• Composed of carbon, oxygen, hydrogen
• two-third of the human diet is composed of
• most carbohydrates are produced by the
photosynthesis in green plants
• the three elements that make up all carbohydrates
are arranged as alcohols, aldehydes, or ketones
•The three main classes of carbohydrates are:
monosaccharides, disaccharides, polysaccharides
• Monosaccharide: a carbohydrate that cannot
be hydrolyzed to a simpler carbohydrate.
– Monosaccharides have the general formula
CnH2nOn, where n varies from 3 to 8.
– Aldose: a monosaccharide containing an aldehyde
– Ketose: a monosaccharide containing a ketone
• also known as
simple sugars can
not be broken down
into smaller CHO
• hexoses and
aldehyde or ketone group
of a straight-chain
monosaccharide will react
reversibly with a hydroxyl
group on a different
carbon atom to form a
hemiacetal or hemiketal
forming a ring with an
oxygen bridge between 2
carbon atoms. Rings with
five carbons are called
furanoses and rings with
6 carbons are call
The red atoms
highlight the aldehyde
group, and the blue
atoms highlight the
furthest from the
this –OH is on the
right of the Fischer
projection, this is a D
• the most
• used directly
by the body for
• oxidized to carbon dioxide and water
• Excess stored as adipose
Galactose is in many plant gums and pectins
• component of the disaccharide lactose
Fructose is the sweetest of all the naturally
• honey, fruits
• component of the disaccharide sucrose
Note the position of
the hydroxyl group
(red or green) on the
relative to the CH2OH
group bound to the
carbon 5: they are
either on the opposite
sides (α), or the same
The α and β anomers of
of DNA and
consist of 2 monosaccharide
• glucose and fructose
• table sugar
• 1/4 of total calories
• glucose and galactose
• glucose and glucose
• germinating grains
• Sucrose (table sugar)
– Sucrose is the most abundant disaccharide in the
biological world; it is obtained principally from the
juice of sugar cane and sugar beets.
– Glucose and fructose linked together with a -1,2glycosidic bond
– Lactose is the principal sugar present in milk; it
makes up about 5 to 8 percent of human milk and
4 to 6 percent of cow's milk.
– Contains glucose and galactose linked together
with a -1,4-glycosidic bond
– Present in malt, the juice from sprouted barley and
other cereal grains.
– Maltose consists of two units of glucose joined by
an -1,4-glycosidic bond.
• Monosaccharides are colorless crystalline
solids, very soluble in water, but only slightly
soluble in ethanol
– Sweetness relative to sucrose:
- a carbohydrate consisting of large numbers
of monosaccharide units joined by glycosidic
-2/3 of the human diet
-Potatoes, rice, wheat, cereal grains
-mixture of amylose and amylopectin
-only storage for glucose in the body
-liver and muscle
-similar in structure to amylopectin but, more
- forms the structural component of the cell
walls of plants
-cotton, paper, linen, rayon
-long unbranched chains of glucose (10010,000 glucose molecules)
- linkages instead of
present in starch
linkages that are
-many herbivores have the enzyme to
• Starch: a polymer of D-glucose.
– Starch can be separated into amylose and amylopectin.
– Amylose is composed of unbranched chains of up to 4000
glucose units joined by -1,4-glycosidic bonds.
– Amylopectin contains chains up to 10,000 D-glucose units
also joined by -1,4-glycosidic bonds; at branch
points, new chains of 24 to 30 units are started by -1,6glycosidic bonds.
• Glycogen is the energy-reserve carbohydrate
– Glycogen is a branched polysaccharide of
approximately 106 glucose units joined by -1,4and -1,6-glycosidic bonds.
– The total amount of glycogen in the body of a wellnourished adult human is about 350 g, divided
almost equally between liver and muscle.
• Cellulose is a linear polysaccharide of Dglucose units joined by -1,4-glycosidic bonds.
– It has an average molecular weight of 400,000
g/mol, corresponding to approximately 2200 glucose units
– Cellulose molecules act like stiff rods and align themselves
side by side into well-organized water-insoluble fibers in
which the OH groups form numerous intermolecular
– This arrangement of parallel chains in bundles gives
cellulose fibers their high mechanical strength.
– It is also the reason why cellulose is insoluble in water.
• Cellulose (cont’d)
– Humans and other animals cannot use cellulose as food
because our digestive systems do not contain glucosidases, enzymes that catalyze hydrolysis of glucosidic bonds.
– Instead, we have only -glucosidases; hence, the
polysaccharides we use as sources of glucose are starch
– Many bacteria and microorganisms have -glucosidases
and can digest cellulose.
– Termites have such bacteria in their intestines and can use
wood as their principal food.
– Ruminants (cud-chewing animals) and horses can also
digest grasses and hay.
• Figure 20.4 Cellulose is a linear polymer
containing as many as 3000 units of D-glucose
joined by -1,4-glycosidic bonds.
1. What are the three main classes of carbohydrates?
2. Cellulose is made from glucose, but humans can not
digest cellulose. Explain.
3. How does your body store carbohydrates?
4. Indicate whether each of the following is a
monosaccharide, disaccharide, or polysaccharide.
5. Starch is a mixture of two types of polysaccharides.
6. Name the 2 monosaccharides in each of the following.
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