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Chapter 41

Chapter 41. Digestion and Human Nutrition A Summary. AP Biology Spring 2011. The Nature of Digestive Systems. A digestive system mechanically and chemically reduces food particles and molecules small enough to be absorbed into the internal environment. Incomplete and Complete Systems.

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Chapter 41

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  1. Chapter 41 Digestion and Human Nutrition A Summary AP Biology Spring 2011

  2. The Nature of Digestive Systems • A digestive system mechanically and chemically reduces food particles and molecules small enough to be absorbed into the internal environment

  3. Incomplete and Complete Systems • Incomplete System: has one opening • Food enters and waste leaves through the same opening • Digestive produces are absorbed directly to the needy tissues • Ex. Flatworm

  4. Incomplete and Complete Systems • Complete Systems: tube with two openings, allowing food to move in one direction through the lumen

  5. Incomplete and Complete Systems • Complete system performs 5 tasks: • Mechanical processing and motility is the breaking up, mixing, and transporting of food material • Secretion is the release of needed enzymes and other substances into the lumen • Digestion is the chemical breakdown of food matter to molecules small enough to cross the gut lining • Absorption is the passage of digested nutrients into the blood or lymph • Elimination is the expulsion of undigested and unabsorbed residues at the end of the gut

  6. Correlations with Feeding Behavior • The digestive system is an internal space or tube with specialized regions for food intake, transport, processing, and storage

  7. Correlations with Feeding Behavior • Regional specializations correlated with feeding behavior • Birds store meals in a stretchable crop and grind the food in a gizzard • Ruminants can eat grass almost continuously and have multiple stomachs to digest cellulose • Carnivores have shorter intestines compared to ruminants, with expandable stomachs to accept a lot of food in a single sitting

  8. Overview of Human Digestion System • The human digestive system is a tube with two openings and many specialized regions • Its overall extended length is 6.5 to 9 meters, comprising of mouth, pharynx, esophagus, stomach, small intestine, colon, rectum, and anus • Accessory glands include salivary glands, liver (with gallbladder), and pancreas

  9. Overview of Human Digestion System • All along tube, mucus coated epithelium for protection and promotes diffusion • Mouth: food partially processed • Tongue- taste buds (chemoreceptor) • Muscles help position food, swallow, and speech

  10. Overview of Human Digestion System • Pharynx: tubular entrance to esophagus and trachea • Epiglottis: keeps food from entering trachea • Esophagus: muscular tube • Sphincter between esophagus and stomach (ring of smooth muscle)

  11. Overview of Human Digestion System • Stomach: breaks apart, mixes, propels food onward • Small intestine: breaks apart, mixes, propels food onward; absorption; digestive enzymes and other secretions enter gut lumen and go to work

  12. Overview of Human Digestion System • Large intestine: (colon) water and ions are absorbed which compacts the undigested residues • Rectum: wastes are briefly stored before being expelled through the anus • Anus: terminal opening

  13. Overview of Human Digestion System

  14. Prepping Food in the Mouth • 32 teeth in humans interact with saliva to mechanically and chemically prepare food in the mouth

  15. Prepping Food in the Mouth • Teeth are hardened jaw appendages with an enamel coat of dentin and calcium with a pulpy matrix • Incisors bite off chunks, canines tear, and premolars and molars grind food • Saliva (from salivary glands) contains salivary amylase to begin carbohydrate digestion, bicarbonate to neutralize acids, and mucins to lubricate

  16. Food Breakdown in the Stomach • Stomach is a muscular sac that stores and mixes food, secretes substances that dissolve and degrade food, and controls the rate at which food enters the small intestine

  17. Food Breakdown in the Stomach • Gastric fluid: includes hydrochloric acid, pepsinogens, and mucus • HCl dissolves bits of food to form a soupy chyme, also converts pepsinogen (inactive) to pepsin (active) • Pepsin begins digestion of proteins

  18. Food Breakdown in the Stomach • Mucus and bicarbonate ions protect the stomach lining • If these are blocked, hydrogen ions stimulate release of histamine, which in turn stimulates release of more HCl, which may result in a peptic ulcer

  19. Food Breakdown in the Stomach • Peristaltic contractions churn the chyme and keep the sphincter of the stomach’s exit closed • Small amounts are released at regular intervals into the small intestine • http://www.youtube.com/watch?v=hpS5kMn_B0I&NR=1

  20. Digestion in the Small Intestine • 3 regions of SI • Duodenum • Jejunum • Ileum

  21. Digestion in the Small Intestine • Secretions from the liver, gallbladder, and pancreas enter via a common duct • Bile (stored in gallbladder) is a secretion consisting of bile salts, pigments, cholesterol, and lecithin • Bile salts speed up fat digestion and emulsification • Triglycerides tend to form large globules, but when smaller fat droplets become coated with bile salts, the negative charges on the droplets repel and cause them to stay separated

  22. Digestion in the Small Intestine

  23. Digestion in the Small Intestine • Contractions known as segmentation mix the chyme and are responsible for coating the fat droplets with bile salts

  24. Controls Over Digestion • Distention of the gut wall after a meal stimulates mechanoreceptors and their neurons, which respond with muscle action and enzyme secretion

  25. Controls Over Digestion • 4 gastrointestinal hormones play roles • Gastrin: produced by stomach lining, stimulates secretion of acids into the stomach • Cholecytokinin (CCK): enhances the actions of secretion and stimulates gallbladder contractions • Secretin: from intestinal lining stimulates insulin secretions from pancreas • GIP (glucose insulinotropic peptide): released in the presence of glucose and fat, stimulates insulin secretion

  26. Absorption From the Small Intestine • From structure to function • Lining of small intestine, mucosa, is not smooth but highly folded • Absorptive surface area increased by fingerlike projections of intestine lining called villi • Cells of villi bear even smaller microvilli • Different cell types secrete mucus, hormones, bicarbonate, or lysozyme • Brush border cells are covered with as many as 1,700 microvilli on their free surface in order to facilitate nutrient absorption

  27. Absorption From the Small Intestine

  28. Absorption From the Small Intestine • Water and Solute Absorption • Absorption of water depends on solute concentration gradients across lining of small intestine • Cotransporters passively shuttle Na+ from lumen into epithelial cells, along with simple sugars and amino acids

  29. Absorption From the Small Intestine • Water and Solute Absorption • An Na+ concentration gradient results, which sets up an osmotic gradient that attracts water from the lumen, into or between interstitial cells, and into blood capillaries in the villus

  30. Absorption From the Small Intestine • Water and Solute Absorption • Brush border cells have several kinds of sodium-dependent transporters • For example, SGLUT-1 transports glucose and sodium into cells together • SGLUT-2 helps glucose, galactose, and fructose diffuse out of brush border cells and into interstitial fluid

  31. Absorption From the Small Intestine • Fat Absorption • Fatty acids and monoglycerides diffuse across the bilayer of brush border cells aided by the bile salts • First products combine with bile salts to form micelles

  32. Absorption From the Small Intestine • Fat Absorption • Diffusion gradients favor movement of products out of micelles and into the epithelial cells of the mucosa • Triglycerides combine to form chylomicrons that leave the cell by exocytosis to enter the lymph vessels mainly

  33. Fat Absorption in SI

  34. Large Intestine • Large intestine: stores and concentrates feces (undigested and unabsorbed material, water, and bacteria) • Begins at cup shaped pouch at its junction with the small intestine (appendix attached here) • Draped across lower abdomen, ends in a rectum (feces storage) that opens to the outside through the anus

  35. Large Intestine

  36. Colon Function • Sodium actively transported out of colon • Sodium concentration drops, water concentration increases, sets up a gradient that results in water moving out by osmosis • Fiber (bulk) in diet is important in moving material in feces through the large intestine at the proper speed

  37. Colon Malfunction • Several factors, including stress and a low-bulk diet, can delay defecation, resulting in constipation • Fecal material lodged in appendix can lead to complications of appendicitis

  38. Colon Malfunction • Some people are genetically predisposed to develop colon polyps, which start as benign growths, but may later become cancerous • Colon cancer is highly curable when detected early enough • Diagnosis can come from a colonoscopy and a virtual colonoscopy, which uses X-rays and a computer to generate an image of the colon

  39. What Happens to Absorbed Organic Compounds • Nutrient molecules are shuffled and reshuffled once they have been absorbed • All cells continually recycle some carbohydrates, lipids, and proteins by breaking them apart • Cells use the products as energy sources and building blocks

  40. What Happens to Absorbed Organic Compounds • When you eat, excess carbohydrates and other organic compounds are converted to fat for storage in adipose tissue or converted to glycogen in the liver and muscle tissue

  41. What Happens to Absorbed Organic Compounds • Between meals, glucose levels are maintained by breakdown of glycogen reserves in the liver and amino acids are converted to glucose • Adipose cells degrade fats to glycerol and fatty acids, all which enter blood stream

  42. What Happens to Absorbed Organic Compounds • Liver stores, converts, and maintains the concentrations of required organic compounds in the blood • Inactivates most hormones, sending them to kidneys for excretion and it removes worn-out RBC and detoxifies chemicals

  43. Human Nutritional Requirements • USDA Dietary Recommendations • Nutritional guidelines have replaced “food pyramid” • Diet recommends lowering intake of refined grains, saturated fats, trans-fatty acids, added sugars or caloric sweetners, and no more than one teaspoon of full salt daily

  44. Human Nutritional Requirements • USDA Dietary Recommendations • More fruits and vegetables with high potassium and fiber content are encouraged with fat-free or low-fat milk products and whole grains; 55% of daily caloric intake is to come from carbohydrates

  45. Human Nutritional Requirements • Carbohydrate-Insulin Connection • Refined sugars and starches have high-glycemic index • Cause a surge in insulin for fast uptake of ingested sugar, which later leads to increased hunger • Excessive intake of high fructose corn syrup is concern because fructose does not stimulate same release of leptin from adipose cells as glucose does, leading to decrease in stomach’s ghrelin output • Calories are the same, hunger remains

  46. Ghrelin • Ghrelin is a hormone produced mainly by P/D1 cells lining the fundus of the human stomach and epsilon cells of the pancreas that stimulates hunger. Ghrelin levels increase before meals and decrease after meals. It is considered the counterpart of the hormone leptin, produced by adipose tissue, which induces satiation when present at higher levels. In some bariatric procedures, the level of ghrelin is reduced in patients, thus causing satiation before it would normally occur.

  47. Human Nutritional Requirements • Good fat, bad fat • Phospholipids and cholesterol are important components of membranes • Fats are energy reserves and provide insulating cushioning • Body needs very little polyunsaturated fat to supply essential fatty acids, those not made by the body itself

  48. Human Nutritional Requirements • Body building proteins • Of the 20 amino acids in proteins, 8 are essential • Most proteins in animal tissues are complete; their amino acid ratios match human nutritional needs • Nearly all plant proteins are incomplete, they lack one or more amino acids that are essential for humans

  49. Human Nutritional Requirements • Alternative Diets • Mediterranean diet emphasizes grains, fruits, and vegetables and limits fat to olive oil • Low-carb diets, which do not result in weight loss but may have undesirable side effects such as ketone production • Low-carb dieters are advised to select lean cuts of meats and fish over fattier, high protein foods

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