13 3 Digestive Systems Animals use their sensory systems to locate food and a variety of specialized mouth parts to capture it and introduce it to their digestive systems Digestion begins in the mouth and the lumen of the GI tract Enzymes hydrolyze macromolecules in the food into smaller parts which can be used to build macromolecules that the cells need or as energy to fuel ATP synthesis Indigestible material gets excreted through the feces and unmetabolizable energy water and salts are excreted through the urine Some energy is used by the cells that carry out digestion and lost as heat What happens between ingestion and egestion as food travels through the digestive system Throughout evolutionary history digestive systems have tended to become more anatomically and functionally specialized With multiple tissue layers came cells dedicated to digestion This early digestive system like that of cnidarians is a simple internal sac that functions as a two way gut it has a single opening at the oral end through which food enters and waste leaves The internal cavity of a radially symmetric animal like a jellyfish or hydra is not a true cavity or coelom When the animal develops it does not form a cavity between its tissue layers or form an outpocketing of one of its layers It simply develops in an umbrella shape which surrounds the seawater Bilaterally symmetric animals have three tissue layers Almost all bilaterally symmetric animals form true cavities coeloms during development with the exception of Platyhelminthes flatworms which are acoelomates The lining of the internal sac may be convoluted in flatworms forming diverticula to increase the surface area for absorption of nutrients and export of waste products During development of the bilaterally symmetric animal groups Nematoda and Rotifera a coelom forms between the endoderm and mesoderm forming a pseudocoel they are pseudocoelomates Most invertebrates except echinoderms are protostomes Recall that when gastrulation occurs during embryonic development the anterior posterior and dorsal ventral axes are defined The tissue layers invaginate on one side In protostomes this defines the mouth The invagination pushes through the embryo and bisects it to form the coelom in protostomes All other bilaterally symmetric animals are coelomates their internal body cavity forms as a cavity within the mesoderm Of the coelomates echinoderms and chordates which includes vertebrates are deuterostomes The initial invagination during gastrulation defines the anus instead of the mouth In chordates the coelom forms when when the mesoderm splits like in protostomes In other deuterostomes the coelom forms when the mesoderm pinches off from the gut during embryonic development The progression from false body cavity to proper coelom corresponds to the evolution of the two way gut into the one way gut The one way gut is a proper gastrointestinal tract in which food enters at one end and waste exits from the other The evolution of the GI tract allowed for further specialization dividing the tract into regions In vertebrates which is what we will focus on for the rest of the lesson the mouth pharynx and esophagus became specialized for the mechanical breakdown of food The stomach became an acidic compartment that uses low pH to denature proteins and other macromolecules The upper small intestine is specialized for digestion and absorption while the lower large intestine is specialized for absorption of water The anus is specialized for the controlled release of indigestible material At several of the junctions between these regions there are muscular rings that encircle the tract called sphincters the contraction and relaxation of which control the passage of food from one section to the other In larger animals there are further divisions of the two intestinal regions based on anatomy and functional specialization Early in embryonic development the gut is derived from endoderm and divided into three large sections the foregut midgut and hindgut Foregut esophagus stomach and anterior part of the duodenum the anterior section of the small intestine Midgut remainder of the small intestine most of the anterior part of the large intestine including the cecum Hindgut posterior part of the large intestine remainder of colon and rectum The length and surface area of each section can vary drastically between species Once digestive enzymes hydrolyze macromolecules into smaller molecules in the lumen of the gut these are absorbed by the cells that line the gut Animals can increase the efficiency of this absorption process by increasing gut length or making the surface of the gut convoluted Large gut lengths can be accommodated by coiling up the intestines inside the animal s abdomen and reflect diets that are more difficult to digest For example carnivores tend to have shorter guts than herbivores because their food is easier to digest Animals with particularly difficult diets may have stomachs that are divided into multiple chambers to further aid in digestion Surface area of the gut epithelia can be increased by three anatomical specializations in order from largest to smallest circular folds that run around the intestine villi and microvilli that form part of the intestinal epithelial layer called the brush border These specializations may increase the gut surface area by several hundred times Digestion efficiency can also be improved by specialized compartments along the GI tract The cells in particular regions may secrete acids that lower the pH of the compartment or transcribe and translate genes for special digestive enzymes The type of secretory and reabsorptive cells that line different parts of the GI tract will determine the function and chemical environment of that region For example even ctenophores radially symmetric animals with only two tissue layers and no coelom create an acidic environment in their pharynx which is just beyond the mouth Animals with one way guts have more regional specializations which are separated by sphincters The first of these compartments is the mouth Many species have salivary glands in the mouth which contain cells that produce and release digestive enzymes Digestion begins with the hydrolytic activity of these enzymes and the mechanical force of the jaws teeth and other mouth parts The secretions of these cells saliva also includes fluid that lubricates and dissolves any polar contents in the food allowing chemicals to bind