Digestion in Animals

Digestion in Animals

• Animal food sources include proteins, fats, and carbohydrates.
• Plants and animals are primarily made up of organic compounds.
• Digestion is a process that breaks down large, complex food molecules into absorbable smaller units using enzymes.
• Digestion can occur inside cells (intracellular), outside cells within the body (extracellular), or outside the body (external).

Types of Digestion

• External digestion: This process involves enzymes being secreted outside the body to break down food (e.g., spiders).
• Intracellular digestion: This occurs within the cell (e.g., protozoans, sponges, and some flatworms).
• Extracellular digestion: This takes place outside cells but within the body, often with a digestive tract (e.g., most animals).

Extracellular Digestion - Digestive Tract Modifications

• Flatworms: Have a gastrovascular cavity with extensive branching that increases surface area for food transport and absorption.
• Amphibians: Typically have long stomachs, a short oesophagus with cilia for food movement, and mucus to aid digestion. They might also have chitinase to break down chitin, which is a protective substance in arthropods. Their digestive tracts might also have structural modifications specific to the type of food they eat.
• Birds: These have a crop (a storage organ) at the base of the oesophagus. The proventriculus secretes acid (HCl) and enzymes (e.g., pepsin) to begin digestion. The gizzard has strong muscular walls to grind hard foods using ingested grit.
• Monogastric organisms: These have a simple, single-chambered stomach.
  • Omnivores (e.g., humans, rats, pigs): Eat both animal and plant materials.
  • Carnivores (e.g., dogs, cats): Eat meat.
  • Herbivores (e.g., horses, rabbits): Eat plants.
  • Hindgut fermenters (e.g., some herbivores): can digest cellulose like ruminants but have a different digestive structure and process.
  • Colonic fermenters and Caecal fermenters are sub-types of hindgut fermenters.
• Herbivores: Their digestive systems are often longer to accommodate the longer digestion time required for plant matter. They may use microbial fermentation for cellulose digestion.
• Ruminants: Have a four-chambered stomach (e.g., cows, sheep, goats). Their digestive system includes specific chambers for microbial fermentation and cud-chewing.
  • Rumen: A large storage space for quickly consumed food. It is also a fermentation vat containing billions of bacteria and protozoa that digest cellulose through anaerobic fermentation.
  • Reticulum: Close to the rumen and mixes contents. Fluid helps separate particles; contractions send larger to the rumen and smaller to the omasum.
  • Omasum: A structure with minimal digestive activity. Water is absorbed here.
  • Abomasum: The "true stomach", similar in function to the stomach of non-ruminants, including secretion of enzymes and acids for nutrient breakdown.

Enzymatic Digestion

• Protein digestion: Enzymes called exopeptidases hydrolyze terminal peptide bonds, while endopeptidases act on interior peptide bonds. Examples include pepsin and trypsin, both produced by vertebrates. Pepsin, is secreted as the precursor pepsinogen, activated in the acidic stomach environment (HCl). Trypsin, initially produced as trypsinogen is activated by enterokinase in the small intestine.

• Fat digestion: Pancreatic lipase in vertebrates and invertebrates, works with bile acids (produced by the liver) and intestinal movements to emulsify fats, making them absorbable by the body.

• Carbohydrate digestion: Amylase, secreted in saliva and the pancreas attacks cooked starches. Cellulose, a complex carbohydrate essential to plants, typically requires symbiotic microorganisms in the gut to be broken down (e.g., in ruminants and some herbivores). In other animals, the products of fermentation are absorbed, and wastes like CO2 and methane exit via belching. Some invertebrates rely on symbiotic microorganisms for cellulose digestion.