Digestion

• Types of feeders


  • Filter feeders
  • Substrate feeders
  • Fluid feeders
  • Bulk feeders
• Filter feeders


  Sift small food particles from the water
• Substrate feeders


  Consume foods on which they live
• Fluid feeders


  Drink nutrient-rich fluids
• Bulk feeders


  Ingest large foods
• Food processing


  1. Ingestion

  2. Digestion

  3. Absorption

  4. Elimination
• Ingestion


  Bringing food resources into the digestive system
• Digestion


  Breaking down the food resources into units (nutrients) small enough for the body to be able to utilize
• Absorption


  Transfer of the nutrients from the digestive tract into cells and/or other body organs/systems
• Elimination


  Removal of any un-digested, unusable or excess food resources
• Mechanical digestion, chewing or grinding, increases the surface area of food
• Chemical digestion splits food into small molecules that can pass through membranes; these are used to build larger molecules
• Enzymatic hydrolysis


  The process of splitting bonds in molecules with the addition of water
• Intracellular digestion


  Occurs in some single-celled eukaryotes and multicellular organisms, where the organism engulfs food particles through phagocytosis or liquids through pinocytosis, and the resulting food vacuole fuses with a lysosome containing digestive enzymes
• Extracellular digestion


  Occurs in a digestive system outside of cells
• Gastrovascular cavity


  • Has 1 opening (mouth)
  • Digestive enzymes are released from a gland cell
  • Food particles are engulfed and digested in food vacuoles
• Alimentary canal


  • Has 2 openings (mouth and anus)
  • Specialized regions carry out digestion and absorption in a stepwise fashion
• Features of extracellular digestion


  • Esophagus: tube linking mouth to site for mechanical digestion
  • Stomach/Crop/Gizzard: site for storage and mechanical digestion
  • Small Intestine/Intestine/Gastric Cecae: site for enzymatic digestion & the site of absorption of nutrients
  • Large Intestine/Hindgut/Rectum/Anus: site for absorption of excess H2O
• The mechanism for extracellular digestion is evolutionarily conserved
• Absorption


  The uptake of small molecules by body cells
• Elimination


  The passage of undigested material out of the digestive system
• Organs of the mammalian digestive system


  • Oral cavity (mouth)
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Rectum
  • Anus
  • Liver
  • Salivary glands
  • Gallbladder
  • Pancreas
• Chemical digestion


  One of the two major digestive processes, where food is split into small molecules that can pass through membranes
• Mechanical digestion


  One of the two major digestive processes, where food is chewed or ground to increase surface area
• Oral cavity (mouth)


  • Food enters here
  • Teeth masticate food (mechanical digestion)
  • Salivary glands secrete saliva which moistens food and contains the enzyme amylase (chemical digestion)
  • Tongue forms a bolus and facilitates swallowing
• Esophagus


  • Moves food to stomach through peristalsis
  • Pharynx is the opening between oral cavity and esophagus
  • Epiglottis prevents bolus from entering trachea
• Stomach


  • Major site of mechanical and chemical digestion
  • Gastric juice has low pH and contains hydrochloric acid and pepsin enzyme
  • Mucus protects stomach lining from gastric juice
• Small intestine

  • Site of majority of nutrient absorption
  • Duodenum is first portion, site of digestion
  • Jejunum is second portion, site of absorption
  • Ileum is third portion, site of absorption
• Nutrient absorption in small intestine


  • Pancreatic secretions and liver bile are secreted into duodenum
  • Nutrients are absorbed through intestinal cells, through villi and microvilli
  • Nutrients absorbed = monomers (monosaccharides, amino acids, fatty acids) and vitamins and minerals
• Small intestine


  • Nutrient absorption, particularly water, amino acids, fatty acids, monosaccharides, vitamins and minerals
  • 3 segments: 1. Duodenum - 1st portion; digestion, 2. Jejunum - 2nd portion; absorption, 3. Ileum - 3rd portion; absorption
• Nutrient absorption


  1. Nutrients absorbed through intestinal cells, through villi and microvilli

  2. Nutrients absorbed = monomers (monosaccharides, amino acids, fatty acids) and vitamins and minerals
• Pancreatic secretions


  • Exocrine (digestive) - amylase, protease, and lipase
  • Endocrine (hormonal) - insulin, glucagon
• Fat digestion


  Pancreatic lipase
• Nucleic acid digestion


  Pancreatic nucleases
• Protein digestion


  1. Pancreatic trypsin and chymotrypsin

  2. Pepsin
• Carbohydrate digestion


  1. Salivary amylase

  2. Pancreatic amylases

  3. Disaccharidases
• Insulin acts on nearly all body cells to stimulate glucose uptake from blood
• Glucagon and Insulin


  • Produced in the islet cells of the pancreas
  • Alpha cells make glucagon, Beta cells make insulin
• Gall bladder


  • Stores bile from liver
  • Bile emulsifies lipids
• Large intestine


  • The colon of the large intestine is connected to the small intestine
  • A major function of the colon is to recover water that has entered the alimentary canal
  • The colon houses bacteria (e.g., Escherichia coli) that live on unabsorbed organic material; some produce vitamins
  • Feces, including undigested material and bacteria, become more solid as they move through the colon