Carbohydrates and Protein Digestion

Question 1

what is digestion

Answer 1

enzymatic conversion of complex dietary substances to a from that can be absorbed

Question 2

how do most digestive processes in the small intestine occur

Answer 2

luminal digestion

membrane digestion

Question 3

how is luminal digestion mediated

Answer 3

by pancreatic enzymes secreted into duodenum

Question 4

how is membrane digestion mediated

Answer 4

by enzymes situated at the brush border of epithelial cells (pepsin)

Question 5

what is absorption

Answer 5

the processes by which the absorbable products of digestion are transferred across both apical and basolateral membranes of enterocytes

Question 6

what are enterocytes

Answer 6

absorptive cells of intestinal epithelium

Question 7

what is assimilation

Answer 7

overall process of digestion and absorption

Question 8

digestion of carbohydrates

Answer 8

all dietary carbohydrates must be converted form polysaccharide to oligosaccharide and finally to monosaccharides for absorption

Question 9

conversion of polysaccharide to oligosaccharide

Answer 9

intraluminal hydrolysis of polysaccharide to oligosaccharide

via alpha-amylase (salivary and pancreatic glands)

Question 10

conversion of oligosaccharides to monosaccharides

Answer 10

membrane digestion (at brush border)

Question 11

function of alpha-amylase

Answer 11

endoenzyme;
breaks down linear internal alpha-1,4 but not terminal α-1,4 linkages - no production of glucose
cannot cleave α-1,6 linkages at branch points (in amylopectin) or α-1,4 linkages adjacent to branch points

Question 12

products of alpha-amylase

Answer 12

linear glucose oligomers (maltotriose, maltose)

α-limit dextrins

Question 13

what are oligosaccharidases

Answer 13

integral membrane proteins with catalytic domain that faces the lumen of the GI tract
e.g. lactase, maltose

Question 14

role of lastase

Answer 14

has one substrate - breaks down lactose to glucose and galactose

Question 15

role of oligosaccharidases (exception of lactase)

Answer 15

cleave terminal alpha-1,4, linkages of maltose, maltotriose and alpha-limit dextrins (to yield glucose)
hydrolysis reactions performed by maltase, sucrose and isomaltase occur at a faster rate than subsequent transport of the released monomers, but for lactase the rate of hydrolysis is rate limiting in assimilation

Question 16

special role of maltase

Answer 16

degrade the alpha-1,4 linkages in straight chain oligomers up to 9 monomers in length

Question 17

special role of sucrase

Answer 17

specifically responsible for hydrolysing sucrose to glucose and fructose

Question 18

special role of isomaltase

Answer 18

only enzyme that splits the branching alpha-1,6 linkages of alpha-limit dextrins

Question 19

final products of carbohydrate digesiton

Answer 19

monosaccharides;
glucose
galactose
fructose

Question 20

where are final products of carbohydrate digestion absorbed

Answer 20

duodenum and jejunum

Question 21

process of absorption of the final products of carbohydrate

Answer 21

2 steps;
entry and exit from enterocytes via apical and basolateral membranes, respectively
exit for all monosaccharides is mediated by facilitated diffusion by GLUT2

Question 22

how are glucose and galactose absorbed

Answer 22

via secondary active transport mediated by SGLT1;

sodium dependent

Question 23

how are fructose absorbed

Answer 23

via facilitated diffusion mediated by GLUT5

Question 24

how are oligopeptides transported across the apical membrane

Answer 24

via the H+/oligopeptide co-transporter, PepT1

Question 25

where are oligopeptides hydrolysed

Answer 25

within cytoplasm

Question 26

how are oligopeptides hydrolysed

Answer 26

hydrolysed to amino acids via peptidases within enterocyte

Question 27

how do amino acids exit the enterocyte

Answer 27

via the basolateral membrane by sodium independent transporters