NAME: ____________________
Lab #7: Carbohydrates Prelab
Introduction:
Of the three main sources of energy and raw materials for cellular function, carbohydrates are
normally ingested in the largest portions, typically 60 to 70% of our diet. They are also the primary fuel,
or source of energy, for the cells in our body. The main categories of carbohydrates are sugars, starches
and cellulose.
Carbohydrates were originally thought to be hydrates (containing water) of carbon, and this was the
derivation of the name. It turned out that they were not hydrates, but did contain the elements C, H and
O expressed in the ratio: Cx(H2O)y. They are now usually classified as polyhydroxy (many OH groups)
aldehydes or ketones.
The simple sugars are the monosaccharides, with the three main dietary ones being glucose (blood
sugar), galactose and fructose (fruit sugar). They all have the general formula C6H12O6, and are isomers
that exist in both open and closed forms. The open chain forms are regularly shown as Fischer
projections and the closed ring forms as Haworth diagrams. These are shown below for the
monosaccharides.
H
O
C
HC
HO
CH2OH
OH
O
CH
HC
OH
H2C
OH
H
OH
OH
OH
α-D-Glucose
H
OH
CH2OH
Glucose
D-Glucose
aldohexose)
(an(an
aldohexose)
HC
HO
HO
CH2OH
C
HO
CH2OH
HC
OH
H2C
OH
CH2OH
Fructose
D-Fructose
ketohexose)
(an(aketohexose)
OH
O
OH
3
4
OH
Β-D-Fructose
beta-D-Fructose
2
CH2OH
1
OH
CH2OH
OH
O
CH
OH
OH
CH
H2C
O
CH
O
C
H
OH
CH2OH
Galactose
D-Galactose
(an aldohexose)
OH
β-D-Galactose
The position of the OH group on the anomeric carbon on the far right of the ring determines whether
the sugar is an alpha (α) or a beta (β) sugar. The OH group points down below the ring in α sugars and
up above the ring in β sugars. These have important consequences later for digestion purposes.
The other sugars are the disaccharides, formed when two monosaccharide residues bond to each
other. The three main dietary disaccharides are maltose, sucrose and lactose. Maltose (malt sugar)
forms when two glucose units are connected with an α-(1è4) glycosidic bond. Sucrose (table sugar) is
composed of a glucose and a fructose reside. Lactose (milk sugar) is composed of a glucose residue and
a galactose residue. To digest these disaccharides, the necessary digestive enzymes, maltase, sucrase,
and lactase, must be present in the small intestine.
If many glucose monosaccharide residues are joined together, then a polysaccharide forms.
Sometimes called complex carbohydrates, the polysaccharides include plant and animal starch, as well
as cellulose. Starch is the main energy storage form for plants, usually in the seeds. In starch, the
glucose residues are joined by α-(1è4) glycosidic bonds and are therefore digestible by humans with the
aid of the amylase enzyme in saliva and pancreatic juices. Starch consists of about 20% amylose and
80% amylopectin. Amylose is linear and hundreds to several thousand glucose units long, whereas
amylopectin is branched and up to 100,000 glucose units.
Glycogen is the main energy storage form for animals, mainly in their liver and muscles. It is often
called animal starch. It consists of up to 1 million glucose units joined by glycosidic bonds and is even
more highly branched than amylopectin.
Cellulose is the main structural component of plant cell walls. It consists of several thousand glucose
units in a linear fashion just like amylose, but has β-(1è4) glycosidic bonds and therefore is not
digestible by humans. Some ruminant animals contain special bacteria in their stomachs that allow
digestion of cellulose β-(1è4) glycosidic bonds. For humans, it acts as dietary fiber, or “roughage”, and
eases defecation.
In this experiment we will become acquainted with some of the common tests/reactions for
carbohydrates. We will look at the reactions of fructose, sucrose, cellulose, lactose, and starch. We
will use these reactions to determine which of the five sugars is present in an unknown.
Part I: Benedict's Test
Benedict’s reagent is a copper compound that will oxidize only aldehyde groups (aldoses) and not
alcohols. If you consider cyclic forms of carbohydrates, hemiacetals give positive tests while acetals
give negative tests. The reason for this is that the cyclic form interconverts (is in equilibrium) with the
linear form that contains an aldehyde!
A sugar that reacts with Benedict’s
solution is called a reducing sugar
since it reduces the ion Cu2+ à Cu+
Part II: Seliwanoff’s Test
The Seliwanoff reagent contains hydrochloric acids which converts fructose to Shydroxymethylfurfural. The reagent also contains resorcinol which reacts with the Shydroxymethylfurfural to give a red color. Complex carbohydrates which contain fructose units can also
give a positive test. Aldohexoses react similarly, but more slowly.
Part III: Iodine test
One of the two molecular components of starch are shaped like very long spiral staircases, inside of
which is just enough space to accommodate iodine molecules. The blue color arises when the electrons
of the entrapped iodine molecules interact with the electrons of the starch molecule and the resulting
complex absorbs visible light (appears dark).
Questions:
1) Why do some disaccharides give positive Benedict’s tests while other disaccharides give negative
tests?
2) Fructose is not an aldose. Why does it give a positive Benedict's test?
3) Is the aldehyde group on a sugar oxidized or reduced in the Benedict’s test? Explain.
4) A carbohydrate that gives a positive Benedict's test is said to be a reducing sugar. Why? What's being
reduced?
5) Cu2+ in Benedicts tests is a weak oxidizing agent. Why don't we use a stronger oxidizing agent?
6) Starch is made up of two components.
a) What are the two components, and how do the two components differ from each other?
b) Which of the two components of starch is similar to glycogen? How does the component differ from
glycogen?
c) Draw a section of a amylose molecule containing two glucose units. Label the linkages (for example
β (1→4)) and place an asterisk (*) next a carbon where branching would occur if it was a amylopectin
molecule.