CARBOHYDRATES
LEARNING OUTCOME

3.1.2 The digestive system provides an
interface with the environment. Digestion
involves enzymic hydrolysis producing
smaller molecules that can be absorbed and
assimilated.
YOU SHOULD KNOW :
That the basic carbohydrate units are
monosaccharides
 The monosaccharides act as monomers and link
to form disaccharides and polysaccharides
 The structure of a-glucose.
 About linking of a-glucose by glycosidic bonds
formed by condensation reactions to form maltose
and starch.
 Disaccharides, sucrose and lactose.
 About lactose intolerance.
 Biochemical tests for reducing sugars, non
reducing sugars and starch

CARBOHYDRATES - SUGARS AND STARCHES

C H 2O

Hydrates of carbon

Functions
Fuel for cell activity
 food reserve (starch)
 structural


part of DNA and RNA (deoxyribose and ribose)
BUILDING LARGER CARBOHYDRATES

Carbohydrates are made up of chains of
individual monomer components joined together
to form a polymer.

Single monomer = monosaccharide.

2 monosaccharides joined = disaccharide.

3 or more = polysaccharide.
MONOSACCHARIDES - SIMPLE SUGARS

Contain from 3 to 7 carbons





Trioses
Tetroses
Pentoses
Hexoses
Heptoses
3 carbons
4 carbons
5 carbons
6 carbons
7 carbons
β-glucose
Beta-glucose.
The hydroxyl
group on
carbon number
1 is UP.
The hydroxyls
alternate “up
and down”
around the
molecule.
DISACCHARIDES - 2
MONOSACCHARIDES

Glucose + Fructose = Sucrose

Glucose + Galactose = Lactose

Glucose + Glucose = Maltose
The
bond between sugars is a
glycosidic bond
POLYSACCHARIDES - A CHAIN OF
MONOSACCHARIDES
1. Glycogen- a storage compound in animals. We
store glycogen in liver and muscle cells. The
monomer unit is α gluclose.
 2. Starch-a storage compound in plants. We eat
this in foods such as bread and potatoes. Also
chains of α gluclose.
 3. Cellulose – makes plant cell walls. When we
eat fruit / vegetables we eat this. It forms
fibre/roughage in our diet. It consists of chains of
β glucose

amylose
Starch - storage form of glucose in
plants-amylopectin
GLYCOGEN
3.
Cellulose - main structural
component of plant cell walls
BENEDICT’S TEST FOR A REDUCING
SUGAR.
Theory
Need to know this for practical exam and written exams.
All monosaccharides and some disaccharides, (like maltose)
are reducing sugars.
Receiving an electron is reduction.
A reducing sugar is a sugar that can donate an electron to
(or reduce), another chemical, in this case Benedict’s
reagent.
Benedict’s contains Copper II sulphate and is alkaline.
Cu2+ ions from the copper sulphate are reduced by the –
CHO Aldose or C=O Ketone groups in reducing sugars to
form Cu+ ions.
When heated with a reducing sugar pale blue
Benedict’s solution forms an insoluble red
precipitate of Copper I oxide.
TEST FOR NON-REDUCING SUGARS

Some disaccharides such as sucrose are nonreducing sugars. This means they do not change
the colour of Benedict’s when heated. To test a
non-reducing sugar it must be broken down into
its monosaccharide components by hydrolysis.
These monosaccharides can then be tested with
Benedicts as a reducing sugar.
METHOD
The non-reducing sugar is first hydrolysed by
boiling with hydrochloric acid so that it will be
broken down into its monosaccharides.
These can then reduce Benedict’s reagent in the
normal way.
So a non-reducing sugar is identified by a negative
reaction to Benedict’s before hydrolysis and a
positive result after hydrolysis.
METHOD
Carry out the reducing sugar test.
If negative then in a boiling tube, add 2cm3 of
dilute hydrochloric acid, to a sample to be tested,
mix the solution and heat in a hot water bath for
2-3 mins
Then add sodium hydroxide, to the boiling tube
until the solution is neutral, Add until the fizzing
stops. (This is important because Benedict’s is
not effective in acid conditions).
Carry out the reducing sugar test again. This time
it will be positive