Chapter 41 Animal Nutrition p. 867

Digestive System

Ingestion- act of eating

Digestion- breaking down the food

Egestion- removal of waste

• Enzymes cleave large macromolecules
  • Fatà glycerol and fatty acids
  • Proteinsàamino acids
  • Polysaccharidesà sugar monomers
  • Nucleic acidsà nucleotides
• Uses enzymatic hydrolysis
  • Adds water; increase exposure to enzymes

 

 

Intracellular Digestion vs Extracellular Digestion

• Intracellular-  accomplished by an individual cell, lysosomes merge with a food vacuole
• Extracellular- food ingested is too large to be engulfed by individual cells.

 

Incomplete vs Complete Digestion

Incomplete – one opening; food and waste enter and exit out the same opening; example Hydra p. 859 Figure 41.11 Hydra with bud

·        Contain a gastrovascular cavity (functions in both digestion and distribution of nutrients)

 

Complete- two openings; An Alimentary Canal (digestive tube) connects the mouth and anus.

1. Single-celled organisms; such as protists move dissolved materials directly across their cell membranes by diffusion. Perform intracellular digestion p. 858 Figure 41.10 paramecium

1.      oral groove- “mouth”

2.      food vacuoles-form and carry food around the cell

3.      hydrolytic enzymes (Digestion)

4.      Anal pore (elimination of waster

2. Simple Animals
   1. Intracellular digestion (digestion within a food vacuole) ex. Hydra
   2. Lysosomes (digestive enzymes) fuse with vacuoles and breakdown food.
   3. Paramecium, food vacuoles, oral groove, anal pore
   4. See p. 859 Fig 41.12 , Earthworm

a.       Alimentary canal (canals for complete digestion

b.      Mouth- food entry

c.       Esophagus- tube connecting mouth to crop

d.      Crop- a storage organ

e.       Gizzard-a grinding organ

f.        Intestine-nutrient absorption

g.       Anus- waste exit

3. Complex animals- extracellular digestion
   1. More evolved digestive tract
   2. Special cavity for digestion
   3. Grasshopper p. 859 Figure 41.12b

1.       mouth

a.       Salivary glands-secrete saliva

2.      esophagus,

3.      crop (a storage organ),

4.      stomach

a.       Gastric caecum-contains digestive enzymes

5.      intestine

6.      rectum and anus

Human Digestive p.860 Figure 41.13

 

Mechanical Digestion

• Mouth (Oral Cavity)-
  • chewing
  • Saliva- secreted by Salivary glands, contain Salivary Amylase.      
  • Begins chemical  digestion by converting starch into maltose (crackers melt in your mouth)
  • Bolus- mushy ball of food
  • Mastication- softening and breaking up of food

 

• Pharynx: intersection for digestive and respiratory systems
  • Epiglottis: blocks entrance to windpipe (trachea)
    • Directs food into esophagus

 

• Esophagus: muscular tube
  • Peristalsis- muscle contractions that force food into the stomach. The same muscle contractions push waste out of the body later.

 

• Stomach
  • Cardiac valve prevents food from moving back into the esophagus
  • Thick muscular sac
  • Partially digests proteins
  • Temporarily stores the ingested food
  • Kills bacteria
  • Just below diaphragm (left side of the abdominal cavity)
  • Folds can expand to hold 2L of food
  • Churns about every 20 minutes (churning is a mechanical process that aids in digestion of food)
  • Chyme-  mushy, soupy mixture that contains food and digestive enzymes and acids (it is the stuff that hits the floor when you vomit)
  • Secretes
    • Gastric juices (contain digestive enzymes and HCl)
      • Pepsin- breaks down proteins into smaller peptides
      • Works best in an acidic environment (HCl lowers stomach pH and activates pepsin)

 

• Mucus: coats the stomach lining and protects from acidic juices

 

• HCl: kills bacteria
  • Also, unfolds large proteins into polypeptide strands. Then proteases cut strands into fragments.

 

• Small Intestine
  • 3 segments: Duodenum, Jejunum and Ileum
  • Pyloric sphincter (valve) prevents intestinal contents from moving back into the stomach.
  • 23 ft in avg man
  • 1 inch in diameter
  • walls produce enzymes to breakdown proteins (enzymatic hydrolysis)
  • All  food  completely digested
  • Products of digestion, amino acids, fatty acids, glycerol, and sugar are absorbed into the bloodstream
  • Duodenum (1^st 25 cm)
    • Nucleases attached to intestinal epithelium (lining) hydrolyze DNA and RNA
    • Neutralizes stomach acid allowing enzymes to work.
    • pancreatic juices, produced by Pancreas are dumped into the small intestine

 

Major Digestive enzymes produced by the Pancreas

 

• Trypsin and Chymotrypsin- breakdown proteins into amino acids
• Pancreatic Lipase- breakdown lipids into fatty acids and glycerol
• Pancreatic amylase- breakdown starches (carbohydrates) into sugars
• Pancreatic duct- connects pancreas to the intestine
• Also, releases bicarbonate (neutralizes acids)
• Pancreas is also an endocrine gland
  • Produces the hormones
    • Insulin; cause glucose levels in the blood to decrease and
    • Glucagen; increases glucose levels

 

• Jejunum and Ileum are:
  • Specialized for absorption
  • Folded with villi with microvilli (increase surface area).
    • Contain capillaries
      • Hepatic portal vein (1liter/min) leads to the liver where molecules are used, stored or converted to different forms…ex. glycogen

 

• The amount of fat in chyme acts a  feedback mechanism
  • Duodenum release enterogastrone (hormone) when fat levels are high… inhibits peristalsis, slows down food movement.
  • Fat must be broken down
  • Liver to the Rescue
    • Produces bile (not a digestive enzyme)
    • Contains bile salts; emulsify fat into droplets
      • Then lipases break down the fat into fatty acids and glycerol
    • Stored in gall bladder
      • Hepatic duct connects the gall bladder to the small intestine

 

• Large Intestine (aka Colon)
  • 1.5 m long
  • purpose is water absorption
    • produces feces; peristalsis moves the feces out of the body
    • Intestinal bacteria live on material in feces.
  • Cecum; point where the small and large intestine meet.
  • Appendix; vestigal organ
  • Rectum- anus