Standard Grade Chemistry
Summary Notes
Topic 15. Carbohydrates
General
Learning Outcomes
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State that photosynthesis is the process by which plants make
carbohydrates from carbon dioxide and water, using light energy in the
presence of chlorophyll, oxygen is released in the process.
State that the role of chlorophyll in photosynthesis is to absorb light energy
State that the carbohydrates made in plants during photosynthesis are an
important food for animals.
State that respiration is the process by which animals and plants obtain a
supply of energy by breaking down carbohydrates using oxygen to give
carbon dioxide and water.
State that carbohydrates release energy, producing carbon dioxide and
water when burned.
Give examples of how energy can be used by animals
Explain the importance of respiration and photosynthesis in maintaining the
balance of carbon dioxide and oxygen in the air.
Explain why the extensive clearing of forests could present dangers to life
on Earth
State that glucose is the carbohydrate which reacts with oxygen during
respiration
State that carbohydrates contain the elements carbon, hydrogen and
oxygen.
State that sugar is sweet and dissolves in water and that starch is not sweet
and does not dissolve well in water
Explain what is seen when a beam of light is passed through :
glucose solution
starch in water
State that glucose is a carbohydrate built up in photosynthesis
State that starch is a polymer made in plants from glucose monomer units
Explain that the joining up of glucose molecules to form starch is an example
of polymerisation
State that it is possible to distinguish starch from other carbohydrates
using iodine solution.
State that Benedicts or Fehlings reagent is used to test for glucose , but
not for sucrose.
State that during digestion starch molecules are broken down in the body
into small glucose molecules which can pass through the gut wall.
State that the breakdown of starch can be carried out in the laboratory
using acid or amylase
State that enzymes, e.g. amylase act as biological catalysts in the breakdown
of complex food molecules into smaller ones in the digestive system.
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Give examples to show that the type of alcoholic drink varies with the plant
source of the carbohydrate.
State that an enzyme produced by yeast, a living organism, acts as a catalyst
for the reaction.
State that fermentation is the breakdown of glucose to form alcohol and
carbon dioxide.
State that distillation is a method of increasing the alcohol concentration of
fermentation products.
Explain why water and alcohol can be separated by distillation.
State that alcohol is a member of the alkanol family and is called ethanol.
Credit
Learning Outcomes
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Explain why the production of carbon dioxide and water, on burning,
indicates the presence of carbon and hydrogen in carbohydrates.
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Explain why glucose/fructose and maltose/sucrose are pairs of isomers
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Write the molecular formulae for monosaccharides and disaccharides.
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State that the glucose molecules join together with loss of water
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Explain that the joining up of glucose molecules to form starch is an example
of condensation polymerisation
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State that Benedicts or Fehlings reagent is used to test for glucose ,
fructose, maltose and other sugars but not for sucrose.
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State that starch molecules break down by reacting with water molecules.
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State what is meant by hydrolysis.
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Explain that the breakdown of starch and sucrose are examples of
hydrolysis.
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Describe the effect of changes in pH and temperature on the optimum
efficiency of an enzyme.
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Explain why there is a limit to the alcohol concentration of fermentation
products.
Carbohydrates
General
The food we eat contains all the nutrients that our bodies need to stay alive. Food is a
complex mixture of nutrients. The study of nutrients that make up food is made easier
by the fact that they can be classified into five types.
The five
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types of nutrient are :
Carbohydrates
Fat
Proteins
Vitamins
Minerals
Different foods contain varying amounts of each nutrient. It is important to eat a
variety of foods to ensure our body gets enough of each nutrient.
Carbohydrates are compounds of carbon, hydrogen and oxygen. The hydrogen and
oxygen are always in the same ratio as in water, i.e. 2 hydrogens for every oxygen.
Examples of carbohydrates include sugars such as glucose, sucrose, maltose and also
the substance starch.
Carbohydrates are made by plants in a process called photosynthesis.
Photosynthesis takes place in the leaves of a plant and requires carbon dioxide, which
comes from the air, and water which comes from the soil.
The photosynthesis reaction produces glucose, the simplest of all the carbohydrate
molecules, and oxygen gas.
For the photosynthesis reaction to take place leaves have to contain chlorophyll.
Chlorophyll, which makes plant leaves green, is the substance that absorbs energy
from sunlight to allow photosynthesis to take place.
The formula equation for photosynthesis is :
CO2 + H2O
sunlight

chlorophyll
C6H12O6 + O2
glucose
In most plants, the glucose that forms in the leaves is immediately converted to
starch. It is easy to test leaves to find out if they contain starch.
Before a leaf is tested for starch, the following have to be done :
1. The waterproof coating on the leaf has to be destroyed
2. The green colouring in the leaf, called chlorophyll, has to be removed
To destroy the waterproof coating
1.
Half fill a 250cm3 beaker with water
and heat until the water boils.
2.
Switch off the Bunsen burner.
3.
Put the leaf in the water and leave
there for one minute.
To remove the chlorophyll
1.
Collect a test tube of alcohol
2.
Remove the leaf from the water using
tongs and place it in the alcohol.
3.
Place the test tube in the hot water
for five minutes.
To test the leaf for starch
1. Remove the leaf from the alcohol. Rinse
the leaf with water and place on a dimple
tile.
2. Return the alcohol.
3. Place 2 or 3 drops of iodine solution on the
leaf and watch what happens.
Tests For Carbohydrates
General/Credit
Starch gives a dark blue/black colour when tested with iodine solution. This is a
specific test for starch and can be used to distinguish it from all other carbohydrates.
Some carbohydrates, such as glucose or sucrose, contain relatively small molecules and
fully dissolve in water to form true solutions. Starch is made up of much larger
molecules and fully dissolve that do not completely dissolve in water. Instead the
larger molecules form a suspension in the water called a colloid.
A light beam test can be used to distinguish a true solution from a colloid. When a
beam of light is shone through some water containing starch, an intense beam of light is
seen. This is due to reflection from the large molecules present. This does not happen
with solutions of smaller molecule sugars such as glucose or sucrose.
Benedict’s test
Glucose, fructose, maltose and lactose, but not sucrose give a positive test with
Benedict’s solution and heat. The positive test changes blue Benedict’s reagent to an
orange-red precipitate.
Summary of tests for carbohydrates
Carbohydrate
Solubility in
Light beam test
water
glucose
fructose
maltose
lactose
sucrose
starch
sweet solution
sweet solution
sweet solution
sweet solution
sweet solution
colloid
Types Of Carbohydrates
no intense beam
no intense beam
no intense beam
no intense beam
no intense beam
intense beam
Test with
iodine
negative
negative
negative
negative
negative
positive
Test with
Benedict’s
reagent + heat
positive
positive
positive
positive
negative
negative
General/Credit
Glucose and fructose are two of the simplest type of carbohydrate molecules. They
are building block molecules because they can join together to form many other
carbohydrates. Glucose and fructose can be called monomers.
A term used to classify glucose and fructose is monosaccharide.
Monosaccharides have six carbon atoms in each molecule and the formula C6H12O6.
Glucose and fructose have different structures, but because they both have the
formula C6H12O6 they can be considered isomers.
Two monosaccharide molecules can join together to form a disaccharide. When this
happens, it is not simply adding one molecule to another. A molecule of water also
forms. This type of reaction is called a condensation reaction. Disaccharides have the
formula C12H22O11.
Sucrose is a disaccharide made when a molecule of glucose and a molecule of fructose
join.
C6H12O6 + C6H12O6  C12H22O11 + H2O
glucose
fructose
sucrose
(monosaccharides)
(disaccharide)
Lactose and maltose are other examples of disaccharides.
Type of carbohydrate
monosaccharide
disaccharide
Molecular Formula
C6H12O6
C12H22O11
Examples
glucose, fructose
sucrose, maltose, lactose
Starch is another type of carbohydrate.
Energy From Carbohydrates
General/Credit
All living things need energy. You have seen that carbohydrates give out a lot of energy
when they burn.
A reaction similar to burning takes place in animals and plants. This releases the
energy in food by burning glucose (using oxygen), and produces carbon dioxide and
water.
Credit
The production of carbon dioxide and water also proves that
carbohydrates contain carbon and hydrogen – as these elements do not
come from the air.
This reaction is much slower than burning and is called respiration.
The word equation for respiration would be as follows :
glucose + oxygen  carbon dioxide + water + energy
Respiration is a very important reaction for all living things because the energy
produced can be used as :
heat – to provide warmth and maintain our body temperature at 37oC
movement – in the muscles of animals, to keep the heart beating, running
electrical – sending messages through the nerves to and from the brain
The Carbon Cycle
General
Plants carry out photosynthesis in the daylight hours only. All living things, both plants
and animals, require energy to live. The energy is provided by respiration.
In the daylight hours plants carry out both respiration and photosynthesis, but
photosynthesis takes place much faster.
A study of the word equations for these reactions shows that they are chemically the
exact opposite of each other.
Photosynthesis
carbon dioxide + water + energy  glucose + oxygen
Respiration
glucose + oxygen  carbon dioxide + water + energy.
Photosynthesis and respiration are two important processes in the Carbon cycle. The
carbon cycle shows how carbon, in carbon dioxide and other carbon compounds, is
recycled through various processes that occur in nature, and out everyday lives.
The Carbon cycle is a very important cycle to man as levels of carbon dioxide and
oxygen in the air are maintained by the reactions involved in the cycle – photosynthesis
and respiration.
In our modern society a lot of carbon based fuels are burned releasing huge amounts of
carbon dioxide into the atmosphere.
Apart from this increase in the amount of carbon dioxide entering the atmosphere
there is major destruction of tropical rainforests. As a result, scientists have
discovered that the amount of carbon dioxide in the atmosphere is increasing.
This increase is though to be contributing to the Greenhouse effect.
Starch
General /Credit
Glucose, made by plants in the photosynthesis reaction, is converted to starch. Starch
is made up of very large molecules.
Starch is in fact a polymer molecule made up of many glucose ,molecules joined
together. Starch is an example of a polysaccharide because it is formed by the joining
together of many monosaccharide units. The common way to write the formula of a
polysaccharide is (C6H10O5)n where n is a very large number ( about 300 for starch).
We can be more descriptive about the type of polymerisation. When glucose molecules
join together water is also formed. This type of polymerisation is called condensation
polymerisation.
Digestion
General/Credit
Digestion is the chemical breakdown of food into smaller molecules that can pass
through the gut wall and be absorbed into the blood.
Digestion of starch in food begins in the mouth where the starch meets saliva. Saliva
contains many chemicals but the one responsible for the digestion of starch is called
amylase. Amylase is an enzyme. Enzymes are molecules, produced by living things, which
catalyse reactions.
After your food is swallowed, digestion continues in the stomach. In the stomach the
food meets hydrochloric acid.
We can carry out digestion of starch in the laboratory using amylase and using
hydrochloric acid.
Experiments show that digestion breaks down large carbohydrate molecules, like
starch, into monosaccharides, like glucose.
Digestion of starch and sucrose (poly and disaccharides) is really the reverse of
building them up from monosaccharides. Breaking down poly and disaccharides will mean
that the water molecules which formed as the monosaccharides joined will have to be
replaced.
The breakdown of a large molecule by addition of the elements of water is called
hydrolysis.
Hydrolysis of a disaccharide requires one water molecule.
C12H22O11
+
disaccharide
H2O 
water
C6H12O6 + C6H12O6
monosaccharide molecules
Hydrolysis of a disaccharide requires one water molecule.
(C6H10O5)n
+
polysaccharide
n H2 O 
water
nC6H12O6
monosaccharides
Alcohol
General/Credit
Alcoholic drinks can be made from any fruit or vegetable that is a source of starch or
sugars. The type of alcoholic drink varies with the type of fruit or vegetable used.
Drink
beer
cider
vodka
whisky
wine
Fruit/ Vegetable
hops
apples
potatoes
barley
grapes
The process used to convert glucose into alcohol has been used for thousands of years
and is called fermentation. Fermentation is the breakdown of glucose to form alcohol
and carbon dioxide. The glucose for the fermentation comes from the breakdown of
the starch or sugars in the fruit or vegetable material.
An essential component of the fermentation process is yeast. Yeast, a living organism,
produces an enzyme that acts as a catalyst for the reaction. The fermentation
reaction is best carried out at about 37oC.
C6H12O6
glucose
yeast

37oC
2C2H5OH
ethanol
+ 2CO2
The ethanol formed in the reaction is a member of the family of alcohols called the
alkanols.
After a few days the rate of the fermentation reaction slows down. As the
concentration of alcohol increases the efficiency of the yeast decreases. The
alcohol effectively poisons the yeast When the concentration of the alcohol reaches
about 10% the yeast stops operating and the fermentation reaction stops. Some
alcohol drinks e.g. beer, cider and wine are made by fermentation only.
Alcoholic drink with a greater concentration of alcohol e.g. whisky at about 40% have an
additional distillation step in their production. The alcohol and water can be separated
by distillation because the water and alcohol have different boiling points.