Digestive System!
Major Components:
Starr and Taggart: Chapter 40!
mouth
pharynx
esophagus
Accessory Organs:
stomach
liver
small intestine
large intestine
(colon)
rectum
anus
!
Epithelial cells line the entire digestive tract, but the cells’ shape varies dramatically with the
different functions of each segment of the tract. !
Digestive enzymes are
secreted by:"
*Salivary glands"
*Stomach (lining)"
*Liver"
*Pancreas"
*Small intestines (lining)!
(Lumen - inside of gut - is at top of slide)!
Water absorption:"
Large intestine"
(Colon )!
gallbladder
pancreas
+ Enzymes !
Food and Nutrients!
Major Classes of Molecules in Food!
•! Food = mixtures of complex substances!
–!
–!
–!
–!
Fats!
Proteins!
Carbohydrates!
Nucleic acids!
•! Only monomers can be absorbed across cell membranes!
–! Fatty acids!
–! Amino acids!
–! Simple sugars!
•! Much of our digestive tract is organized to digest foods!
•! Digestion: "
–! The process of turning food into absorbable nutrients"
•! Absorption:"
•! Carbohydrates!
–! Based on sugars!
–! C, H, and O!
•! Proteins!
–! Built up from amino acids!
–! Contain N!
•! Lipids!
–! Built up from triglycerides!
•! Nucleic Acids!
–! Sugar/phosphate backbone!
–! Bases!
–! Movement of nutrients across cell membranes into the blood"
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Consider the Big Mac"
"
2 beef patties -- proteins, triglycerides (neutral fat)!
Special sauce (Triglycerides? Sugars?)!
Lettuce -- cellulose: not assimilated !
Cheese -- proteins, triglycerides!
Pickles -- cellulose, starch !
Onions -- cellulose, starch !
Sesame seed bun -- starch!
Energy content!
200 gram Big Mac = !
–! 25 grams of protein!
–! 28 grams of fat!
–! 47 grams of carbohydrate!
! = 530 Calories!
Digestion is the process by which complex foodstuffs are broken !
down and gain access to the bloodstream, so their components can be used by cells.!
Digestion and Absorption!
•! Absorption occurs after digestion!
•! D
" igestion is mediated by enzymes!
–! mostly in small intestines!
•! Stepwise!
•! Monomers are absorbed!
–! protein --> polypetide --> aa!
–! simple sugars!
–! Starch, glycogen --> disaccharide
–! amino acids!
--> monosaccharide!
–! Nucleosides (minus the sugar)!
–! Fat --> fatty acids!
–! fatty acids!
•! Regional!
•! Substances that are not absorbed
become the feces!
–! mouth !
•! carbohydrates!
–! stomach !
•! proteins, nucleic acids!
–! small intestine!
•! fats!
–! Fiber!
–! Olestra!
–! Water !
•! added in secretions of enzymes!
•! reabsorbed in large intestine
(colon)!
–! Some excreted materials are added
to the unabsorbed material!
Digestion of
Carbohydrates!
•! Begins in the mouth!
•! Enzymes in saliva include !
–! Amylases !
•! Break carbohydrates into disaccharides!
–! Disaccharidases!
•! Convert disaccharides to monosaccharides --simple sugars!
•! Chitin and cellulose!
–! Cannot be digested by vertebrate enzymes!
–! Ruminant digestive tracts contain symbiotic protozoans that
digest cellulose!
Digestion of
Proteins!
•! Begins in the stomach!
–! Pepsin cleaves proteins into small peptides!
•! In the small intestine cleavage is continued by!
–! Trypsin"
–! Chymotrypsin"
–! Carboxy- and amino-peptidases cleave single amino acids
(from the acid and amino ends, respectively) of peptides!
•! Nitrogen in proteins poses problems for excretory system !
Digestion of Nucleic Acids!
•! Begins in stomach:!
–! sugar phosphate backbones are cut by low pH!
•! In the small intestine!
–! Endo- and exo-nucleases continue degradation!
–! May cleave bonds between bases and sugars!
Digestion of Lipids!
•!
•!
•!
•!
Begins in the small intestine !
Problematic because fats are not water soluble!
Liver secretes bile into the gall bladder!
Bile !
–! Is derived from cholesterol"
–! Consists of salts of cholic acid!
–! is secreted into duodenum (small intestine)!
–! attaches to and emulsifies fat globules !
•! Emulsified fats can be digested by lipases"
! ----> fatty acids!
Absorption of Nutrients!
epithelium
villi
blood
capillaries
•! Occurs mostly in the small intestine!
–! Duodenum(1st segment) is still primarily digestive!
–! Jejunum (2nd segment) is mostly absorptive!
•! Lined with villi!
•! Villi have outer surfaces composed of microvilli!
•! Microvilli hugely increase surface area of jejunum!
–! Greatly facilitate absorption!
lymph
vessel
connective
tissue
vesicles
artery
vein
lymph
vessel
Villi on one of the folds,
longitudinal section
The Large Intestine!
•! Colon"
–! Continued absorption of ions and water"
•! Appendix"
•! Conditions"
–! Appendicitis"
–! Constipation"
–! Diarrhea"
–! Cancer"
•! Screening saves lives"
One villus
Human Nutritional Requirements!
•! Food Pyramid"
–! Carbohydrates for energy"
–! Proteins for building, repair, structures"
–! Fats for insulation, some vitamins, linoleic acid"
•! Caloric intake must equal expenditure to maintain body weight"
•! Remember that we eat foods, not nutrients. !
•! Michael Pollan: “Eat food. Not too much. Mostly from plants.”"
Vitamins and Minerals!
•!
Vitamins"
–! Fat Soluble"
•! A"
•! D"
•! E"
•! K"
–! Water Soluble"
•! B1, B2, B6"
Harvard Public School of Health!
Healthy Eating Pyramid!
•! Minerals"
–! Essential"
•! Calcium, Chloride, Copper,
Fluoride, Iodine, Iron,
Magnesium, Zinc, Phosphates,
Potassium, Sodium, Sulfur,
Selenium"
–! Purely toxic"
•! B12 - absent in vegan diet"
•! Lead"
•! Folate or folic acid"
•! Cadmium"
•! Pantothenic acid or B5"
•! Mercury"
•! Biotin"
•! C"
http://www.hsph.harvard.edu/nutritionsource/pyramids.html!
Amino acids in vegetarian diets: by eating foods from different !
columns, adequate intake of all essential amino acids is assured.!
“Functional” Foods!
•! Functional food or medicinal food "
–! any fresh or processed food
claimed to have a healthpromoting and/or diseasepreventing property beyond the
basic nutritional function of
supplying nutrients."
–! there is no consensus on an exact
definition of the term."
•! Examples"
–! flavenoids; polyphenols; antioxidants; resveratrol; fiber; monounsaturated fats; vitamins"
Calories Count!
•! Atkins and other ‘fad’ diets !
–! Cause weight loss!
•! IFF you eat less than you need for your activities!
–! May not be healthy!
•! Exercise alone won’t keep weight off:!
–! You can’t exercise enough to work off what you can eat.!
•! Dieting alone won’t keep weight off:!
–! Couch potatoes can’t eat little enough to stay thin.!
•! Exercise PLUS a vegetable-rich diet PLUS portion control will work!
….. if you keep them up permanently!
•! Summary:!
•! Eat anything you want, but not as much of it as you want"
•! Losing weight is harder than not gaining it in the first place."
Fads, Fallacies, & Nutrition!
•! Vitamins!
–! Are essential!
–! Can be toxic!
–! A, D, E, K !
•! are fat soluble!
•! Are stored if taken in excess!
•! Don’t overdose on these vitamins!"
–! Folate is extremely important"
•! To prevent birth defects (and miscarriages?)!
•! To prevent heart attacks!