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Carbohydrates > Carbohydrates > Flashcards

Carbohydrates Flashcards

1
Q

Carbohydrates (5)

-characteristics

A
  1. most abundant organic molecule in nature
  2. important source of energy
  3. cell- membrane components mediating intercellular communication
  4. part of body’s extracellular substance
  5. part of RNA, DNA and several coenzymes
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2
Q

Carbohydrates

-they are:

A
  • polyhydroxy aldehydes or ketones

- substances that yield such compounds on hydrolysis

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3
Q

Types of carbohydrates

A
  • monosaccharides –> the simplest
  • oligosaccharides –> consist of few monosaccharides (disaccharides)
  • polysaccharides –> contain many monosaccharides
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4
Q

Monosaccharides (3)

A
  • colorless, insoluble in nonpolar solvents, soluble in water, most have a sweet taste
  • backbones are usually unbranched carbon chains
  • one of the carbon is double- bonded to an oxygen atom to from a carbonyl group (aldose, ketose)
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5
Q

Monosaccharides nomenclature

A

-carbon number + functional group
ex: glucose –> referred as an aldohexose
fructose –> referred as an ketohexose

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6
Q

Isomers

A

-have the same molecular formula but different structures, two classes: stereoisomers and constitutional isomers (different orders of attachment of atoms)

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7
Q

Stereoisomers

A
  • atoms are connected in the same orders but different conformations
  • Enantiomers or diastereoisomers
  • ex: glyceraldehyde
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8
Q

D and L configuration (3)

A
  • configuration of the chiral center (carbon with 4 different groups) most distant from the carbonyl carbon
  • if the lowest OH groups is on the RIGHT –> D sugar
  • if the lowest OH group is on the LEFT –> L sugar
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9
Q

Epimers (2)

A
  • two sugars that differ only in configuration around one carbon atom
  • ex: D- glucose, mannose and galactose
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10
Q

Cyclic structure of monosaccharides

A
  • exist in solutions mainly as ring structures –> carbonyl group has reacted with a hydroxyl group
  • carbon atom becomes asymmetric, it is called anomeric carbon and anomeric hydroxyl group
  • α- anomer –> anomeric hydroxyl is below the ring plane
  • β- anomer –> anomeric hydroxyl is above the ring plane
  • start counting from the first oxygen, clockwise
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11
Q

Mutarotation

A
  • spontaneous conversion of one anomer to the other, because of a change in equilibrium
  • β –> d- glucose –> α
  • α –> d- glucose –> β
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12
Q

All monosaccharides are…

-reducing or oxidizing sugars?

A

Reducing, because they have an aldehyde group (if they are aldoses) or they are able to form an aldehyde group in solution (if they are ketoses)

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13
Q

Important monosaccharides

A

D-glucose: found in large quantities, primary fuel for living cells, is the preferred energy source of brain cells and cells that have little or no mitochondria
Fructose: fruit sugar, imp member of ketose family of sugars, twice as sweet as sucrose
Galactose: necessary to synthesize a variety of biomolecules (ex: lactose, glycoproteins, proteoglycans), it is synthesized from glucose 1- phosphate

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14
Q

Oxidation derivatives

A

Oxidation of aldehyde group –> aldonic acid
Oxidation of terminal group CH2OH –> uronic acid
Oxidation of both aldehyde and CH2OH group –> aldaric acid

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15
Q

Reduction derivatives

A

Reduction of the aldehyde and ketone groups –> sugar alcohols (alditols)
-Sorbitos (D- glucitol) –> improves the life shelf of candy because it helps to prevent moisture lost

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16
Q

Glycosidic bond (2)

A
  • the hydroxyl group of the anomeric carbon of a monosaccharide can react with an -OH or an -NH group to form another compound, form a glycosidic bond
  • the end of a chain with a free anomeric carbon (on the right) is the reducing end (non- reducing end is usually on the left)
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17
Q

Disaccharide (2)

A
  • two monosaccharides joined together by an o-glycosidic bond
  • maltose, sucrose, lactose
18
Q

Maltose
Sucrose
Lactose

A

Maltose: glucose + glucose –> α 1,4
Sucrose: glucose = fructose –> α1,-2 - formed by plants but not by animals, contains NO FREE anomeric carbon atom, therefore it is a non- reducing sugar
Lactose: galactose + glucose –> β 1,4 - anomeric carbon of the glucose residue is available, lactose is a reducing disaccharide

19
Q

Oligosaccharides (2)

A
  • small polymers attached to polypeptides in glycoproteins and glycolipids
  • two classes: N- linked and O-linked
20
Q

N- and O- linked oligosaccharides

A

N –> N- glycosidic bond with the side chain amide group of asparagine
O –> by the side chain hydroxyl group of serine or threonine or hydroxyl group of membrane lipids

21
Q

Polysaccharides (3)

A
  • polymers of medium to high molecular weight
  • homo –> contain only a single type of monomer
  • hetero –> contain two or more different kinds
22
Q

Homo- polysaccharides (3)

A
  • serve as storage form of monosaccharides that are used as fuels
  • starch in plant cells and glycogen in animal cells
  • starch and glycogen groups are heavily hydrated, because they have many exposed hydroxyl groups available to H bond with water
23
Q

Starch (5)

A
  • contains two types of glucose amylose and amylopectin (either branched or unbranched)
  • Amylose –> consists of long, unbranched chains of D- glucose connected by α1–>4 linkages
  • Amylopectin –> highly branched, the glycosidic linkages are α1–>4 linkages, the branchpoints are α1–>6
  • found in rice, noodles, potatos
  • digestive enzyme: α-amylase
24
Q

Glycogen (5)

A
  • the main storage polysaccharide of animal cells
  • is a polymer of α1–>4 linked subunits of glucose, with α1–>6 linked branches
  • more extensively branched
  • found in fish and meat
  • digestive enzyme: pancreatic α-amylase
25
Q

Cellulose (4)

A
  • fibrous, tough, water- insoluble substance
  • found in the cell walls of plants
  • constitutes much of the mass of wood and cotton is almost pure cellulose
  • linear, unbranched, β1–>4 linkage
26
Q

Difference between glycogen and starch

A

Branch points occur more frequently in glycogen (every 8-12 residues) than in starch (every 24-30 residues)

27
Q

Why humans cannot digest cellulose?

A

Because we do not have the enzymes necessary to breakdown β1–>4 glycosidic bonds

28
Q

Hetero-polysaccharides (3)

A
  • high molecular weight polysaccharide
  • contain more than one kind of monosaccharide
  • Glycosaminoglycans (GAG) –> linear polymers composed of repeating disaccharides units.

amino sugar (N-acetylglucosamine or N-acetlygalactosamine-irudonic acid) + uronic acid (D-glucuronic or L- iduronic acid)

29
Q

Dietary carbohydrates (6)

A
  • largest source of calories
  • plant starches amylopectin and amylose
  • major sugar is sucrose
  • major of animal origin is lactose
  • all cells require glucose for metabolic function, it is NOT REQUIRED in the diet. Glucose can be synthesized from many amino acids (dietary protein)
  • fructose, galactose and xylose –> can synthesize glucose
30
Q

Digestion of dietary carbohydrates (3)

A
  • principal sites are: mouth and intestinal lumen
  • digestion is catalyzed by enzymes –> glycoside hydrolases (glycosidades) that hydrolyze glyosidic bonds
  • the final products are monosaccharides (glucose, galactose, fructose) which are absorbed by cells of the small intestine
31
Q

Glycosidades (3)

A

Endoglycosidades –> hydrolyze polysaccharides and oligosaccharides
Disaccharidases - hydrolyse tri and disaccharides into their reducing sugar components

32
Q

Digestion in mouth (5)

A
  • digestion of starch begins in the mouth, where chewing mixes the food with saliva.
  • salivary glands secrete salivary α-amylase.
  • α-amylase is an endoglucosidase - it hydrolyzes internal α-1,4 bonds between glucosyl residues at random intervals
  • the shortened polysasccharides chain that are formed are called α-dextrins
  • salivary α-amylase is inactivated by the acidity of the stomach contents
33
Q

Digestion in the intestine

A

Pancreatic α-amylase continues to hydrolyze the starches and glycogen forming: disaccharide maltose, trisaccharide maltotriose, olisaccharides containing one or more β- 1,6 branches

34
Q

Final carbohydrate digestion (2)

A
  • occurs at the mucosal lining of the upper jejunum
  • dietary disaccharides are converted to monosaccharides by glycosidases (disaccharidases) attached to the membrane in the brush-border of absorptive cells
35
Q

Disaccharidases: glucoamylase (3)

A
  • it is an exoglucosidase that is specific for the α-1,4 bonds between glucosyl residues
  • begins at the non-reducing end
  • hydrolyzes the bonds to release glucose monosaccharides
36
Q

Sucrose- isomaltase complex (3)

A
  • is a glucosidase enzyme located on the brush border of the small intestine.
  • Sucrase –> splits sucrose, maltose and maltotriose
  • Isomaltase –> splits α-1,6 bonds in number of limit dextrins
37
Q

β- glycosidase complex (2)

A
  • Lactase catalytic side hydrolyzes the β-bond connecting glucose and galactose in lactose
  • Glucosyl- ceramidase splits β- glycosidic bonds between glucose and galactose and hydrophobic residue
38
Q

Absorption of monosaccharides (5)

A
  • duodenum and upper jejunum absorb the bulk of the dietary sugars
  • different sugars have different mechanisms of absorption
  • galactose and glucose are transported into the mucosal cells, by sodium- dependent glucose transporter (SGLT-1), requires energy and sodium uptake,
  • Fructose uptake requires sodium-independent monosaccharide transporter (GLUT-5),
  • all 3 are transported from the intestinal mucosal cells into the portal circulation by GLUT- 2
39
Q

Lactose intolerance (4)

A
  • pain, nausea and flatulence after ingestion of food containing lactose
  • can be caused by low levels of lactase or intestinal injury
  • the lactose that is not absorbed is converted by colonic bacteria to lactic acid, methane gas and H2 gas
  • can be detected by measuring glucose levels in blodo and urine
40
Q

Example of monosaccharide derived

A
  • glucose-6-phosphate

- oxidized forms (aldehyde group is oxidized to aldonic acid)

Carbohydrates (4 decks)

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