Download Hydrolysis of Starch

UNIT 2 - NATURAL PRODUCTS
CARBOHYDRATES
Carbohydrates are one of the most important
groups of compounds in the world. All living
things require them for energy. The name
‘carbohydrate’ tells us the elements found in
these compounds.
‘carbo’ = carbon
‘hydr’ = hydrogen
‘ate’ = oxygen
Sugars are carbohydrates. Alkanols contain
the 3 same elements but are not
carbohydrates. In a carbohydrate the ratio of
hydrogen:oxygen must be 2:1 (as in water,
H2O).
Photosynthesis
(Making Carbohydrates)
This is a process by which green plants make
the carbohydrate glucose. This is required for
energy for the plant to live and grow. Animals
then feed on these plants to provide their own
carbohydrate needs.
The green pigment in plants is called
chlorophyll. This traps the energy from the
sun to catalyse the photosynthesis reaction.
Equation:
light
carbon dioxide + water
glucose + oxygen
chlorophyll
Formula equation:
CO2
+
H2O
C6H12O6 + O2
The glucose is stored in green plants as
starch.
Reactions of Carbohydrates
1. Dehydration
Heating sugar (sucrose) with concentrated
sulphuric acid takes the water out of the
sugar.
What is left?
Equation:
sucrose
C12H22O11
c.H2SO4
carbon + water
C
+
H 2O
2. Respiration
This is the process by which all living things
get their energy. Carbohydrates are the main
starting materials in this reaction. They are
the ‘fuels’ we all need to survive.
Getting energy from Carbohydrates
What do we get when we burn sugar?
Answer:
carbon dioxide + water
Equation:
carbohydrate + oxygen
carbon dioxide
+ water + energy
This is also the equation for RESPIRATION.
glucose + oxygen
carbon dioxide + water
Formula equation:
C6H12O6 + O2
CO2
+
H2O
Note that this is the reverse of the equation
for photosynthesis. These 2 opposite
reactions form a major part of the carbon
cycle.
NB. Even though green plants photosynthesise
during daylight, producing oxygen, they still
require some oxygen to meet their own energy
needs through respiration.
Testing for Starch - Iodine
In a dimple tile, add 2 drops of the solution
being tested to a dimple. Then add 2 drops of
iodine solution and note any changes to the
colour.
Carbohydrate
Glucose
Sucrose
Starch
Fructose
Lactose
Result
The test for starch is that it turns iodine
solution from brown to blue/black.
Testing For Reducing Sugars
Benedict’s Solution
Method:
1. Pour 3ml of Benedict’s solution into 5
test tubes.
2. To each test tube add a spatula of one of
the carbohydrates being tested.
3. Pour hot water into a 250ml beaker.
4. Place your test tubes in the hot water
for 5min.
Results:
Carbohydrate Colour at start Colour at end
Starch
Glucose
Sucrose
Fructose
Lactose
Conclusion:
Benedict’s reagent (solution) gives a positive
test (blue
orange or brick red) with:
Glucose
Maltose
Lactose
These sugars are known as reducing sugars.
Sucrose and starch do not give a positive test.
Sucrose is the only common carbohydrate
which gives a negative result with both the
Iodine and Benedict’s tests for
carbohydrates.
Testing Foods For Carbohydrates
Food
Does it
have
starch?
Does it have
reducing sugars?
COMPARING STARCH AND GLUCOSE
Tyndall’s Beam
When you shine light on a glucose solution, the
beam just passes through.
However, when you shine light on a starch
solution it appears to glow. This is because
starch molecules are so big they reflect the
light beam.
Also, starch does not dissolve easily in water
because starch molecules are large, non-polar
and covalent while water is polar.
Structures of Carbohydrates
There are different types of carbohydrates
(saccharides) depending on their size.
monosaccharides
disaccharides
polysaccharides
C6H12O6
fructose
galactose
glucose
C12H22O11
lactose
maltose
sucrose
(C6H10O5)n
cellulose
glycogen
starch
The simplest carbohydrates are the
monosaccharides, containing just one sugar
unit (formula C6H12O6). The polysaccharides
contain over 300 of these sugar units joined
together.
Glucose  Starch
Glucose is turned into starch by condensation
polymerisation.
HO
G
OH HO
G
OH HO
HO
G
O
G
O
G
G
OH + 2H2O
OH
This is called CONDENSATION
POLYMERISATION because for every
monomer which joins the polymer a water
molecule is produced.
General equation:
nC6H12O6
(C6H10O5)n + (n-1)H2O
Hydrolysis of Starch
Starch is broken up into its monomers in the
lab and also during digestion in animals by
hydrolysis reactions.
1. Digestion
Starch is made and stored in plants such as
potatoes and wheat. When we eat these foods
we need to break down the starch into smaller
molecules to allow the carbohydrates to pass
through the wall of the gut into the blood.
starch
G
G
+
H 2O
G
G +
G+
glucose
In digestion this reaction is catalysed by
enzymes such as amylase (found in saliva).
G
This works best at 37°C and at near neutral
pH.
Extreme temperatures and pH can
permanently change the shape of enzymes.
This stops them working for good and they
are said to be denatured.
2. In the laboratory
In the lab starch can be broken up into its
monomers by heating it with an acid.
Testing with iodine shows starch is no longer
present, while testing with Benedict’s solution
shows the formation of reducing sugars
(maltose and glucose).
Note that the hydrolysis of starch is the
opposite reaction of the formation of starch.
condensation polymerisation
glucose
starch + water
acid or enzyme hydrolysis
Proteins
All proteins contain the elements:
 Carbon
 Hydrogen
 Oxygen
 Nitrogen
Proteins are essential for us as they make up:
•
•
•
•
•
Muscle fibres
Skin
Hair
Nails
Enzymes
Proteins are natural polymers made up from
smaller molecules called amino acids. The
number of amino acids joined can range from
just 51 (insulin) to hundreds (haemoglobin) to
thousands (urease).
These amino acids contain 2 functional groups:
1. an amine group, -NH2.
2. a carboxyl group, -COOH.
X = H, alkyl group, or
a group containing
OH, S, P or N.
Natural amino acids must have the 2
functional groups attached to the same
carbon atom. These are called -amino acids
and there are about 20 of them. Glycine and
alanine are the 2 simplest.
Amino Acids in Nature
1.
2.
3.
26 amino acids are used to make
proteins.
Proteins are made of many amino acids
linked in a specific order.
Some amino acids cannot be synthesised
in the body so have to be taken in
through our diet.
Making Proteins
Condensation Polymerisation of Amino acids
As with polyamides, proteins are made by
condensation reactions between amine groups
(-NH2) and carboxyl groups (-COOH). This
produces the same linking group of atoms.
However, in proteins this is known as a
peptide link rather than an amide link.
Hydrolysis of Proteins
As with all condensation polymers proteins can
be hydrolysed to give their monomers, the
amino acids.
In the body this is carried out by enzymes.
The amino acids can pass through the gut wall
into the bloodstream. They are then taken to
cells which build them back up into the
proteins needed by the body.
[Diagram]
Hydrolysis is done in the lab by heating with
fairly concentrated hydrochloric acid under
‘reflux’.
Once hydrolysed the amino acids can be
separated and identified using
chromatography.
Fats and Oils
These are high energy food compounds. They
produce about twice as much energy as an
equal mass of carbohydrate.
Fats tend to come from land animals while oils
come from plants and marine animals.
Both are broken down to give glycerol and 3
fatty acids but fats are solids at room
temperature while oils are liquids. So there
must be differences in their structure to
cause the different physical states.
The main difference is the higher level of
unsaturation in oils.
Oils have much more C=C double bonds than
fats.
This means they would decolourise more
bromine solution than an equal mass of fat.
Consequences of Unsaturation
The shape of both oil and fat molecules is
roughly that of a tuning fork. If there are no
double bonds the molecules can fit into one
another.
This creates an ordered structure with lots
of Van der Waals forces between parallel
chains and neighbouring molecules.
However, as double bonds appear, the chains
cannot pack as closely together.
This means less Van der Waals forces
between molecules and so, lower melting and
boiling points. Hence, the saturated fats are
solids and the unsaturated oils are liquids at
room temperature.
Hardening of Oils
If the cause of oils being liquids is the high
level of unsaturation it seems reasonable to
think that making them more saturated would
make them more solid, or harden them.
Oils are hardened by hydrogenation – addition
of hydrogen across the C=C double bonds.
Margarines are made by partial hydrogenation
of oils over a nickel catalyst.
This is how alternatives to butter are
produced from vegetable oils.
The Structure of Fats and Oils
Fats and oils are triesters, formed from a
triol, glycerol, and 3 long-chain carboxylic
acids, called fatty acids.
glycerol
It is in the acid chains that we find any double
bonds. Some of the most common fatty acids
are:
Palmitic
acid
Stearic acid
Oleic acid
Linoleic acid
CH3(CH2)14COOH
CH3(CH2)16COOH
CH3(CH2)7CH=CH(CH2)7COOH
CH3(CH2)3(CH2CH=CH)2(CH2)7COOH
Fats and oils can come from 3 of the same
fatty acids, but more commonly a combination
of 3 fatty acids.
The 3 acids form ester linkages with the
glycerol by condensation reactions.
Example
Hydrolysis of Fats and Oils
(Digestion)
Esters can undergo a hydrolysis reaction to
give an alcohol and a carboxylic acid.
Fat and oil molecules can be hydrolysed to
give glycerol and 3 fatty acids. This can be
done by treatment with superheated steam.
In the lab it is normally done using aqueous
acid or alkali.
Fats and Health
The intake of saturated fat in the diet is
linked to heart disease in many countries,
including Scotland. Mediterranean countries
tend to have more unsaturated oils in their
diet and the incidence of heart disease in
these countries is much lower.