Biological Digestion Processes and Animal Classification

Carbohydrate Digestion and Enzyme Action

Digestion begins in the mouth, where salivary amylase converts starch polysaccharides into smaller fragments: dextrin, maltose, and glucose. In the stomach, amylase is denatured, but a slow hydrolysis of polysaccharides continues through the action of hydrochloric acid.

In the small intestine, pancreatic amylase from pancreatic juice supplements the action of salivary amylase. There are also other enzymes in the juice to hydrolyze other polysaccharides. Intestinal juice contains enzymes that convert disaccharides into monosaccharides: maltase, lactase, and sucrase, which respectively hydrolyze maltose, lactose, and sucrose. There are also enzymes that convert various monosaccharides into glucose, the main monosaccharide absorbed by the intestinal villi.

Protein Digestion and Proteolytic Enzymes

Protein digestion begins in the stomach with pepsin action, which is favored by the presence of hydrochloric acid. The proteins are hydrolyzed into peptides of lower molecular mass. In the intestine, pancreatic juice provides trypsin, chymotrypsin, and carboxypeptidases, which degrade the peptides into increasingly smaller fragments.

Intestinal juice contains aminopeptidases, which conclude the degradation of peptides into amino acids to be absorbed by the intestinal villi. The intestinal juice also secretes enterokinase, which converts trypsinogen to trypsin. Trypsinogen is the inactive form of the enzyme trypsin, which is secreted by the pancreas to avoid the risk of autodigestion of the gland.

Fat Digestion and Lipid Absorption

Fat digestion starts in the small intestine and requires the action of bile, which causes the emulsion of fats present in the chyme. This creates dispersed droplets suspended in water, allowing digestive enzymes to act effectively. The digestion process is slow; to facilitate this, the pancreas secretes a hormone that slows the release of chyme into the duodenum. Consequently, the digestion of fatty foods is considered slow and heavy.

Pancreatic juice provides pancreatic lipase, and intestinal juice provides enterolipase (or intestinal lipase). These enzymes hydrolyze fats into glycerol and fatty acids that are absorbed by the intestinal villi. In intestinal epithelial cells, the glycerol and fatty acids recombine, producing fat droplets that are absorbed by the lymphatic capillaries. Later, the lymphatic system delivers the fat droplets into the bloodstream.

Classification of Animals

Non-Arthropod Invertebrates

Non-arthropod invertebrates: Porifera (sponges), Coelenterata (polyps and jellyfish), Platyhelminthes, Nematodes, Annelids, Molluscs (bivalves, gastropods, and cephalopods), and Echinodermata.

Arthropoda

Arthropoda: Class Insecta, Class Crustacea, the Chelicerata group, and Myriapoda.

Chordata

Chordata: Agnates or cyclostomes (jawless fish), Chondrichthyes (cartilaginous fish: sharks), Osteichthyes (bony fish), amphibians, reptiles, birds, and mammals (including placental mammals).