Animal Nutrition and Digestive Systems

Animal Nutrition

• Plants are autotrophs, producing their own food via photosynthesis. Animals are heterotrophs, needing to consume other organisms for nutrition.
• Animals have dietary categories: herbivores (plant-eaters), carnivores (meat-eaters), and omnivores (both plants and animals).
• Detritivores (detritus feeders) consume dead organic matter (detritus), like earthworms and certain insects. Scavengers also eat dead organisms, but typically larger chunks.
• Single-celled organisms digest food intracellularly within food vacuoles. Multicellular organisms typically digest food extracellularly within a digestive cavity.

Types of Digestive Systems

• Single-celled organisms use intracellular digestion within food vacuoles.
• Multicellular organisms employ extracellular digestion, processing food outside cells within a digestive cavity (incomplete or complete).
• Incomplete digestive systems (e.g., Hydra) have a single opening for ingestion and egestion, with all cells participating in digestion.
• Complete digestive systems (common in most animals) have a tube running through the body, with specialized areas for different stages of digestion (e.g., mouth, stomach, intestine).

Feeding Mechanisms

• Suspension feeding is where animals filter food particles from water (e.g., some whales, clams).
• Filter feeding is a subtype of suspension feeding, sifting small food particles from water.
• Substrate feeding (e.g., earthworms) involves consuming the material within a substrate (like soil).
• Fluid feeders ingest nutrient-rich fluids from a living host (e.g., mosquitoes).
• Bulk feeders ingest large pieces of food (e.g., most mammals).

Nutrient Uptake in Cells

• Cells use bulk transport mechanisms for macromolecules that cannot pass through carrier proteins.
• Phagocytosis ("cell eating") is a process where large solid particles are engulfed by the cell membrane.
• Pinocytosis ("cell drinking") is used to ingest fluids and dissolved substances. Receptor-mediated endocytosis involves specific molecules binding to receptors on the cell surface before being brought inside.

Stages of Food Processing

• Ingestion: Intake of food. Mechanical and chemical digestion break down food.
• Mechanical digestion physically breaks down food (e.g., chewing). Chemical digestion uses enzymes to break down large molecules into smaller ones.
• Absorption: Small molecules are absorbed into the bloodstream.
• Elimination: Undigested matter is removed from the body.

Human Digestive System

• The human digestive system comprises a tubular alimentary canal and accessory glands including the mouth, esophagus, stomach, small intestine, large intestine, liver, pancreas, and others.
• Mouth: Mechanical and chemical digestion begins. Teeth mechanically break down food, saliva contains amylase to start chemical digestion.
• Esophagus: Food moves using peristalsis.
• Stomach: Food is churned and mixed with gastric juices, which contain pepsin (protein digestion) and hydrochloric acid (pH ~1.5-2.5).
• Small intestine: Most chemical digestion and nutrient absorption occurs here, through the use of secretions from the pancreas and liver.
• Large intestine: The large intestine absorbs water and remaining nutrients from waste material. Waste is eliminated via the anus.