Carbohydrates
Carbohydrates
Carbohydrates are the most abundant organic
molecules in nature .
Carbohydrates formula : (CH2O)n
Functions of carbohydrates:
• Energy in the diet
• Storage form of energy in the body
• Cell membrane components
• Structural component of many organisms
Classification of carbohydrates
• Monosaccharides : simple sugars
• Disaccharides : contain two monosaccharides
• Polysaccharides : contain many monosaccharides
Origin:
Carbohydrates are formed in plants by a process called
photosynthesis
Compounds that have same chemical formula but have
different structures are called isomers
Monosaccharides
(Simple sugars)
• Aldoses
• Ketoses
depending on they contain an aldehyde or a ketone
group
Hexoses (six-carbon monosaccharides):
are the most common monosaccharides
• Glucose
• Fructose
• Galactose
Glucose :
- Is an aldohexose
- Is commonly Known as dextrose
- Is the most important of all monosaccharides and is normally
found in the bloodstream and in the tissue fluids
Galactose:
- Is an aldohexose
- Is one of the constituents of lactose
Fructose :
- Is a ketohexose
- Is commonly known as fruit sugar
- Is one of the constituents of sucrose
Monosaccharides and its chemical
reactions
• The hexoses either aldehydes or ketones can
act as reducing agents
Hexose + alkaline Cu complex (Fehling s solution or Benedict s solution) →
A red-orange precipitate of Cu2O is formed
This reaction is the basis for the test for sugar in the
urine
• Hexoses will ferment in the presence of
enzymes found in yeast
Disaccharides
• Sucrose :(table sugar)
Glucose + Fructose
• Maltose :(obtained by the partial hydrolysis of
starch)
Glucose + Glucose
• Lactose :(sugar in milk)
Glucose + Galactose
Disaccharides and its chemical
reactions
• Of the three disaccharides only maltose and
lactose show reducing properties with alkaline
Cu complex ions.
• Sucrose is not a reducing sugar .
• Sucrose and maltose will ferment with yeast
due to the presence of the enzymes sucrase
and maltase.
• Lactose will not ferment with yeast because of
the absence of the enzyme lactase
Polysaccharides
- Polysaccharides are polymers of monosaccharides
- Hydrolysis of Polysaccharides → monosaccharides
- Three common polysaccharides:
• Starch
• Cellulose
• Glycogen
- Plants store their food as starch
- Plants use cellulose as supporting and structural parts
- Animals use glycogen as a reserve supply of
carbohydrate.
Properties of polysaccharides
•
•
•
•
Polysaccharides have a high molecular mass
Insoluble in water
Tasteless
Give negative tests for reducing sugars
These properties are the opposite of those for
monosaccharides and disaccharides
•
A carbohydrate that can be given
intravenously is
a. Cellulose
b. Sucrose
c. Lactose
d. Glucose
• An example of a disaccharide is
a. Glucose
b. Maltose
• Cellulose
d. Starch
• An example of hexose is
a. Sucrose
b. lactose
• Galactose
d. Ribose
• Animals store carbohydrates in the form of
a. Cellulose
b. Glucose
b. Starch
d. Glycogen
• An example of a polysaccharide is
a. Starch
b. Glycogen
b. Cellulose
d. All of these
•
a.
b.
c.
d.
When sucrose is hydrolyzed, it yields
Glucose only
Glucose and fructose
Glucose and galactose
Ribose and galactose
•
Alkaline Cu complex ions are used to test for
the presence of
a. Aldehydes
b. Ketones
b. Acids
d. Oxidizing sugars
• An example of a pentose is
a. Maltose
b. talose
b. Glyceraldehyde
d. Ribose