Digestive System

• Dietary Categories


  • Herbivores - plant eaters
  • Carnivores - meat eaters
  • Omnivores - meat and plant eaters
  • Saprophagous - feeds on decaying matter
• Feeding Mechanisms


  • Suspension Feeders
  • Deposit Feeders
  • Substrate Feeder
  • Fluid Feeders
  • Bulk Feeders
• Suspension Feeders


  • Sifts through food particles in the water
  • Possess filtering devices that strain food from water as it passes through them
  • Herring and other suspending-feeding fishes use gill rakers to strain plankton
  • Baleen whales filter out planktons, mainly crustaceans called krill
  • Filters water with hairy fringes called a whalebone or baleen plates
• Deposit Feeders


  • Eats its way through dirt or sediments and extract partially decayed organic material (detritus) consumed along with the soil or sediments
• Substrate Feeder


  • Lives in or on its food source, eating its way through the food
• Fluid Feeders


  • Sucks nutrient-rich fluids from a living host and is considered a parasite
  • Has proboscis
• Bulk Feeders


  • Eats relatively large pieces of food
• Types of Digestive System


  • Complete Digestive System (2 openings)
  • Incomplete Digestive System
• Complete Digestive System


  • There is a mouth opening and an anus
  • Complete digestive system has alimentary canals
• Incomplete Digestive System


  • There is only one opening, no anus
  • Unicellular organisms like paramecium: digestion occurs via phagocytosis and lysosomal degradation
  • Hydra: Digestion occurs within the cells of the gastrovascular cavity
  • Planaria: Gastrovascular cavity
• Four Main Stages of Food Processing


  • Ingestion - the act of eating
  • Digestion - (Two mechanisms - Intracellular or Extracellular) - breaking down of foods into molecules small enough for the body to absorb
  • Absorption - small molecules are taken in by the animal's cells
  • Elimination - undigested material passes out of the digestive compartment
• Digestive Stage of Food Processing


  • Intracellular
  • Extracellular
  • Both Intracelullar and Extracellular
• Intracellular Digestion


  • Digestion is entirely intracellular in protozoa and sponges (digestion occurs within the cells)
  • Involves phagocytosis
  • Lysosome digest foods
• Extracellular Digestion


  • Certain cells lining the lumen of alimentary canals form digestive secretions (digestive enzymes); other cells function in absorption
  • Digestion occurs in the lumen of the alimentary system
  • For arthropod and vertebrate digestion is almost extracellular
• Both Intracellular and Extracellular Digestion


  • Radiates, turbellarian, flatworms, and ribbon worms (nemerteans) practice both intracellular digestion and extracellular digestion
  • Extracellular digestion can occur in gastrovascular cavity
• Human Digestive System


  • Human Oral Cavity and Esophagus
  • Human Stomach
  • Near Human Small Intestine (Liver and Gallbladder)
  • Near Human Small Intestine (Pancreas)
  • Human Small Intestine
  • Human Large Intestines
• Human Oral Cavity and Esophagus


  • Food is called bolus if it passes through the esophagus
  • Contraction of muscles moves the bolus down the stomach (peristalsis)
  • Uses salivary amylase in the oral cavity to pharynx to esophagus to break down carbohydrates
  • Polysaccharides are digested into smaller polysaccharides
• Human Stomach


  • Food is called chyme which is formed in the stomach
  • Most proteins are broken down here
  • The stomach is more acidic than the intestines
  • Proteins are broken down by pepsin (active enzyme) into small polypeptides (string of amino acids)
  • Pepsinogen and HCl are introduced into the lumen of the stomach
  • HCl converts pepsinogen(inactive) to pepsin
  • Pepsin then activates more pepsinogen, starting a chain reaction. Pepsin begins the chemical digestion of proteins
  • Chief cells secrete pepsinogen
  • Parietal cells secrete HCl
  • Mucous cells secrete mucus to lubricate and protect the linings of the stomach
  • Gastric glands are the storage of chief, parietal and mucous cells' secretion
• Near Human Small Intestine (Liver and Gallbladder)


  • Gallbladder is connected to the liver
  • Liver produces and secretes bile into gall bladder(storage of bile)
  • After a meal, bile is released from gall bladder into duodenum to break down fatty macromolecules
  • Fat globules → bile salts → fat droplets → pancreatic lipase → glycerol, fatty acids, monoglcyerides
  • Liver is needed to further process the nutrients broken down into forms that is easier for the body to use
• Near Human Small Intestine (Pancreas)


  • Pancreas secretes enzymes: inactive trypsinogen, inactive chymotrypsin or chymotrypsinogen, and inactive carboxypeptidase or procarboxypeptidase
  • Pancreas: endocrine cells secrete hormones into blood vessels; exocrine cells secrete pancreatic enzymes into the pancreatic duct
  • In the lumen of duodenum, the intestinal enzyme, membrane bound enteropeptidase, converts inactive trypsinogen into active trypsin - break down proteins
  • Trypsin activates chymotrypsin and carboxypeptidase - all break down proteins
• Human Small Intestine


  • Duodenum
  • Jejunum
  • Ileum
• Duodenum


  • Nearest to the stomach, 26 cm in length
  • Food is called chyle once it enters the small intestine
  • Contains bruner's glands
  • produces mucus-rich alkaline secretion with bicarbonate to
  • protect the duodenum from the acidic content of chyme
  • provide alkaline condition for the intestinal enzymes to be active; and
  • lubricates the intestinal wall
• Jejunum


  • Comes after duodenum, 2.5 m in length
• Ileum


  • Last segment of the small intestine, 3.5 m in length
  • Contains Peyer Patches:
  • lymphoid tissues found in the epithelial layer of the ileum
  • Contain's WBC to survey from any pathogenic microorganisms in the intestinal lumen
  • Fights pathogenic bacteria
• Human Large Intestines


  • The colon recovers water that has entered the alimentary canal as the solvent to various digestive juices
  • The large intestine harbors a rich flora of mostly harmless bacteria
  • The terminal portion of the colon is called rectum, where feces are stored until they can be eliminated