DigestionChapter 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. Ingestion2. Digestion3. Absorption4. 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

• 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?

…a collapsible, muscular, j-shaped bag with many folds and ridges

Stomach acids, enzymesHCl: • 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– Starchesdisaccharidesmonosaccharides

• 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