Carbohydrates

Question 1

What are the most abundant organic molecules in nature and our diet?

Answer 1

Carbohydrates (saccharide)

Question 2

What are some of the many functions of carbohydrates?

Answer 2

Dietary calories
Energy storage (glycogen)
Cell membrane intercellular communication

Question 3

What is the empiric formula of a carbohydrate?

Answer 3

(CH2O)n where n is greater than or equal to 3

Question 4

What 2 things are monosaccharides classified by?

Answer 4

1) Number of carbon atoms (3 carbon = triose; 4 carbon = tetrose; 5 carbon = pentose; 6 carbon = hexose; 7 carbon = heptose; 9 carbon = nonose)
2) Carbonyl group (aldose = aldehyde group; ketose = ketone group)

Question 5

Monosaccharides can be linked by ______ bonds, created by _____.

Answer 5

glycosidic bonds; glycosyltransferases

**1->4 connection makes a straight line ; 1->6 makes a branch

Question 6

What is a disaccharide? An oligosaccharide? A polysaccharide?

Answer 6

Di -> 2 sugars
Oligo -> 3-10 sugars
Poly -> 10+ sugars

Question 7

What is an enantiomer?

Answer 7

Mirror images of each other

-all sugars in humans are in D-configuration (-OH farthest from the carbonyl carbon is on the right -> “derecha”)

Question 8

How can a straight chain monosaccharide become cyclic?

Answer 8

The aldehyde/ketone in the chain will react with an -OH group in the same chain

• if -OH (on the anomeric carbon/C-1) above the plane structure -> alpha-C
• if -OH is below the plane structure -> beta-C

Question 9

What can a colorimetric test detect?

Answer 9

Reducing sugar in urine

-positive result -> pathology (because sugars are not normally present in urine)

Question 10

What are some important disaccharides?

Answer 10

• Lactose = galactose + glucose
• Sucrose = glucose + fructose
• Maltose = glucose + glucose

Question 11

What are some important polysaccharides?

Answer 11

• Branched glycogen (animal source)
• Starch (plant source)
• Unbranched cellulose (plant source)

Question 12

What is an N-glycosidic bond?

Answer 12

Sugar is bound to a nitrogen

-type of bond in ATP

Question 13

What is an O-glycosidic bond?

Answer 13

Sugar is bound to an oxygen

-type of bond in lactose

Question 14

What is a glycoprotein?

Answer 14

Sugar attached to a protein

-can also have N vs O linkage

Question 15

Where are carbohydrates digested?

Answer 15

Mouth and intestines (monosaccharides absorbed by small intestine)

Question 16

What are carbohydrates digested by?

Answer 16

Glycoside hydrolyses

1) Endoglycosidase - hydrolyze polysaccharides (cut from the center)
2) Disaccharidase - hydrolyze disaccharides into single sugar components

Question 17

What is alpha-amylase?

Answer 17

Salivary or pancreatic enzymes (different ones required because salivary one will be destroyed in the acidity of the stomach) that aid in digestion of carbohydrates

Question 18

What does salivary alpha amylase hydrolyze?

Answer 18

Salivary acts briefly on dietary starch and glycogen, hydrolyzing random alpha (1->4 bonds aka straight lines)

• will not break down branches (1->6
• product = DEXTRINS (short, branched oligosaccharides)

Question 19

T or F: Carb digestion halts temporarily in stomach, because high acidity inactivates salivary alpha-amylase.

Answer 19

True; neutralized by pancreas’ bicarbonate -> pancreatic alpha-amylase continues process in small intestine

Question 20

What are the key disaccharidases in upper jejunum (lining small intestine)?

Answer 20

1) Maltase - cleaves 1,4 maltose -> glucose + glucose
2) Sucrase - cleaves 1,2 sucrose -> glucose + fructose
3) Lactase - cleaves 1,4 lactose -> glucose + galactose
- all products are monosaccharides

Question 21

Where does intestinal absorption occur?

Answer 21

Mainly in duodenum and upper jejunum

Question 22

What is the mechanism of absorption in GI tract -> SI tissue?

Answer 22

• Glucose/Galactose: co-transport with Na+ through SGLT-1 (active, energy-dependent)
• Fructose: facilitated diffusion through GLUT-5 (Na+ independent transporter)

Question 23

What is the mechanism of absorption in SI tissue -> blood?

Answer 23

Glucose, fructose and galactose: facilitated diffusion through GLUT-2 transporter

Question 24

What happens if someone has a genetic or acquired disaccharidase deficiency?

Answer 24

Passage of undigested carbohydrate into large intestine -> osmotic gradient that draws water from tissues into large intenstine -> osmotic diarrhea

• Bacterial fermentation of carbohydrates occurs
• Creates large volume of CO2 and H2 gas -> abdominal cramps, diarrhea, flatulence
• Causes: genetic, intestinal disease, malnutrition, drugs that injure mucosa of small intestine

Question 25

What occurs in lactose intolerance?

Answer 25

Body lacks the enzyme lactase

• More than 70% of the world’s adults are lactose intolerant
• Up to 90% of African or Asian descent are affected