Digestion, Absorption and Transport of Carbohydrates

Question 1

What is the enzyme involved in salivary digestion?

Answer 1

Salivary α-amylase (α-1,4 endoglucosidase)

Question 2

What does salivary α-amylase (α-1,4 endoglucosidase) hydrolyze?

Answer 2

The interior α-1,4 bonds between glucose residues within amylopectin, amylose, and glycogen, converting the large polysaccharides to smaller entities.

Question 3

What are the products after salivary α-amylase hydrolyzes amylose?

Answer 3

Maltose (2 C’s), Maltotriose (3 C’s), and oligosaccharides (4-9 C’s and is the most common result)

Question 4

What isomeric form are naturally occurring sugars?

Answer 4

D-isomers

Question 5

What isomeric form are enzymes?

Answer 5

L-isomers

Question 6

Does the α configuration have the -OH group up or down?

Answer 6

Downwards (while beta is opposite)

Question 7

What is sucrose composed of?

Answer 7

It is composed of glucose linked to fructose by α,β-1,2 bonds. (Not reducing sugar because both anomeric carbons are linked)

Question 8

What is lactose composed of?

Answer 8

It is composed of galactose linked to glucose by β-1,4 bonds. (It is reducing because the anomeric carbon on glucose is free)

Question 9

What is trehalose composed of?

Answer 9

It is composed of two glucose molecules linked by their anomeric carbons, αα -1,1). (Not reducing)

Question 10

What are the products after salivary α-amylase hydrolyzes amylopectin?

Answer 10

α-dextrin, maltotriose, and maltose

Question 11

What carbohydrates are hydrolyzed in the stomach?

Answer 11

Amylase is denatured and there may be some limited acid (nonenzymatic) hydrolysis of sucrose to yield fructose and glucose.

Question 12

What does cholecystokinin do?

Answer 12

It acts on pancreas to release digestive enzymes including pancreatic α-amylase.

Question 13

What does secretin do?

Answer 13

It causes pancreas to release bicarbonate to neutralize pH.

Question 14

What are final products of pancreatic α-amylase?

Answer 14

Same as salivary amylase. Maltose, maltotriose and branched dextrins.

Question 15

Where is lactose broken down and what breaks it down?

Answer 15

The brush-border membrane of epithelial cells in the intestinal villi and by the lactase (β-galactosidase) which is specific for the β-1,4 bond between galactose and glucose in lactose

Question 16

Where is trehalose broken down and what breaks it down?

Answer 16

The brush-border membrane of epithelial cells in the intestinal villi and by trehalase which hydrolyzes the (α, α -1, 1bond) to yield two molecules off glucose.

Question 17

Where are oligosaccharides, maltose, and maltotriose broken down and what breaks it down?

Answer 17

The brush-border membrane of epithelial cells in the intestinal villi and the glucoamylase complex cleaves the α-1,4 linkages between glucose units in oligosaccharides, starting from the nonreducing end of the chains.

Question 18

Where is sucrose broken down and what breaks it down”

Answer 18

The brush-border membrane of epithelial cells in the intestinal villi and sucrase-isomaltase. The sucrase site hydrolyzes sucrose to glucose and fructose plus maltase activity, while the isomaltase site hydrolyzes the α-1,6 bonds between glucose residues plus maltase activity.

Question 19

How is starch linked together?

Answer 19

Starch is a polymer of α-D-glucose units linked together through α-1,4 and α-1,6 glycosidic bonds. All starches are mixtures of amylose and amylopectin.

Question 20

How is glycogen linked together?

Answer 20

Glycogen, the carbohydrate storage form in animals, is a polymer of α-D-glucose units linked together through α-1,4 and α-1,6 glycosidic bonds.

Question 21

How is cellulose linked together?

Answer 21

Cellulose is an unbranched polymer of β-D-glucose units linked together through β-1,4 bonds.

Question 22

What are the active transporters for the intestinal epithelial cells?

Answer 22

SGLT-1 is the protein that allows glucose and galactose to be transported into intestinal epithelial cells by an active, energy-dependent process that requires a concurrent uptake of sodium ions. (SGLT-2 is for the kidneys)

Question 23

What are the GLUT transporters?

Answer 23

GLUTs are all transmembrane proteins that alternate between conformations in order to bind and release a monosaccharide on opposite sides of the membrane.

Question 24

What are the GLUT-1 details?

Answer 24

It’s expressed in cell types with barrier functions. They are found in RBC’s and the blood-brain barrier.

Question 25

What are the GLUT-2 details?

Answer 25

They’re found in the liver, Pancreatic β-cells, and contraluminal membrane of intestinal epithelial cells. They accept all 3 monosaccharides and help them pass out of cell after SGLT-1 allows them in.

Question 26

What are the GLUT-3 details?

Answer 26

Major transporter in the CNS and is found in brain (neurons)

Question 27

What are the GLUT-4 details?

Answer 27

Found in adipose tissue, skeletal and heart muscle. It is insulin-sensitive transporter.

Question 28

What are the GLUT-5 details?

Answer 28

Found in intestinal epithelial cells and sperm. It’s a fructose transporter.

Question 29

What is the time table for blood glucose levels?

Answer 29

Within 30-60 minutes after a meal, the blood glucose levels reach a peak and then decrease to fasting levels in two hours. The disposition of blood glucose is dependent on its utilization in biochemical processes.