Lecture 7 Review of CHO Digestion & Absorption

Question 1

What are the two major steps of CHO metabolism?

Answer 1

1. intraluminal hydrolysis (amylases)
2. Membrane digestion (brush border glycohydrases)

Question 2

Basic functions that occur in intraluminal hydrolysis

Answer 2

• intial digestion - salivary 𝝰-amylase & continues with pancreatic 𝝰-amylases (endosaccharidases)
• Salivary 𝝰-amylase is deactivated by stomach acid
• Pancreatic juices in SI neutralize acid, 𝝰-amylase continues hydrolysis

Question 3

Basic function of membrane digestion

Answer 3

Maltose, maltotriose (isomaltose), trisaccharides, oligosaccharides, and 𝝰-limit dextrins (ex. starches) require further breakdown

Question 4

Basic steps of complex CHO digestion

Answer 4

1. Intraluminal hydrolysis of poly- & oligosaccharides to oligosaccharides
2. Brush border membrane digestion of oligosaccharides to tri-, di and monosaccharides (absorbed)
3. Brush border membrane digestion of remaining disaccharides, and other disaccharides (sucrose, lactose, trehalose) to monosaccharides (absorbed)

Question 5

Digestion of amylose and amylopectin in the mouth

Answer 5

𝝰-1,4 glycosidic bonds only breaking down linear structure but not branches
* Amylose: salivary glands release salivary 𝝰-amylase, which hydrolyzes 𝝰-1,4 glycosidic bonds in amylose, forming dextrins
* Amylopectin: salivary glands release salivary 𝝰-amylase, which hydrolyzes 𝝰-1,4 glycosidic bonds in amylose, forming dextrins

Question 6

Digestion of amylose and amylopectin in the stomach

Answer 6

• Amylose: Acidity of gastric juice destroys the enzymatic activity of salivary 𝝰-amylase. The dextrins pass unchanged into the SI.
• Amylopectin: Acidity of gastric juice destroys the enzymatic activity of salivary 𝝰-amylase. The dextrins pass unchanged into the SI.

Question 7

Digestion of amylose and amylopection in the SI

Answer 7

𝝰-1,4 glycosidic bonds only breaking down linear structure but not branches
* Amylose: The pancreas releases pancreatic 𝝰-amylase in the SI, which hydrolyzes 𝝰-1,4 glycosidic bonds. Dextrins are broken down into maltose.
* Amylopectin: The pancreas releases pancreatic 𝝰-amylase in the SI, which hydrolyzes 𝝰-1,4 glycosidic bonds. Dextrins are broken down into maltose and limit dextrins

Question 8

Digestion of Amylose and Amylopection on the brush border of the SI

Answer 8

𝝰-1,4 glycosidic bonds only
* Amylose: Maltose is hydrolyzed by maltase, a brush border enzyme, forming free glucose.
* Amylopectin: Maltose is hydrolyzed by maltase, a brush border enzyme, forming free glucose. The 𝝰-1,6 glycosidic bonds in limit dextrins are hydrolyzed by 𝝰-dextrinase, forming glucose.

Question 9

What are the products of amylose and amylopection after hydrolysis by 𝝰-amylase (salivary & pancreatic)?

Answer 9

• Maltotriose & maltoase result from amylose hydrolysis
• α-limit dextrins (or α-dextrins), along with maltose and maltotriose, are final products of amylopectin hydrolysis

Question 10

amylase levels throughout growth

Answer 10

Both salivary and pancreatic amylase levels are low at birth but typically reach adult levels before 1 year of age.

Question 11

What is the action of intestinal surface membrane oligosaccharides & disaccharides on a remaining 𝝰-dextrin from amylopectin?

Answer 11

• Remove glucose units from the nonreducing end (exoglucosidases) whereby Glc is readily absorbed since transporters are most concentrated near the enzyme complexes

Question 12

Fructose

• typical source
• bonds
• brush-border membrane enzymes
• monosaccharide products

Answer 12

• typical source: fruit & honey
• bonds: non
• brush-border membrane enzymes: none
• monosaccharide products: fructose

Question 13

Glucose

• typical source
• bonds
• brush-border membrane enzymes
• monosaccharide products

Answer 13

• typical source: fruit, honey, & grapes
• bonds: none
• brush-border membrane enzymes: none
• monosaccharide products: glucose

Question 14

Amylopectin

• typical source
• bonds
• brush-border membrane enzymes
• monosaccharide products

Answer 14

• typical source: potatoes, rice, corn, & bread
• bonds: 𝝰-1,4 linear & 𝝰-1,6 branches
• brush-border membrane enzymes: maltase, glucoamylase, isomaltase
• monosaccharide products: glucose

Question 15

Amylose

• typical source
• bonds
• brush-border membrane enzymes
• monosaccharide products

Answer 15

• typical source: potatoes, rice, corn, & bread
• bonds: 𝝰-1,4 linear
• brush-border membrane enzymes: maltase, glucoamylase
• monosaccharide products: glucose

Question 16

Sucrose

• typical source
• bonds
• brush-border membrane enzymes
• monosaccharide products

Answer 16

• typical source: table sugar and dessert
• bonds: 𝝰-1,2
• brush-border membrane enzymes: sucrase
• monosaccharide products: glucose & fructose

non-reducing

Question 17

Trehalose

• typical source
• bonds
• brush-border membrane enzymes
• monosaccharide products

Answer 17

• typical source: young mushrooms
• bonds: 𝝰-1,1
• brush-border membrane enzymes: trehalase
• monosaccharide products: Glucose

Question 18

Lactose

• typical source
• bonds
• brush-border membrane enzymes
• monosaccharide products

Answer 18

• typical source: milk & milk products
• bonds: β-1,4
• brush-border membrane enzymes: lactase
• monosaccharide products: glucose and galactose

Question 18

Lactose

• typical source
• bonds
• brush-border membrane enzymes
• monosaccharide products

Answer 18

• typical source: milk & milk products
• bonds: β-1,4
• brush-border membrane enzymes: lactase
• monosaccharide products: glucose and galactose

Question 19

Maltose

• typical source
• bonds
• brush-border membrane enzymes
• monosaccharide products

Answer 19

• typical source: grains, sweet potato
• bonds: 𝝰-1,4 glucose
• brush-border membrane enzymes: maltase
• monosaccharide products: glucose

Question 20

Galactose

• typical source
• bonds
• brush-border membrane enzymes
• monosaccharide products

Answer 20

• typical source: milk products as lactose
• bonds: none
• brush-border membrane enzymes: none
• monosaccharide products: galactose

Question 21

Absorption of glucose and galactose

Answer 21

glucose and galactose are transported into enterocytes via energy requiring, active transport mechanisms
* transported from lower to higher concentration.
* cotransport with Na (renewed via Na-K ATPase)

Question 22

Absorption of fructose

Answer 22

fructose is transported into enterocytes via facilitated diffusion
* transported from high to lower concentration

Question 23

What monosaccharide has a higher glycemic index?

Answer 23

fructose because it is more readily absorbed since it takes less energy

Question 24

Where do monosaccharides go once absorbed?

Answer 24

Hepatic portal vein transports the absorbed nutrients to the liver
* enter capillary from entercyte via transport pathways located in the apical membrane of the SI villous epithelial cells

Question 25

Absorption of monosaccharides in low-sugar meal

Answer 25

hexoses enter enterocytes via SGLT1 and GLUT5, and then exit the cell via GLUT2 to enter bloodstream

Question 26

Absorption of monosaccharides in sugar-rich meals

Answer 26

Excess monosaccharides saturates SGLT1 and GLUT5 which can result in the recruitment of GLUT2 into the apical membrane from its intracellular pool increasing the uptake by enterocytes to faciliate absorption. GLUT2 apical translocation is maximal within 10 min after consumption, then is internalized back and becomes similar to low-sugar meal.

Question 26

What does the rate of CHO assimilation depend on?

Answer 26

1. enzymatic hydrolysis at the brush border membrane - how quickly we can get it into monosaccharide form
2. How quickly the monosaccharide can be transported

How CHO is presented dictates how well this occurs

Question 27

What is the RLS of CHO digestion?

Answer 27

Transport across the epithelium

Question 28

What happens to undigested lactose?

Answer 28

fermented by bacteria (hypolactasia)
* diagnosed by hydrogen breath test or stoll acidity test
* hydrolysis defect not absorption defect
* experience bloating, watery diarrhea and malasborption