SM_143b: Absorption of Carbohydrates and Proteins

Question 1

Carbohydrates must be ____ before absorption

Answer 1

Carbohydrates must be hydrolyzed before absorption

Question 2

Starch is hydrolyzed by ____ of salivary and pancreatic secretions, which attacks interior 1,6-bonds

Answer 2

Starch is hydrolyzed by alpha amylase of salivary and pancreatic secretions, which attacks interior 1,6-bonds

Question 3

Simple sugars are broken down by ____ into monosaccharides on the brush border

Answer 3

Simple sugars are broken down by disaccharidases into monosaccharides on the brush border (contact digestion)

Question 4

Disaccharidases are located in the ____, have a short ____, and come from ____

Answer 4

Disaccharidases are located in the glycocalyx, have a short half life, and come from enterocytes

(short half life because destroyed by digestive proteins)

Question 5

Disaccharidases are subject to ____, which prevents the buildup of osmotically active materoals in the lumen

Answer 5

Disaccharidases are subject to end product feedback inhibition, which prevents the buildup of osmotically active materoals in the lumen

Question 6

Lactase converts ____ to ____ and ____

Answer 6

Lactase converts lactose to glucose and galactose

Question 7

Maltase breaks ____ into ____

Answer 7

Maltase breaks terminal 1,4 bonds of saccharides into glucose

Question 8

Isomaltase breaks ____ into ____

Answer 8

Isomaltase breaks terminal 1,6 bonds of saccharides into glucose

Question 9

Sucrase converts

____ to ____

____ to ____

Answer 9

Sucrase converts

• Sucrose to glucose and fructose
• Maltose to glucose and glucose

Question 10

Lactose intolerance results from ____

Answer 10

Lactose intolerance results from lactase deficiency

Question 11

Describe transport of glucose and galactose

Answer 11

Transport of glucose and galactose

• Transport requires simultaneous transport of Na+, which is moving down the electrochemical gradient established by basolateral Na+/K+ ATPase (2 sodium / 1 glucose)
• Glucose and galactose leave the cell by facilitated diffusion at the basolateral membrane and diffuse into capillaries
• Glucose and galactose absorption is negligible in absence of Na+ transport
• Glucose absorption involves substantial water and electrolye absorption (useful for rehydration)

Question 12

GLUT1, GLUT2, GLUT3, GLUT4, and GLUT5 transport hexoses ____

Answer 12

GLUT1, GLUT2, GLUT3, GLUT4, and GLUT5 transport hexoses down a concentration gradient

Question 13

SGLUT1 transports hexoses ____

Answer 13

SGLUT1 transports hexoses down a concentration gradient using energy provided by an electrochemical gradient of sodium, which is co-transported

Question 14

Describe the affinities of the transporters for glucose and other hexoses

Answer 14

Affinities of the transporters for glucose and other hexoses

• GLUT1, GLUT3, GLUT4 have high affinity for glucose and function at maximal rate under physiological glucose
• GLUT2 has low affinity for glucose, allowing alteration of transport rate in proportion to increased glucose after meals
• GLUT5 has high affinity for fructose

Question 15

Fructose absorption occurs via ____

Answer 15

Fructose absorption occurs via facilitated diffusion

(goes down concentration gradient but carrier is specific for fructose)

Question 16

Hydrolysis of proteins is initiated by ____ followed by ____

Answer 16

Hydrolysis of proteins is initiated by gastric pepsin followed by pancreatic proteases

(yielding amino acids and small polypeptides)

Question 17

Pepsinogen becomes ____

Answer 17

Pepsinogen becomes pepsin

(via H+ and pepsin itself)

Question 18

Trypsinogen becomes ____

Answer 18

Trypsinogen becomes trypsin

(via enterokinase and trypsin itself)

Question 19

Chymotrypsinogen becomes ____

Answer 19

Chymotrypsinogen becomes chymotrypsin

(via trypsin)

Question 20

Final hydrolysis of proteins is by ____

Answer 20

Final hydrolysis of proteins is by brush border peptidases

• Yields tripeptides, dipeptides, and amino acids
• Peptides hydrolyzed to amino acids in enterocytes
• Peptidases are subject to end product inhibition

Question 21

Amino acids are absorbed by ____ and ____ which include ____, ____, and ____

Answer 21

Amino acids are absorbed by Na+-co-transport and 3 carrier systems, which include neutral system I, basic system, and neutral system II

• Neutral system I: for neutral amino acids
• Basic system: for basic amino acids (e.g. arginine, ornithine, and lysine)
• Neutral system II (imino system): for proline, hydroxyproline, sarcosine, and other N-methyl substituted glycines

Question 22

___ is released by the presence of fat and protein in the small intestine and helps decrease gastric emptying

Answer 22

CCK is released by the presence of fat and protein in the small intestine and helps decrease gastric emptying

Question 23

The gastric zymogen pepsinogen is converted to the active enzyme pepsin by ___

Answer 23

The gastric zymogen pepsinogen is converted to the active enzyme pepsin by gastric HCl

Question 24

Describe general mechanisms of digestion and absorption

Answer 24

General mechanisms of digestion and absorption

Question 25

Describe carbohydrate digestion and absorption

Answer 25

Carbohydrate digestion and absorption

Question 26

Describe digestion in the mouth

Answer 26

Digestion in the mouth

• Mastication and high water content of saliva increase surface area and solubility of food articles to enhance the ability of digestive enzymes to work
• Starch digestion begins through action of salivary a-amlyase

Question 27

Alpha amylase converts starch to ____, ____, and ____

Answer 27

Alpha amylase converts starch to maltose, maltotriose, and alpha dextrins

(hydrolyzes internal a-1,4-linkages)

Question 28

Describe the pancreatic / intestinal phase of carbohydrate digestion

Answer 28

Pancreatic / intestinal phase of carbohydrate digestion

1. Stomach contents reach small intestine
2. Acid is neutralized by bicarbonate secreted by pancreas: pancreatic alpha amylase has same specificity as salivary alpha amylase but total activity of pancreatic enzyme is much greater
3. Further digestion of oligosaccharides occurs by enzymes located on the brush border membrane of epithelial cells lining the small intestine: activity is highest in upper jejunum and gradually declines through the rest of the small intestine

Question 29

____ and ____ have the highest capacity to absorb sugars

Answer 29

Duodenum and upper jejunum have the highest capacity to absorb sugars

Question 30

Monosaccharides are absorbed by ____ and ____

Answer 30

Monosaccharides are absorbed by active transporters (major mechanism) and diffusion

• Only glucose, galactose, and fructose

Question 31

Describe the mechanism of digestion of carbohydrates to monosaccharides and absorption

Answer 31

Mechanism of digestion of carbohydrates to monosaccharides and absorption

Question 32

SGLUT1 is expressed in ____

Answer 32

SGLUT1 is expressed in the intestinal mucosa

Question 33

GLUT2 is found in ____

Answer 33

GLUT2 is found in pancreatic beta cells

Question 34

GLUT4 is ____

Answer 34

GLUT4 is insulin-responsive

Question 35

Phases of protein digestion include ____, ____, and ____

Answer 35

Phases of protein digestion include gastric, pancreatic, and intestinal

• Essentially all ingested prrotein is digested and absorbed
• Ingested protein is almost completely absorbed by the time the meal has traversed the jejunum

Question 36

Describe the phases of digestion and absorption of protein and its degradative products

Answer 36

Phases of digestion and absorption of protein and its degradative products

Question 37

In the gastric phase, pepsinogen secreted by stomach mucosal cells is converted to ___ by ___

Answer 37

In the gastric phase, pepsinogen secreted by stomach mucosal cells is converted to active enzyme pepsin by acidic pH

• Pepsin hydrolyzes 15% of dietary protein
• Pepsin (endopeptidase): cleaves polypeptide chains of proteins into large peptide fragments

Question 38

Describe the pancreatic phase of protein digestion

Answer 38

Pancreatic phase of protein digestion

1. CCK and secretin released from intestinal endocrine cells
2. Stimulate secretion of propeptidases from pancreas into small intestine and release of NaHCO3 to neutralize stomach acid
3. Inactive propeptidases converted to active peptidases in intestine

Question 39

Describe process of enteropeptidase

Answer 39

Enteropeptidase

1. CCK stimulation
2. Enteropeptidase secreted by intestinal mucosal epithelial cells into lumen of small intestine
3. Converts trypsinogen to trypsin
4. Trypsin converts other trypsinogen molecules and other inactive proenzymes into the active forms (endopeptidases, exopeptidases)
5. Endopeptidseases break down large peptide fragments to smaller peptides
6. Carboxypeptidases remove amino acids sequentially from carboxy terminal
7. Free amino acids and small peptides produces

Question 40

In the small intestinal phase of protein digestion, ____ on the luminal surface of intestinal epithelial cells remove amino acids sequentially from the NH2 terminal end of peptides, producing free amino acids and di- and tri-peptides

Answer 40

In the small intestinal phase of protein digestion, aminopeptidases on the luminal surface of intestinal epithelial cells remove amino acids sequentially from the NH2 terminal end of peptides, producing free amino acids and di- and tri-peptides

Question 41

Describe protein digestion and absorption

Answer 41

Protein digestion and absorption