Food Procurement: Digestion in Animals

• Classification of animals according to diet

  • Herbivores
  • Carnivores
  • Omnivores
• Herbivore classifications


  • Frugivores (fruit-eaters)
  • Granivores (seed eaters)
  • Nectivores (nectar feeders)
  • Folivores (leaf eaters)
• Carnivore classifications


  • Obligate carnivores – rely entirely on animal flesh (ex. Lions, cheetah)
  • Facultative carnivores – eat non-animal food in addition to animal food (ex. dog)
• Omnivores


  • Vertebrate omnivores: humans, bears, and chickens
  • Invertebrate omnivores: Cockroaches, and crayfish
• Gastrovascular Cavity


  • Simplest digestive systems
  • Typically a cavity with only one opening (the "mouth" also serves as an "anus")
  • Found in organisms such as Platyhelminthes (flatworms), Ctenophora (comb jellies), and Cnidaria (coral, jelly fish, and sea anemones)
• Alimentary Canal


  • More advanced system than gastrovascular system
  • Consists of one tube with a mouth at one end and an anus at the other
  • Found in some invertebrates and in vertebrates
• Invertebrates with alimentary canal

  • Earthworms and nematodes
• Vertebrates with alimentary canal


  • Humans, avian, ruminants, etc.
• Monogastric: Single-chambered Stomach


  • Consists of one ("mono") stomach chamber ("gastric")
  • Found in humans
• Multi-Chambered Stomachs


  • Found in birds and ruminants
• Avian Digestive System


  • Birds do not have teeth and so their digestive system must be able to process un-masticated food
  • Birds have evolved a variety of beak types that reflect the vast variety in their diet
  • The metabolic rates of birds are high in order to efficiently process food and keep their body weight low
  • Crop - stores food
  • Proventriculus – the 1st stomach where gastric juices are produced to digest the food before it enters
  • Gizzard - where the food is stored, soaked, and mechanically grounded (stones are swallowed and stored in the gizzard to aid the grinding process)
  • Small Intestines – where chemical digestion and absorption happens
  • Large intestine – where some uric acid from the kidneys is secreted into the and combined with waste from the digestive process
  • Cloaca - expanded, tubular structure that serves as the common opening of the digestive, reproductive and urinary systems to the outside of the bird
• Ruminant Digestive System


  • Ruminants are mainly herbivores whose entire diet consists of eating large amounts of roughage or fiber
  • Ruminants do not have upper incisor teeth
  • Stomach is multi-chambered organ
  • Rumen - First stomach compartment, contains prokaryotes and protists
  • Reticulum - Second stomach compartment, contains prokaryotes and protists, food is regurgitated
  • Omasum - Third stomach compartment, where water is removed, food is transformed into cud
  • Abomasum - The fourth and the "true" stomach, where digestive enzymes are produced

  
• Pseudoruminant Digestive System


  • Found in camels and alpacas whose diet consists of eating large amounts of roughage
  • Consist of three chamber stomach: rumen, reticulum, and omasum

  
• Parts of the Digestive System

  • Oral Cavity
  • Pharynx
  • Esophagus
  • Stomach
  • Small Intestine
  • Large Intestine

  
• Oral Cavity


  • Point of entry of food into the digestive system
  • Food consumed is broken into smaller particles by mastication
  • Food is mixed with saliva and prepared into a mass called the bolus for swallowing
  • 3 Major Glands that secrete saliva: Parotid, Submandibula, Sublingual
  • Enzymes: salivary amylase (saliva) that begins the process of converting starches in the food into a disaccharide called maltose, Lipase (produces by the cells in the tongue) – breaks down triglycerides

  
• Pharynx


  • A small passage way of the bolus/food
  • Opens to two passageways: the trachea - which leads to the lungs, and the esophagus – which leads to the stomach
  • Glottis – opening to trachea
  • Epiglottis – cartilaginous flap that covers the glottis
  • When swallowing, the epiglottis closes the glottis and food passes into the esophagus and not the trachea
• Esophagus


  • A tubular organ that connects the mouth to the stomach
  • Peristalsis - a series of wave like movements that push the food toward the stomach and is unidirectional

  
• Stomach


  • A saclike organ that secretes gastric digestive juices
  • Can expand to up to 20 times its resting size when filled with food
  • Churns food while chemical digestion takes place
  • pH in the stomach is between 1.5 and 2.5
  • Parietal cells – secrete Hydrogen and Chloride ions (forms HCl)
  • HCl - convert the inactive pepsinogen to pepsin; kills many microorganisms in the food; helps in the hydrolysis of protein in the food
  • Pepsin (Pepsinogen is the inactive form) - breaks peptide bonds and cleaves proteins into smaller polypeptides
  • Chyme - partially digested food and gastric juice mixture
  • The stomach has a thick mucus lining that protects the underlying tissue from the action of the digestive juices
  • Ulcer - open wounds in or on an organ caused by bacteria (Helicobacter pylori) when the mucus lining is ruptured and fails to reform
• Small Intestine


  • Long tube-like organ where the digestion of protein, fats, and carbohydrates is completed
  • Has a highly folded surface containing fingerlike projections called the villi
  • Each villus, in the apical surface has many microscopic projections called microvilli
  • Villi and microvilli increase the surface area of the intestine and increase absorption efficiency of the nutrients
  • Duodenum - separated from the stomach by the pyloric sphincter, chyme is mixed with pancreatic juices
  • Bile - produced in the liver and stored and concentrated in the gallbladder, functions to emulsify lipids
  • Pancreatic juice – contains enzymes that catabolize starches, disaccharides, proteins, and fats
  • Jejunum - hydrolysis of nutrients is continued while most of the carbohydrates and amino acids are absorbed through the intestinal lining
  • Ileum - last part of the small intestine and where the bile salts and vitamins are absorbed into blood stream, undigested food is sent to the colon

  
• Large Intestine


  • Reabsorbs the water from the undigested food material and processes the waste material
  • Cecum - joins the ileum to the colon and is the receiving pouch for the waste matter
  • Colon - home to many bacteria or "intestinal flora" that aid in the digestive processes, into four regions: ascending, transverse, descending, and sigmoid
  • Rectum - terminal end of the large intestine which primarily store the feces until defecation
  • Anus - an opening at the far-end of the digestive tract and is the exit point for the waste material, has two sphincters: inner (involuntary) and outer (voluntary)
• Accessory Organs


  • Liver
  • Pancreas
  • Gallbladder
• Liver


  • Largest internal organ in humans
  • Helps in the digestion of fats and in the detoxification of blood
  • Processes the vitamins and fats and synthesizes many plasma proteins
  • Produces bile, a digestive juice that is required for the breakdown of fatty components of the food in the duodenum

  
• Pancreas


  • Gland that secretes digestive juices: pancreatic juices
  • Contain high levels of bicarbonate (alkali) that neutralizes the acidic chyme
  • The pancreatic juices contain a large variety of enzymes that helps in the digestion of protein and carbohydrates

  
• Gallbladder


  Aids the liver by storing bile and concentrating bile salts which will be secreted into the duodenum