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.
2.
3.
4.
Ingestion
Digestion
Absorption
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