Animal Nutrition and Digestion: Study Guide

Animal Nutrition and Digestion

Basic Principles of Animal Nutrition

Animals are heterotrophs, meaning they must obtain organic molecules and energy by consuming other organisms. Their diets must provide all necessary components for survival, growth, and reproduction.

• Chemical energy: Required for cellular processes and activities.

• Organic molecules: Serve as building blocks for macromolecules (e.g., amino acids, fatty acids).

• Essential nutrients: Substances that cannot be synthesized by the animal and must be obtained from the diet (e.g., vitamins, minerals, some amino acids, and fatty acids).

Three Nutritional Needs

• Fuel for cellular work: Energy for ATP production.

• Organic raw materials: For biosynthesis of macromolecules.

• Essential nutrients: Required for normal physiological function but cannot be synthesized by the organism.

Classes of Essential Nutrients

• Amino acids: Some are essential and must be obtained from food.

• Fatty acids: Certain unsaturated fatty acids are essential.

• Vitamins: Organic molecules required in small amounts.

• Minerals: Inorganic nutrients (e.g., iron, calcium).

Feeding Mechanisms in Animals

Main Feeding Mechanisms

• Suspension feeders: Extract food particles suspended in water (e.g., baleen whales).

• Filter feeders: Strain small organisms or food particles from the surrounding medium.

• Substrate feeders: Live in or on their food source (e.g., caterpillars).

• Fluid feeders: Suck nutrient-rich fluid from a living host (e.g., mosquitoes).

• Bulk feeders: Eat relatively large pieces of food (e.g., humans, most mammals).

Stages of Food Processing

Major Stages

• Ingestion: The act of eating or feeding.

• Digestion: Breaking down food into molecules small enough to absorb. Involves mechanical and chemical processes.

• Absorption: Uptake of small nutrient molecules by body cells.

• Elimination: Removal of undigested material from the digestive tract.

Intracellular vs. Extracellular Digestion

• Intracellular digestion: Food particles are engulfed by phagocytosis and digested within food vacuoles (e.g., sponges).

• Extracellular digestion: Breakdown of food in compartments continuous with the outside of the animal's body (e.g., gastrovascular cavity, alimentary canal).

Gastrovascular Cavity vs. Alimentary Canal

• Gastrovascular cavity: Single opening; functions in both digestion and distribution of nutrients (e.g., cnidarians).

• Alimentary canal: Complete digestive tract with two openings (mouth and anus); allows for specialized regions for digestion and absorption.

Mammalian Digestive System

Structure and Function of Digestive Organs

• Oral cavity: Mechanical breakdown of food; saliva (from salivary glands) contains amylase to begin starch digestion.

• Pharynx: Passageway for food from mouth to esophagus.

• Esophagus: Conducts food to stomach via peristalsis (waves of muscle contractions).

• Stomach: Stores food and secretes gastric juice (contains hydrochloric acid (HCl) and pepsin for protein digestion); churns food to produce chyme.

• Small intestine: Major site of digestion and absorption; divided into duodenum (where digestive juices are mixed), jejunum, and ileum.

• Large intestine: Includes cecum, colon, and rectum; absorbs water and forms feces.

Accessory (Exocrine) Glands

• Salivary glands: Secrete saliva (contains amylase) into the oral cavity.

• Pancreas: Secretes bicarbonate (HCO3-) to neutralize stomach acid and digestive enzymes (e.g., pancreatic lipase, proteases) into the duodenum.

• Liver: Produces bile (emulsifies fats); stores glycogen.

• Gall bladder: Stores and releases bile into the small intestine.

Structure of the Small Intestine and Nutrient Absorption

• Villi: Finger-like projections that increase surface area for absorption.

• Microvilli: Microscopic extensions on villi cells, further increasing surface area.

• Large surface area enhances nutrient absorption efficiency.

Regulation of Digestion

Neural and Hormonal Regulation

• Neural responses: The enteric nervous system coordinates peristalsis and secretion.

• Hormonal responses: Hormones such as gastrin (stimulates gastric juice secretion), secretin, and cholecystokinin (CCK) regulate digestive processes as food moves through the alimentary canal.

Feedback Circuits in Digestion and Energy Storage

• Insulin: Secreted by the pancreas in response to high blood glucose; promotes uptake of glucose by cells and storage as glycogen in the liver.

• Glucagon: Secreted by the pancreas when blood glucose is low; stimulates breakdown of glycogen to glucose in the liver.

Adaptations of Vertebrate Digestive Systems

Correlation with Diet

• Length of alimentary canal: Herbivores generally have longer alimentary canals than carnivores, allowing more time for digestion of plant material.

• Mutualistic microorganisms: Many herbivores house bacteria or protists in their digestive tracts to help break down cellulose (e.g., ruminants).

• Dentition: Teeth structure varies with diet—herbivores have flat molars for grinding, carnivores have sharp canines for tearing.

Key Vocabulary

Term	Definition
Herbivore	Animal that eats mainly plants or algae
Carnivore	Animal that eats other animals
Omnivore	Animal that eats both plants and animals
Essential nutrients	Nutrients required for normal body functioning that cannot be synthesized by the body
Ingestion	Act of eating or taking in food
Digestion	Process of breaking down food into absorbable units
Absorption	Uptake of nutrients by body cells
Elimination	Removal of undigested material from the body
Peristalsis	Waves of muscle contractions that move food along the digestive tract
Amylase	Enzyme that breaks down starch
Pepsin	Enzyme in the stomach that digests proteins
Bile	Substance produced by the liver that emulsifies fats
Proteases	Enzymes that digest proteins
Gastrin	Hormone that stimulates secretion of gastric juice
Insulin	Hormone that lowers blood glucose levels
Glucagon	Hormone that raises blood glucose levels
Glycogen	Storage form of glucose in animals
Mutualistic microorganism	Microbes that live in the digestive tract and aid in digestion

Summary Table: Comparison of Gastrovascular Cavity and Alimentary Canal

Feature	Gastrovascular Cavity	Alimentary Canal
Number of Openings	One	Two (mouth and anus)
Function	Digestion and distribution of nutrients	Specialized regions for digestion and absorption
Examples	Cnidarians, flatworms	Most animals (e.g., mammals, birds, insects)

Example: Ruminant Digestion

• Ruminants (e.g., cows) have a complex, multi-chambered stomach that houses mutualistic microorganisms to digest cellulose.

• This adaptation allows efficient extraction of nutrients from plant material.

Additional info: The regulation of appetite is a complex process involving hormones such as leptin and ghrelin, but this topic is not required for this section.