and short peptides
  • Lipids are absorbed as fatty acids and monoglycerides
  • Vitamins, minerals, and water are also absorbed in the small intestine

Nutrient Requirements and Functions

• Carbohydrates are the primary energy source for most animals
  • Complex carbohydrates (starches, fiber) are broken down into simple sugars (glucose) for energy
• Proteins are essential for growth, repair, and maintenance of tissues
  • Amino acids are the building blocks of proteins and are required for synthesis of enzymes, hormones, and other proteins
• Lipids provide energy, insulation, and protection for organs
  • Essential fatty acids (omega-3 and omega-6) are required for proper growth and development
• Vitamins are organic compounds required in small amounts for normal metabolism
  • Fat-soluble vitamins (A, D, E, K) are stored in the body and can be toxic in excess
  • Water-soluble vitamins (B-complex, C) are not stored and must be consumed regularly
• Minerals are inorganic elements required for various physiological processes
  • Macrominerals (calcium, phosphorus, magnesium, sodium, potassium, chloride, sulfur) are required in larger amounts
  • Microminerals or trace elements (iron, iodine, zinc, copper, fluoride) are required in smaller amounts
• Water is essential for life and is involved in many physiological processes
  • Water is required for transportation of nutrients, removal of waste, and regulation of body temperature

Energy Metabolism in Animals

• Energy metabolism involves the breakdown of nutrients to release energy (catabolism) and the use of energy to synthesize compounds (anabolism)
• Cellular respiration is the process of breaking down glucose in the presence of oxygen to release energy
  • Glycolysis breaks down glucose into pyruvate in the cytoplasm
  • Citric acid cycle (Krebs cycle) oxidizes acetyl-CoA to release carbon dioxide in the mitochondrial matrix
  • Electron transport chain and oxidative phosphorylation generate ATP through a proton gradient in the mitochondrial inner membrane
• ATP (adenosine triphosphate) is the primary energy currency of the cell
  • ATP is generated through substrate-level phosphorylation and oxidative phosphorylation
• Fermentation is the breakdown of glucose in the absence of oxygen
  • Lactic acid fermentation occurs in muscle cells during intense exercise
  • Alcoholic fermentation occurs in yeast and some bacteria
• Thermoregulation is the maintenance of a stable internal body temperature
  • Endotherms (mammals and birds) maintain a constant body temperature through metabolic heat production
  • Ectotherms (reptiles, amphibians, fish) rely on external sources of heat to regulate body temperature

Digestive Adaptations Across Species

• Herbivores have adaptations for digesting plant material, which is high in cellulose
  • Longer digestive tracts and enlarged compartments (rumen, cecum) for microbial fermentation of cellulose
  • Specialized teeth for grinding plant material (molars)
• Carnivores have adaptations for digesting animal tissue, which is high in protein and fat
  • Shorter digestive tracts to quickly pass animal tissue
  • Specialized teeth for tearing and slicing meat (canines)
• Omnivores have adaptations for digesting both plant and animal material
  • Intermediate length digestive tracts and tooth structure
• Insectivores have adaptations for digesting insects, which have a tough exoskeleton
  • Specialized teeth and digestive enzymes for breaking down chitin
• Filter feeders have adaptations for straining small food particles from water
  • Baleen plates in some whales, gill rakers in fish
• Detritivores have adaptations for digesting dead organic matter
  • Specialized digestive enzymes and symbiotic microorganisms for breaking down detritus

Disorders and Diseases of the Digestive System

• Gastroesophageal reflux disease (GERD) occurs when stomach acid flows back into the esophagus, causing heartburn and damage to the esophageal lining
• Peptic ulcers are sores that develop in the lining of the stomach or duodenum, often caused by Helicobacter pylori bacteria or excessive use of NSAIDs
• Inflammatory bowel disease (IBD) includes Crohn's disease and ulcerative colitis, which cause chronic inflammation of the digestive tract
• Celiac disease is an autoimmune disorder triggered by the ingestion of gluten, leading to damage of the small intestine lining
• Gallstones are hardened deposits of bile that can cause pain and blockages in the gallbladder or bile ducts
• Pancreatitis is inflammation of the pancreas, often caused by gallstones or excessive alcohol consumption
• Colorectal cancer is cancer that develops in the colon or rectum, often associated with a diet low in fiber and high in processed meats
• Hemorrhoids are swollen veins in the anus and lower rectum, often caused by straining during bowel movements or pregnancy
• Malnutrition occurs when there is an imbalance between nutrient intake and requirements, leading to deficiencies or excesses of specific nutrients