Digestion And Absorption Of Carbohydrates

Front 1

What are the three common monosaccrarides?

Back 1

Glucose (an aldose)
Galactose (an aldose)
Fructose (a ketose)

Front 2

How is a sugar determined to be in the D or L form?

Back 2

Based on the position of the OH group farthest from the carbonyl carbon
D sugars - OH group on right
L sugars - OH group on right
Most sugars present in human tissues belong to D series

Front 3

Why do the open chains of glucose, fructose and many other sugars in solution cyclize into ring forms?

Back 3

The ring forms are energetically more stable

The oxygen that was on the OH group is now part of the ring
The original carbonyl carbon contains an OH group and becomes the anomeric carbon

Front 4

What is the difference between an alpha and beta anomer of glucose?

Back 4

Alpha: OH group at C-1 is on opposite side of the ring from the CH2OH
Beta: OH group at C-1 is on same side of ring as CH2OH

For fructose, OH group attached at C-2 determines alpha or beta

Front 5

What is starch and what are the two forms?

Back 5

Starch is a polysaccharide of glucose
1. Amylose - unbranched form, and glucose units form a straight chain linked via alpha 1,4-glycosidic bonds
2. Amylopectin - branched form, and alpha 1,4-chains contain branches connected via alpha 1,6-glycosidic bonds

Front 6

What is sucrose?

Back 6

Disaccharide of glucose and fructose
Natural sweetener found in fruits and vegetables

Front 7

What is dietary fiber?

Back 7

Plant polysaccharides and a polymer called lignan
Part of diet that cannot be digested by human enzyme
In colon, can be converted to gases (H2, CO2, CH4) and short-chain fatty acids by bacteria

Front 8

What are pectin and cellulose?

Back 8

Pectin: (a soluble fiber) slows the movement of food
through GI tract, allowing better digestion and absorption of nutrients
Cellulose: increases the rate at which digestion products
pass through the large intestines, may minimize exposure to toxins in our diet

Front 9

What is lactose?

Back 9

Major dietary carb of animal origin
Disaccharide of glucose and galactose

Front 10

What enzymes are responsible for the digestion of starch?

Back 10

Salivary alpha amylase hydrolyzes starch to alpha dextrins
Pancreatic alpha amylase (secreted by exocrine pancres into lumen of SI) continues hydrolysis of alpha dextrins, converting them to disaccharides (maltose), trisaccharides (maltotriose) and limited dextrins

Front 11

How are disaccharides such as lactose and sucrose digested into monosaccharides?

Back 11

Disaccharidases attached to the membrane surface of the brush border (microvilli)
Include alpha-glucosidase, sucrase-isomaltase, lactase, and trehalase

Front 12

How are glucose, galactose, and fructose absorbed?

Back 12

Transported into absorptive epithelial cells of SI by Na+ dependent active transport and facilitated diffusion
Monosaccharides transported from these cells to blood and circulate to liver and peripheral tissues, where taken up by facilitated transporters (GLUT 1-5)

Front 13

What is the action of amylase?

Back 13

Endoglucosidase that hydrolyzes internal alpha-1,4 bonds
Does not cleave the branched alpha-1,6 bond at branch point and has little activity for the alpha-1,4 bond at the non-reducing end of the chain

Front 14

Describe the action of the four types of disaccharidases?

Back 14

1. Alpha-glucosidase (maltase) - exoglycosidase digests maltose, maltotriose, limited dextrin on alpha-1,4 bond, begins at non-reducing end to yield glucose
2. Sucrase-isomaltase - hydrolyzes sucrose, maltose, and maltotriose to yield glucose and fructose
3. Trehalase - digests trehalose into glucoses (glucose + glucose linked by alpha-1,1 bond)
4. Lactase - split the beta-1,4 bond b/w glucose and galactose

Front 15

What is lactose intolerance and how is it caused?

Back 15

Condition of pain, nausea, and flatulence after ingestion of foods containing lactose
Caused by deficiency of lactase (primary) or intestinal ijury (secondary)

Front 16

How is the transport of glucose and galactose an indirect active process?

Back 16

Glucose and galactose are pumped into cell through Na+-dependent transporter located on luminal side
Na+-K+ ATPase on serosal (blood) side of cell removes Na+ from the cells in exchange for K+, with concomitant hydrolysis of ATP

Front 17

How does fructose enter and leave cells?

Back 17

Enters cells by facilitated diffusion via GLUT 5 located at luminal side
Exported by GLUT 2 located at serosal side

Front 18

What is the tissue distribution for each of the five facilitative transporters?

Back 18

GLUT 1 - RBCs, blood-brain, retinal barrier
GLUT 2 - serosal side of intestinal epithelial cells, liver, kidney, pancreas
GLUT 3 - brain (neuron)
GLUT 4 - adipose tissue, skeletal muscle
GLUT 5 - luminal side of intestinal epithelial cells

Front 19

Why for most cell types is the rate of glucose transport not rate-limiting for glucose metabolism?

Back 19

Low Km of transporter proteins

GLUT 2 has high Km so liver will convert glucose to other storage forms only when blood glucose levels are high

Front 20

What stimulates glucose transport in muscle and adipose tissue?

Back 20

Insulin - increased translocation of GLUT 4 to the plasma membrane

Front 21

What happens in fructose metabolism?

Back 21

Primarily in liver
Conversion to G3P and DHP, intermediates of glycolysis
G3P and DHP can proceed through it to pyruvate, TCA cycle and fatty acid synthesis
GSP and DHP can also be converted o glucose by gluconeogenesis

Front 22

What is the function of aldolase B?

Back 22

Rate-limiting step of fructose metabolism
Low affinity and is very slow in cleaving fructose-1-phosphate
Indigestion of high dose of fructose leads to accumulation of fructose-1-phosphate in liver (slowly converted to glycolytic intermediates)

Front 23

What is hereditary fructose intolerance?

Back 23

Aldolase B deficiency
Causes inhibition of glycogenolysis and gluconeogenesis, leading to hypoglycemia
Accumulation of fructose-1-phosphate depletes phosphate pools, leading to converting AMP to IMP, which is degraded to uric acid

Front 24

What happens in galactose metabolism?

Back 24

Galactose converted of UDP-galactose and then glucose-1-phosphate (expense of 1 ATP)
Glucose-1-phosphate then converted to glucose

Front 25

What is classical galactosemia? Nonclassical?

Back 25

Classical: deficiency of galactose-1-phosphate uridyl transferase
Nonclassical: deficiency of galactokinase

Front 26

What are the presentations for galactosmia?

Back 26

Infants fail to develop
Vomit, diarrhea after consuming milk
Enlargement of liver and jaundice
Cataracts will form (galactitol or dulcitol)
Lethargy and retarded mental development