Carbohydrates
BIOL 313
4-8-2014
Trioses
tetroses
Simple Carbohydrates
 sugars
– monosaccharides – single sugars
– disaccharides – 2 monosaccharides
Glucose




mild sweet flavor
known as blood sugar
essential energy source
found in every
disaccharide and
polysaccharide
Fructose
 sweetest sugar
 found in fruits and honey
 added to soft drinks,
cereals, deserts
Galactose
 hardly tastes sweet
 rarely found
naturally as a single
sugar
•they differ only in the configuration
around one carbon atom.
To possess optical activity a compound
hah to be assymetric
How many stereoisomers are possible for
a ketopentose such as ribulose?
A) 2
B) 4
C) 8
D) 16
E) 32
When the linear form of glucose cyclizes,
the product is a(n): hemiacetal
Which of the following pairs is interconverted in the process of
mutarotation?
a-D-glucose and b-D-glucose
anomeric pair
a-D-glucose and b-D-glucose
D-Glucose is called a reducing sugar because it undergoes an
oxidation-reduction reaction at the anomeric carbon. One of
the products of this reaction is:
1. D-galactose.
2. D-gluconate.
3. D-glucuronate.
4. D-ribose.
5. muramic acid.
A reducing sugar is any sugar that either has
an aldehyde group or is capable of forming one in
solution through isomerism. The aldehyde functional
group allows the sugar to act as a reducing agent, for
example in the Tollens' test or Benedict's test, or
the Maillard reaction, important in the browning of
many foods. The cyclic hemiacetal forms of aldoses
can open to reveal an aldehyde and
certain ketoses can undergo tautomerization to
become aldoses. However, acetals, including those
found in polysaccharide linkages, cannot easily
become free aldehydes. A sugar is classified as a
reducing sugar only if it has an open-chain form with
an aldehyde group or a free hemiacetal group.
Categorize each of the following as an aldose, a ketose,
or neither.
H
H
H
H
C
OH
O
C
C
O
H
C
C
OH
HO
C
H
H
H
H
C
OH
HO
C
H
OH
HO
C
H
H
C
OH
H
HO
C
H
C
O
C
O
H
C
OH
H
C
H
H
H
H
H
H
(a)
(b)
(c)
(d)
(e)
Disaccharides
 pairs of the monosaccharides
– glucose is always present
– 2nd of the pair could be fructose, galactose
or another glucose
– taken apart by hydrolysis
– put together by condensation
– hydrolysis and condensation occur with all
energy nutrients
– maltose, sucrose, lactose
Condensation
 making a disaccharide
– chemical reaction linking 2
monosaccharides
Hydrolysis
 breaking a disaccharide
– water molecule splits
– occurs during digestion
Lactose
 glucose and galactose
 main carbohydrate in
milk
– known as milk sugar
Maltose
 2 glucose units
 produced when starch breaks down
 not abundant
Explain why all mono- and
disaccharides are soluble in
water.
Ans: These compounds have
many hydroxyl groups, each of
which can hydrogen bond with
water.
Sucrose
 fructose and
glucose
 tastes sweet
– fruit, vegetables,
grains
 table sugar is
refined sugarcane
and sugar beets
 brown, white,
powdered
Explain why sucrose is not a reducing sugar,
even though both glucose and fructose are.
Sucrose has no anomers
From the abbreviated name of the compound
Gal(b1
4)Glc, we know that:
Explain why all mono- and disaccharides are soluble in water.
These compounds have many hydroxyl groups, each of which
can hydrogen bond with water.
Define "reducing sugar." (b) Sucrose is a disaccharide
composed of glucose and fructose
(Glc(a1 2)Fru). Explain why sucrose is not a reducing sugar,
even though both glucose and fructose are.
Ans: (a) A reducing sugar is one with a free carbonyl carbon that
can be oxidized by Cu2+ or Fe3+. (b) The carbonyl carbon is C1 of glucose and C-2 of fructose. When the carbonyl carbon is
involved in a glycosidic linkage, it is no longer accessible to
oxidizing agents. In sucrose (Glc(a1 ® 2)Fru), both oxidizable
carbons are involved in the glycosidic linkage.
Define each in 20 words or less:
(a) anomeric carbon;
(b) enantiomers;
(c) furanose and pyranose;
(d) glycoside;
(e) epimers;
(f) aldose and ketose.
Define each in 20 words or less:
(a) anomeric carbon;
(b) enantiomers;
(c) furanose and pyranose;
(d) glycoside;
(e) epimers;
(f) aldose and ketose.
Ans: (a) The anomeric carbon is the carbonyl carbon atom of a
sugar, which is involved in ring formation. (b) Enantiomers are
stereoisomers that are nonsuperimposable mirror images of each
other. (c) Furanose is a sugar with a five-membered ring;
pyranose is a sugar with a six-membered ring. (d) A glycoside is
an acetal formed between a sugar anomeric carbon hemi-acetal
and an alcohol, which may be part of a second sugar. (e)
Epimers are stereoisomers differing in configuration at only one
asymmetric carbon. (f) An aldose is a sugar with an aldehyde
carbonyl group; a ketose is a sugar with a ketone carbonyl group.
Which of the following is
not a reducing sugar?
1. Fructose
2. Glucose
3. Glyceraldehyde
4. Ribose
5. Sucrose
Which of the following is the most abundant
disaccharide?
A)
B)
C)
D)
E)
lactose
cellulose
α-amylose
maltose
sucrose
Complex Carbohydrates
 starches and fibers
 polysaccharides
– chains of monosaccharides
Complex Carbohydrates
 polysaccharides
– glycogen and starch
 built entirely of glucose
– fiber
 variety of monosaccharides and other
carbohydrate derivatives
Starches
 stored in plant cells
 body hydrolyzes plant starch to glucose
Glycogen
 limited in meat and not found in plants
– not an important dietary source of
carbohydrate
 BUT
– all glucose is stored as glycogen
– long chains allow for
hydrolysis and release
of energy
Cellulose
Physical Properties of Cellulose and Glycogen
Native cellulose consists of glucose units linked by (beta1 4) glycosidic bonds. The
beta linkages force the polymer chain into an extended conformation. Parallel series of
these extended
chains can form intermolecular hydrogen bonds, thus aggregating into long, tough,
insoluble fibers. Glycogen consists of glucose units linked by (a1n4) glycosidic bonds.
The a linkages
cause bends in the chain, and glycogen forms helical structures with intramolecular
hydrogen bonding; it cannot form long fibers. In addition, glycogen is highly branched
and,
because many of its hydroxyl groups are exposed to water, is highly hydrated and
therefore
very water-soluble. It can be extracted as a dispersion in hot water.
The physical properties of the two polymers are well suited to their biological roles.
Cellulose
serves as a structural material in plants, consistent with the side-by-side aggregation of
long molecules into tough, insoluble fibers. Glycogen is a storage fuel in animals. The
highly
hydrated glycogen granules, with their abundance of free, nonreducing ends, can be
rapidly
hydrolyzed by glycogen phosphorylase to release glucose 1-phosphate, available for
oxidation
and energy production.
Fiber
 structural parts of plants
– found in all plant derived food
 bonds of fibers cannot be broken down
during the digestive process
– minimal or no energy available
Fiber types
 cellulose
 pectins
 lignins
 resistant starches
– classified as fibers
– escape digestion and
absorption
Fiber Characteristics
 soluble fibers, viscous, fermentable
– easily digested by bacteria in colon
– associated with protection against heart
disease and diabetes
 lower cholesterol and glucose levels
– found in legumes and fruits
The number of structurally different polysaccharides that can be
made with 20 different monosaccharides is far greater than the
number of different polypeptides that can be made with 20
different amino acids, if both polymers contain an equal number
(say 100) of total residues. Explain why.
Describe one biological advantage of storing glucose units in
branched polymers (glycogen, amylopectin) rather than in linear
polymers.
Because virtually all peptides are linear (i.e., are formed with
peptide bonds between the a-carboxyl and a-amino groups),
the variability of peptides is limited by the number of different
subunits. Polysaccharides can be linear or branched, can be
a- or b-linked, and can be joined 1 - 4, 1 - 3, 1 - 6, etc. The
number of different ways to arrange 20 different sugars in a
branched oligosaccharide is therefore much larger than the
number of different ways a peptide could be made with an
equal number of residues
The enzymes that act on these polymers to mobilize glucose
for metabolism act only on their nonreducing ends. With
extensive branching, there are more such ends for enzymatic
attack than would be present in the same quantity of glucose
stored in a linear polymer. In effect, branched polymers
increase the substrate concentration for these enzymes.
Explain how it is possible that a polysaccharide molecule,
such as glycogen, may have only one reducing end, and yet
have many nonreducing ends.
Describe one biological advantage of storing glucose units in
branched polymers (glycogen, amylopectin) rather than in
linear polymers.
Explain how it is possible that a polysaccharide molecule, such as
glycogen, may have only one reducing end, and yet have many
nonreducing ends.
Ans: The molecule is branched, with each branch ending in a
nonreducing end.
Describe one biological advantage of storing glucose units in
branched polymers (glycogen, amylopectin) rather than in linear
polymers.
Ans: The enzymes that act on these polymers to mobilize glucose
for metabolism act only on their nonreducing ends. With
extensive branching, there are more such ends for enzymatic
attack than would be present in the same quantity of glucose
stored in a linear polymer. In effect, branched polymers increase
the substrate concentration for these enzymes.
Fiber
 insoluble and not easily fermented
– promote bowel movements
– alleviate constipation
– found in grains and vegetables
Lactose Intolerance
 more lactose is consumed than can be
digested
– lactose molecules attract water
 cause floating, abdominal discomfort,
diarrhea
– intestinal bacteria feed on undigested
lactose
 produce acid and gas
Lactose Intolerance
 age, damage, medication, diarrhea,
malnutrition
 management requires dietary change
– 6 grams (1/2 cup) usually tolerable
– take in gradually
– hard cheeses & cottage cheese
– enzyme drops or tablets
 lactose free diet is extremely difficult to
accomplish
Carbohydrate Metabolism
1/3 of body’s glycogen is stored in liver
– released as glucose to bloodstream
1. eat – intake glucose
2. liver condenses extra glucose to glycogen
3. blood glucose falls
4. liver hydrolyzes glycogen to glucose

Glycogen is bulky, so we store only so much:
short term energy supply
Fat is the long term energy supply.
Glucose for Energy
 enzymes break apart glucose – yielding
energy
 inadequate supply of carbohydrates
– ketone bodies (fat fragments) are an
alternate energy source during starvation
– excess ketones can lead to ketosis:
imbalance of acids in body
 minimum of 50 – 100 grams of carbs/day
are needed to avoid ketosis
Glucose Homeostasis
 maintaining an even balance of glucose is
controlled by insulin and glucagon
– insulin
 moves glucose into the blood
– glucagon
 brings glucose out of storage
Imbalance
 diabetes
– after food intake, blood glucose rises and
is not regulated because insulin is
inadequate
 hypoglycemia
– blood glucose drops dramatically
 too much insulin, activity, inadequate
food intake, illness
 diet adjustment includes fiber-rich carbs
and protein
Sugar
 ½ comes from natural sources, ½ from
refined and added
– sucrose, corn syrup, honey
 excess can lead to nutrient deficiencies
and tooth decay
– empty calories
– sugar and starch break down in the
mouth
Sugar
 recommended intake
– added sugar = no more than 10% of energy
intake
Starch and Fiber
 diet that includes starch, fiber and natural
sugars
– whole grains, vegetables, legumes, fruits
 may protect against heart disease and
stroke
 reduces the risk of type 2 diabetes
 enhances the health of the large
intestine
 can promote weight loss
Starch and Fiber
 starch intake
– 45-65%
– 225 – 325 grams (DV is 300
grams)
– 900-1300 kcal/2000 kcal
– RDA is 130 grams
 fiber intake
– Daily Value is 25 grams/2000
kcal
Groceries
 grains: 1 serving = 15 grams
 vegetables
– ½ cup starchy = 15 grams
– ½ cup nonstarchy = 5 grams
 fruit: 1 serving = 15 grams
 milk: 1 cup = 12 grams
 meat: none or little
 legumes: ½ cup = 15 grams
Artificial Sweeteners
 help keep sugar and energy intake down
 anything we eat has FDA approval
– saccharin
– aspartame
– acesulfame potassium
– sucralose
– neotame
Sugar Replacers
 sugar alcohols
– provide bulk and sweetness
 cookies, gum, candy, jelly
– do contain minimal kcal
– low glycemic response
 absorbed slowly
– do not cause dental caries