DIGESTION AND ABSORPTION

Question 1

Starch

Answer 1

• the most digestible carbohydrate
• cellulose is indigestible
• starch is first digested to:

1. Oligosaccharides up to eight glucose residues long
2. Then into the disaccharide maltose
3. Finally to glucose which is absorbed by the small intestine

Question 2

Salivary amylase

Answer 2

• hydrolyzes starch into oligosaccharides
• amylase works best at pH of 6.8 to 7.0 of oral cavity
• amylase quickly denatured on contact with stomach acid and digested by pepsin
• salivary amylase stops working in stomach at pH less than 4.5–50% of dietary starch digested before it reaches small intestine

Question 3

Lactose Intolerance

Answer 3

• lactose passes undigested into large intestine
• increases osmolarity of intestinal contents
• causes water retention in the colon and diarrhea
• gas production by bacterial fermentation of the lactose

Question 4

Sodium–glucose transport (SGLT) proteins

Answer 4

• 80% of absorbed sugar is glucose
• glucose is transported out the base of absorptive cell into ECF by facilitated diffusion
• sugar entering ECF increases its osmolarity
• draws water osmotically from the lumen of the intestine, through now leaky tight junctions between epithelial cells
• water carries more glucose and other nutrients with it by solvent drag
• SGLT absorbs galactose, fructose is absorbed by facilitated diffusion
• glucose, galactose, and any remaining fructose are transported out of the base of the cell by facilitated diffusion
• absorbed by blood capillaries in the villus
• hepatic portal system delivers them to the liver

Question 5

Proteins

Answer 5

• proteases (peptidases)—enzymes that digest proteins
• begin their work in the stomach in optimum pH of 1.5 to 3.5
• pepsin digests 10% to 15% of dietary protein into shorter peptides and some free amino acids
• pepsin inactivated when it passes into the duodenum and mixes with the alkaline pancreatic juice (pH 8)
• pancreatic enzymes trypsin and chymotrypsin take over the process
• hydrolyzing polypeptides into even shorter oligopeptides

Question 6

Lipids

Answer 6

• lipases—fat-digesting enzymes
• lingual lipase secreted by the salivary glands of the tongue
• inactive in mouth, but more active in stomach along with gastric lipase (infants)
• 10% to 15% of lipids digested before reaching duodenum
• pancreatic lipase: in the small intestine; digests most of the fats
• fat enters duodenum as large globules exposed to lipase only at their surface
• globules broken up into smaller emulsification droplets by certain components of bile
• lecithin and bile acids

Question 7

Nucleic Acids

Answer 7

nucleases (deoxyribonuclease and ribonuclease)from pancreatic juice hydrolyze DNA and RNA to nucleotides

nucleosidases and phosphatases of brush border split them into phosphate ions, ribose or deoxyribose sugar, and nitrogenous bases

transported into intestinal epithelium by membrane carriers and enters the blood

Question 8

Vitamins

Answer 8

1.fat-soluble vitamins: A, D, E, and K absorbed with other lipids

-if they are ingested without fat-containing food, they are not absorbed at all, but are passed in the feces and wasted

2.water-soluble vitamins, B complex and C, absorbed by simple diffusion and B₁₂ if bound to intrinsic factor from the stomach

Question 9

Minerals

Answer 9

• absorbed all along small intestine
• Na⁺ cotransported with sugars and amino acids
• Cl⁻ exchanged for bicarbonate reversing chloride
• bicarbonate exchange that occurs in the stomach
• iron and calcium absorbed as needed
• iron absorption is stimulated by liver hormone hepcidin

Question 10

Water

Answer 10

• digestive tract receives about 9 L of water/day
• 0.7 L in food, 1.6 L in drink, 6.7 L in gastrointestinal secretions
• 8 L is absorbed by small intestine and 0.8 L by large intestine
• 0.2 L voided in daily fecal output