Animal Nutrition: Herbivores, Carnivores, and Omnivores

• Animals are categorized into herbivores, carnivores, and omnivores based on their food preference
• Herbivores primarily eat plants
• Carnivores primarily eat other animals
• Omnivores eat both plants and animals

Reasons for Eating

• To obtain energy: Chemical energy from food fuels ATP synthesis and regulates body heat
• To obtain raw materials: Carbon, nitrogen, and other elements from food make up cells, tissues, and organs
• To obtain essential nutrients: Raw materials, substrates, and cofactors from food support biosynthesis

Essential Nutrients

• These cannot be synthesized by the animal
• Amino acids: 8 of 20 are essential for humans
• Fatty acids: Those produced by plants
• Vitamins: Organic compounds that have many uses and functions, required in small amounts
• Minerals: Inorganic compounds that have many uses and functions, required in small amounts

Food Processing

• Ingestion: The act of taking in food
• Suspension feeding: Sponges, bivalves, and blue whales filter food from water
• Substrate feeding: Caterpillars, earthworms, and snails eat their way through a substrate
• Fluid feeding: Mosquitoes, hummingbirds, and aphids consume nutrient-rich fluids
• Bulk feeding: Humans and most animals eat relatively large pieces of food
• Digestion: Food breaks down into small molecules like amino acids, monosaccharides, and fatty acids
• Mechanical digestion: Chewing and grinding that physically breaks down food
• Chemical digestion: Cleaving large molecules into smaller building blocks by breaking chemical bonds
• Absorption: Cells take up the small molecules
• Elimination: Undigested material is removed from the body

Digestion Types

• Intracellular digestion: Occurs inside the cell, limits the size of ingestible food, and is used exclusively by sponges
• Extracellular digestion: Occurs in digestive compartments
• Gastrovascular cavity: An incomplete gut with one opening by cnidarians and flatworms
• Alimentary canal: "Complete" digestive tract with a separate mouth and anus, separate compartments by most bilaterians

Mammalian Digestive System

• Food passes through a series of specialized compartments and organs
• Digestion is facilitated by accessory organs and glands:
• Salivary glands
• Liver
• Gall bladder
• Pancreas
• Sequence of food passage:
• Oral cavity
• Pharynx
• Esophagus
• Stomach
• Small intestine
• Duodenum
• Jejunum
• Ileum
• Large intestine
• Cecum
• Colon
• Rectum

Oral Cavity

• The mechanical breakdown and lubrication of food occurs, forming a bolus for easier swallowing
• Salivary glands: They secrete saliva, which lubricates food and contains digestive enzymes
• Amylase: Begins the chemical breakdown of starch

Pharynx

• Connects the oral cavity to the esophagus and the windpipe (trachea)

Esophagus

• A muscular tube connecting the pharynx to the stomach
• Food moves through the esophagus via peristalsis, rhythmic contractions of muscles

Stomach

• Functions include food storage and digestion, which involves churning food with gastric juice
• Gastric juice contains:
• Hydrochloric acid (HCl): A strong acid that helps denature proteins and kill bacteria
• Protease: An enzyme breaking down proteins
• Chyme is a mixture of partially digested food and gastric juice that passes to the small intestine

Small Intestine Digestion

• Most chemical digestion occurs
• Duodenum: Receives chyme from the stomach, neutralizes its acidity, and mixes it with digestive enzymes from the pancreas
• Jejunum: Digestion continues
• Further breakdown of:
• Proteins using protease
• Carbohydrates using amylase
• Fats using lipase
• Bile: Produced by the liver and stored in the gall bladder, it breaks down fat into smaller pieces

Small Intestine Absorption

• Most nutrient absorption happens here in the jejunum and ileum
• Lining: Highly folded and projects into villi to increase surface area
• Epithelial cells: Also called as villi which have microvilli to further increase the surface area
• Nutrients: Enter epithelial cells actively or passively and then enter the bloodstream

Large Intestine

• Cecum: Involved in the fermentation of plant tissue
• Colon: Reabsorbs water and forms feces
• Rectum: Stores feces until elimination

Evolutionary Adaptations

• Heterodont dentition: Mammals have teeth with different shapes and sizes for various functions
• Carnivore vs. herbivore alimentary canals: Herbivores need help with plant material (cellulose) because it is hard to digest and requires symbiotic bacteria
• Fermentation: Breakdown of plant matter bacteria
• Ruminant digestion: Fermentation occurs before food passes to the stomach by cattle, deer, sheep and giraffes