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					   Digestion
   Chapter 41
Campbell & Reece
  49-1: Nutritional Requirements
Cells need:
• Water
• Carbohydrates
• Lipids
• Proteins
• Nucleic acids
• Vitamins-- Study Table 49-1 pg. 980
• Minerals - Table 49-2 pg. 981
Nutrition involves the following steps:
    1.   Ingestion
    2.   Digestion
    3.   Absorption
    4.   Distribution


Digestion: the breakdown of ingested food
  materials into molecules that can be delivered
  to and utilized by the individual cells.
• Can be both mechanical and chemical.
• human anatomy tutorials, labeled with explanations (all systems) link
  The “Gut” (Gastrointestinal Tract):
                    Fig.49-2 pg. 983
• Made of 4 layers (see pictures on
  page 985)

• Mucosa: (innermost) -epithelial,
  connective, some smooth muscle
• Submucosa: connective, nerve
  fibers, blood & lymph vessels
• Muscularis Externa: muscle tissue
  (circular & longitudinal)
• Serosa: outer covering of
  connective tissue
• Peristalsis: coordinated contractions of
  circular muscles along digestive tract which
  move food along



• Sphincters: thickened, heavy bands of muscle
  along tract that act as valves by contracting
  (closing) and relaxing (opening) to control of
  materials
    The Oral Cavity: Initial Processing:
Mechanical Digestion
• Tongue - swallowing (microscopic picture)
• Teeth: “mastication” (chewing)
   – incisors- cutting, clipping
   – canines- tearing, stabbing
   – premolars- "bicuspids" 2 cusps (points)
   – molars- 4 or 5 cusps (for grinding)

• Other Animals lack teeth, have alternatives:
   – rasping tongue: ex- snails, lamprey, frogs
   – grinding organ (gizzard): ex- birds, earthworms

Chemical Digestion (enzymatic hydrolysis): saliva, mucous
• 3 pairs of salivary glands and buccal glands in jaw: lubricates, softens food
• Enzyme- salivary amylase for hydrolysis of starch into smaller sugar
   molecules; slightly alkaline (sodium bicarbonate)
       The Pharynx & Esophagus:
              Swallowing
Swallowing: passage of food to the esophagus --
  --> stomach
• begins as voluntary action (striated muscles in
  upper part of esophagus) lower portion
  involuntary (smooth muscle)= "peristalsis"
  Esophagus opens into Stomach
• Abdominal cavity lined with peritonium /
  organs are suspended by mesentaries
(animation)
  The Stomach: Storage & Liquefaction
  …a collapsible, muscular, j-shaped bag with
  many folds and ridges
• mucosal lining folds into gastric pits
  (microscopic picture)
• lined with mucous-secreting
  epithelial cells
• gastric glands secrete HCl and
  pepsinogen (precurser to pepsin)
• pH between 1.5 & 2.5 (mucous
  barrier between acids + stomach
  lining )- ulcers?
         Stomach acids, enzymes
HCl:
• kills bacteria loosens tissues for digestion converts
  pepsinogen to pepsin
Pepsin: breaks down proteins into smaller peptides
• Semi-liquid mass "Chyme" moves to lower end of
  stomach => Pyloric Sphincter (takes about 4 hrs.)
• Rate of leaving stomach = H2O,carbs, proteins, fat,
  protein/fat combo
• The three phases of gastric secretion animation
              The Small Intestine:
           Digestion and absorption
• lined with VILLI ( microscopic
  fingerlike projections) and tiny
  microvilli on surface of epithelial
  cells
• about 21 feet long (adult), total
  surface area about 300 m2
Digestion:
• Duodenum- 1st 25 cm (10
  inches)
   • most digestion takes place here;
   • receives several digestive
     juices/enzymes from ducts
     leading from ACCESSORY GLANDS
     (liver pancreas, gall bladder)
                      Accessory Glands

• neutralize
stomach acids with
alkaline secretions
• produces several
enzymes to
chemically digest
nutrients
               The Pancreas
Enzymes made here enter the duodenum
  via the pancreatic duct
• Amylase: continues starch digestion
  – Starchesdisaccharidesmonosaccharides
• Trypsin: breaks down proteins  amino acids
• Lipase: hydrolyzes fats
   The Liver: an accessory gland
Bile-
• produced by liver
• stored in gall bladder
• contains salts (emulsify fats)
• sodium bicarbonate (neutralize pH to
  optimum 7 or 8)
       Absorption of Nutrients:
• food is absorbed through epithelial cells of
  intestinal mucosa into the bloodstream, for
  distribution to all cells
• Active Transport: Monosaccharides,
  Dipeptides, Amino Acids
• Diffusion: fatty acids
• Lipids resynthesize into triglycerides,
  phospholipids (blood & lymph vessels),
  Cholesterol
The Large Intestine: Water Absorption
            & Elimination
The Large Intestine (colon)continues absorption of
  water, sodium, and other minerals (which begins
  in small intestine)
• E. coli bacteria: makes Vit.K, amino acid synthesis
• Appendix: vestigial organ (gastric caecum?)
• Waste: "fecal matter"
   – water, bacteria, dead cells, indigestible matter
     (cellulose)
   – lubricated with mucus
   – temporary storage in rectum

				
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