Animal Nutrition and Digestive Systems

Nutrition Definition
- Process of acquiring, breaking down, and absorbing food.
- Animals are classified as heterotrophs.
Types of Heterotrophs:
- Herbivores: Consume plants and algae.
- Carnivores: Eat other animals.
- Omnivores: Eat both plants and animals.
Importance of Food:
- Source of energy, building materials, and essential nutrients.
- Energy is primarily derived from ATP through cellular respiration to:
- Fuel muscle contractions.
- Active transport across cell membranes.
- Drive endergonic reactions for molecule production.

Essential Amino Acids:
- Total of 20 amino acids; about half cannot be synthesized by animals and must be obtained from diet.
- Complete Proteins: Found in animals, contain all essential amino acids.
- Incomplete Proteins: Found in plants, lack one or more essential amino acids.
Essential Fatty Acids:
- Must be obtained from diet; e.g., linoleic acid, which serves as a precursor for various lipids.
Vitamins:
- Organic molecules needed in small amounts.
- Function as coenzymes and support mineral absorption; they also act as antioxidants.
- Example: Folic acid (Vitamin B9) is crucial for neural tube development; deficiency can lead to spina bifida or anencephaly.
Minerals:
- Inorganic molecules required in trace amounts.
- Help assemble molecules and structures, serve as ion sources.
- Example: Vitamin D and Calcium deficiency can lead to rickets; Iodine deficiency can cause goiter.

Stages of Digestion:
1. Ingestion: Eating, involving various specialized structures.
2. Digestion: Breaking down materials, can be intracellular or extracellular.
- Mechanical digestion: Involves physical breakdown of food.
- Chemical digestion: Involves hydrolysis reactions.
- Absorption of nutrients typically requires a circulatory system.
1. Elimination: Removing undigested waste.
Digestive Strategies:
- Filter feeding: Sieve food from water.
- Substrate feeding: Live in or on food source.
- Fluid feeding: Suck liquids from hosts.
- Bulk feeding: Consume large solid food items.

Components:
- Oral Cavity: Initial mechanical and chemical digestion.
  - Teeth increase surface area; salivary amylase begins starch digestion.
  - Saliva also contains water, mucus, buffers, and enzymes.
- Stomach:
  - Mechanical mixing and chemical digestion using gastric juices that contain pepsin (activated by H+ from parietal cells) to break peptide bonds.
  - Mucus protects the stomach lining; chyme is released into the small intestine.
- Small Intestine:
  - Composed of the duodenum, jejunum, and ileum.
  - Duodenum: Chyme mixes with bile (from liver) and pancreatic enzymes for further digestion.
  - Villi and microvilli in jejunum and ileum increase absorption area, allowing for both passive and active transport of nutrients.
- Large Intestine:
  - Larger diameter; primary function to absorb water and salts.
  - Ends with rectum and anus; the cecum serves as a pouch for microbial action and leads to the appendix.

Comparative Anatomy focuses on different organismal adaptations:
- Physical Adaptations: Variations in mouthparts, dentition (e.g., gizzards in herbivores).
- Functional Adaptations: Differences in microbiome, foregut vs. hindgut fermenters, and behaviors like coprophagy.

Hormonal Regulation:
- Controlled through hormones during the cephalic, gastric, and intestinal phases, which regulate secretions.
Energy Storage:
- Blood sugar levels maintained stable; excess glucose is stored as glycogen.
- Insulin: Reduces blood glucose by promoting uptake into cells.
- Glucagon: Increases blood glucose by releasing stored glucose from cells.
Hunger Regulation:
- Influenced by hormones and environmental factors.
- Appetite Stimulants: Ghrelin.
- Appetite Suppressants: Leptin, insulin, PYY.
- Medications: Ozempic/semaglutide mimic hormones that induce satiety, assisting weight loss.