Chapter 33

Food is processed in animals through three stages:
- Ingestion (taking in food)
- Digestion (breaking down food)
- Absorption (uptake of nutrients)
- Elimination (waste disposal)

Animals can be categorized into three main dietary groups:
- Herbivores: Primarily consume plants and algae.
- Carnivores: Primarily consume other animals.
- Omnivores: Consume both animals and plants.
- Many animals are opportunistic feeders, taking advantage of a variety of food sources.

Essential nutrients must be obtained through diet. Four main classes:
1. Essential Amino Acids: Out of 20 amino acids, 9 are essential and must be obtained from food.
2. Essential Fatty Acids: Cannot be synthesized by animals; typically obtained from dietary sources.
3. Vitamins: Organic molecules required in small amounts; 13 essential vitamins for humans, categorized into fat-soluble and water-soluble.
4. Minerals: Inorganic nutrients, often acting as cofactors in biological reactions, required in small amounts; excessive intake can harm health.

Stages of Food Processing:
1. Ingestion: Intake of food via various feeding strategies.
2. Digestion: Process of breaking down food, which includes:
- Mechanical Digestion: Involves physical breakdown (e.g., chewing).
- Chemical Digestion: Involves enzymatic reactions that break food into small molecules (e.g., hydrolysis).
1. Absorption: Uptake of nutrients into body cells from the intestinal lumen.
2. Elimination: Removal of undigested materials from the body.

Intracellular Digestion: Food particles engulfed by cells and broken down internally (phagocytosis).
Extracellular Digestion: Occurs outside cells in digestive compartments; includes:
- Gastrovascular Cavity: Simple organisms digest and distribute nutrients.
- Alimentary Canal: More complex animals have a complete digestive tract (mouth to anus) with specialized regions.

Consists of an alimentary canal and accessory glands (e.g., salivary glands, pancreas).
Mechanical and Chemical Digestion begin in the oral cavity:
- Salivary amylase initiates carbohydrate breakdown.

The stomach secretes gastric juice containing enzymes and hydrochloric acid (HCl), converting food into chyme.
Pepsin: An active enzyme that breaks down proteins; initially secreted as pepsinogen.
Stomach muscles churn contents and sphincters regulate passage into the small intestine after several hours.

Major organ for digestion and nutrient absorption:
- Duodenum: First segment where chyme mixes with digestive juices.
- Pancreas produces enzymes (proteases) and alkaline solution to neutralize chyme.
- Bile aids in fat digestion, produced by the liver, stored in the gallbladder.

The small intestine’s surface is increased by villi and microvilli, greatly enhancing absorption capacity.
Nutrients absorbed enter the bloodstream via the hepatic portal vein to the liver for processing.

Major functions include:
- Water recovery from undigested food.
- Housing of bacteria that aid in digestion, producing vitamins.
- Feces formation as water is absorbed.

Dental Adaptations: Teeth structure is varied based on dietary needs, reflecting an animal's diet.
Digestive System Length: Carnivores tend to have shorter digestive tracts compared to herbivores, who require longer canals for digesting plant material.
Mutualistic Relationships: Herbivores often have fermentation chambers inhabited by microorganisms that digest cellulose.

Metabolic rate: the energy an animal uses over time; depends on activity levels and physiological states.
Storage of excess energy mainly in the form of glycogen and fat; mobilized when needed.
Insulin and glucagon regulate blood glucose levels, maintaining homeostasis.