Download Chemicals of life

Do you think the bodies of the
following animals are composed
of similar chemical constituents?
What are the chemical constituents that might exist
in the bodies of animals?
Do you think the bodies of plants and other
organisms also contain these chemical constituents?
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Introduction
• A variety of organisms are made up of
similar chemical constituents
• The chemical constituents are divided into:
- Organic substances
- Inorganic substances
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• Organic substances are composed of carbon
and other elements
• Inorganic substances do not contain any carbon
Chemical constituents
Organic substances
Carbohydrates
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Lipids
Proteins
Inorganic substances
DNA
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Water
Minerals
What are carbohydrates?
• Organic substances composed of carbon, hydrogen
and oxygen
• The ratio of hydrogen to oxygen = 2:1
• General formula : Cx(H2O)y (where x and y are
whole numbers)
• Chemical formula of glucose : C6H12O6
• Energy-rich substances  important energy
sources for organisms
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Simple sugars
• The simplest form of
carbohydrates
• Characteristics:
– Sweet in taste
– Soluble in water
– Have reducing power
Glucose is used in
supplements
• Glucose is the most common simple sugar found
in organisms
• Other common example: fructose (in fruits)
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Double sugars
• Formed by the combination of two simple sugar
molecules
• Common examples: maltose (malt sugar),
sucrose (cane sugar) and lactose (milk sugar)
Maltose (used as
a kind of candy)
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Sucrose (cane sugar, rock
sugar and table sugar)
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• A double sugar molecule can be
converted to two simple sugar molecules
by specific digestive enzymes in the body.
• Characteristics:
– Sweet in taste
– Soluble in water
– Have reducing power (except sucrose)
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How can we determine the
presence of reducing
sugar or glucose in a
sample?
Practical 3.1
Testing the presence of reducing sugar
using the Benedict’s test
Practical 3.2
Testing the presence of glucose using
Clinistix paper
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Testing the presence of reducing
sugar using Benedict’s test
Practical 3.1
Testing the presence of reducing sugar using the
Benedict’s test
Introduction
Benedict’s test can be used
to test the presence of a
reducing sugar in a solution.
 Wear safety
spectacles when
heating Benedict’s
solution.
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Procedure
1. Label two test tubes as A and B.
2. Pour 2 cm3 of glucose solution into tube
A, and 2 cm3 of distilled water into tube B.
3. Add 2 cm3 of Benedict’s solution to
each tube and shake gently.
4. Boil the test tubes in a water bath for
5 minutes. Shake the tubes at intervals.
Note the colour changes of the mixtures.
5. Stand the test tubes for a few minutes.
Note any precipitate formed at the bottom
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of each tube.
Analysis
1. Compare the results of tube A and tube B.
What colour changes can be observed? How
would you make use of Benedict’s solution to
find out the content of a food sample?
Ans: Brick-red precipitate formed in tube A; no precipitate
formed in tube B. The colour of Benedict’s solution changes
from blue to green to yellow and then brick red and is used
for testing the presence of reducing sugars
2. Why should we test distilled water as a control
experiment in this practical?
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Ans: To ensure that the experimental result is only due
to the presence of reducing sugar
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2 Testing the presence of
glucose using Clinistix paper
Practical 3.2
Testing the presence of glucose using
Clinistix paper
Introduction
Clinistix paper
is commonly used to
test for the presence
of glucose.
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Procedure
1. Put a drop of glucose
solution on a spot tile.
2. Dip a piece of Clinistix
paper into the glucose
solution. Note any colour
changes.
3. Repeat the above test
with distilled water and
then solid glucose. Note
any
colour
changes.
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Analysis
1. What is the purpose of testing distilled water?
Ans: As a control experiment; ensure that the colour change
is due to glucose but not water
2. What is the purpose of testing solid glucose?
Ans: To indicate that Clinistix paper can be applied for
liquid samples only
3. What is the use of Clinistix paper?
Ans: The change from pink to blue colour of Clinistix paper
indicates the presence of glucose
Further investigation
People suffering from diabetes may use Clinistix paper
to test their urine. Try to find out why they do so.
Ans: To test whether glucose is present in the urine or not.
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Polysaccharides
• Made up of a large number
of simple sugar units
• Characteristics:
– Do not give a
sweet
taste
– Insoluble or very slightly
soluble in water
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• Common examples:
– Starch – the main form of carbohydrate
storage
in plants
– Cellulose – the main component of the
plant
cell walls Where comes the glucose
for forming the starch
and cellulose in plants?
– Glycogen – a common form of
carbohydrate
storage in animals. In mammals,
it is mainly
stored in the liver and skeletal
muscles
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What processes are involved in building
up and breaking down sugar molecules?
Condensation
2 C6H12O6 Condensation
simple sugar
C12H22O11 + H2O
Double sugar Water
e.g. A large number of
A polysaccharide
Condensation
17 sugar molecules
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simple
molecule
Hydrolysis
C12H22O11 + H2O Hydrolysis 2 C6H12O6
Double sugar Water
Simple sugar
A polysaccharide
molecule
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Hydrolysis
A large number of
simple sugar molecules
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3 Testing the presence of
starch using the iodine test
Practical 3.3
Testing the presence of starch using
the iodine test
Introduction
Iodine solution is commonly used to test
for the presence of starch.
 Handle iodine
solution carefully
as it is irritant.
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Procedure
1. Add a drop of starch
solution on a spot tile.
2. Add a drop of iodine
solution to the starch
solution.
3. Note any colour
change of the mixture.
4. Design and perform a
control
experiment.
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Analysis
From the results, what can you say about the use
of the iodine test?
Ans: The change from brown to blueblack colour of iodine solution indicates
the presence of starch
Further investigation
How would you perform the iodine test on solid food?
Ans: Just add a few drops of iodine
solution directly on solid starch and observe the
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colour
change © Manhattan Press (H.K.) Ltd.
Functions of Carbohydrates
1) Glucose releases energy when
broken
down during respiration.
Double sugars
and polysaccharides
have to be
converted into glucose or
other simple
sugars before being used
in respiration.
2) Cellulose is the main component
of
plant cell walls, which make
them
strong and rigid.
3) In animals, cellulose, in the form
of 22
dietary©fibres,
stimulates
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Ltd.
What are lipids?
• Organic substances composed of carbon, hydrogen
and oxygen
• The ratio of hydrogen to oxygen is always  2:1
Example: a common animal fat has the
chemical formula C54H110O3 where H:O d 37:1
• Characteristics:
– Insoluble in water
– Readily soluble in organic solvents such
as
alcohol, ether and chloroform
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Those which exist as
solids or semi-solids at
room temperature are
called fats.
Those which exist as
liquid at room temperature
are called oil
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The structure of a fat molecule
Note that the three fatty acid molecules
may be the same or different
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Testing the presence of fat
using the grease spot test
Practical 3.4
Testing the presence of fat using the grease
spot test
Introduction
The grease spot test can be used to test for the
presence of fat.
Procedure
1. Place a small drop of peanut
oil on a small piece of filter
paper.
2. Alongside the oil spot place a
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drop of distilled water.
3. Hold the paper up against light to examine the
oil spot.
4. Leave the paper in the air to dry for 10 minutes.
Examine the oil spot against light.
5. Dip the filter paper into ether and shake. After
drying it in air, examine the oil spot against light.
The test using ether should be carried
out in the fume cupboard.
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Analysis
1. Consider the results of the test, how do we know
whether a liquid sample contains fat?
Ans: The presence of a translucent spot
on a filter paper after drying indicates
the presence of fat
2. Describe the effect of ether on fat.
Ans: Translucent spot disappears
after dipping in ether; because fat
dissolves in ether (an organic solvent)
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What are the functions of fats?
1) Fats are important sources of energy for our
body, releasing twice as much energy
as carbohydrates and proteins during respiration
2) They act as energy reserves
A mammal with a large amount of fat stored in the body
3) They act as a heat-insulating layer under the
skin
and help to reduce heat loss from the
body
surface
4) Fat around internal organs absorbs shock
and
protects them from injury
5) In plants, fat stored in the form of oil in
seeds
serve as energy reserve.
6) Fats are major components of cell
membrane
and certain animal hormones
To know more about fats, visit:
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• http://www.bbc.co.uk/education/
asguru/biology/02biologicalmol
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ecules/03lipids/index.shtml
What are proteins?
• Large organic molecules composed of carbon,
hydrogen, oxygen and nitrogen
• Some of them also contain sulphur
• The basic units of proteins are amino acids
• In the human body, there are 20 different
kinds of amino acids
• The amino acid molecules combine in
different numbers and sequences  to form a
large variety of proteins
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• Structure of an amino acid:
• The side chains specify different amino acids
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Two amino acids condense to form a dipeptide
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• Many amino acid molecules  polypeptide
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One or more
polypeptide
chains  a
protein
molecule
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5 Testing the presence of
protein using Albustix paper
Practical 3.5
Testing the presence of protein using
Albustix paper
Introduction
Albustix paper is often
used to test for the
presence of protein.
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Procedure
1. Put a drop of egg white
on a spot tile.
2. Dip a piece of Albustix
paper into the egg white.
Note any colour
changes.
3. Repeat the above test
with peanut oil and
distilled water. Note any
colour
changes.© Manhattan Press (H.K.) Ltd.
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Analysis
1. Why do we test distilled water with Albustix paper?
Ans: As a control experiment
2. Why do we test peanut oil with Albustix paper?
Ans: Protein is present in peanut. It
is tested to see whether it has the
same effect on Albustix paper
3. What can you tell about the use of Albustix paper?
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Ans: The change from yellow to
green colour of Albustix paper indicates
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(H.K.) Ltd.
the presence
ofPress
protein
What are the functions of proteins?
• Important components of cell membrane and
cytoplasm, hence they are also
important for the formation of new cells
required for body growth and repair of wornout parts in the body
At what stages does a person need a
greater supply of proteins in their
diet?
•Proteins also provide energy for body activities
To know more about proteins, visit:
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• http://www.bbc.co.uk/education/asguru/biology/0
2biologicalmolecules/01proteins/index.shtml
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• Many important molecules in the body of
organisms are proteins, for example:
– Enzymes: act as catalyst for
chemical
reactions in the cells
– Antibodies: important for protecting
the
organisms from invading pathogens
– Hormones: involved in controlling various
processes in the body
Can you name one
hormone in your body
that is a protein?
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What is DNA?
• DNA stands for
deoxyribonucleic acid
• Structure: double helix
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What are the functions of DNA?
• Contains the information that controls the
activities of the cells
• The information governs inherited
characteristics
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What is the role of water in the body
of organisms?
• A major component of cytoplasm, blood and
body fluids
• A good solvent for many substances
• Water acts as:
– the medium for chemical reactions in cells
– the medium for transporting water-soluble
substances (e.g. nutrient and wastes) in
the
body
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What is the role of water in breathing?
• In the gas exchange of organisms on land:
oxygen and carbon dioxide  dissolves in
a film of water lining the respiratory
surface  enters or leaves the body
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What is the role of water in reactions
in the body of organisms?
• Water takes part in many cellular reactions
For example:
– Photosynthesis
– The breaking down of large molecules
into
smaller molecules in digestion
• Water is important for activating the enzymes
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What are other functions of water
in the body of organisms?
In plants
• The medium for transport of nutrients
and minerals inside the plant body
• Acts as a cooling agent through evaporation
• Maintains the turgidity of the cells in nonwoody plants
• Provides buoyancy to aquatic plants
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In mammals
• The medium for
transport of substances
inside the body
• Acts as a cooling agent
through the
evaporation of sweat
 regulation of body
temperature
• Acts as a lubricant, e.g. Water provides buoyancy to aquatic
animal
in swallowing
• Provides buoyancy to
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aquatic
animals © Manhattan Press (H.K.) Ltd.
Minerals
• Minerals are inorganic substances
• Important for the normal functioning of the cells
• Present in all living cells
In mammals
• Require a variety of minerals
• For example: calcium, sodium, potassium,
magnesium, phosphorus, iron and iodine
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In plants
• Also needed by plants for healthy growth
• Required in small amounts
• Land plants obtain minerals from soil water
Characteristics:
• Absorbed from food in the gut
• Have no energy value
• Usually needed in small amounts
• Some minerals are components of body
structures
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The following statements contain some wrong concepts commonly
held by students. Point them out and give correct statements.
1. Carbohydrates and fats are the only energy-giving
food substances.
Ans: Proteins are also energy-giving food substances
2. The appearance of a brick-red precipitate in Benedict’s test
shows the presence of all sugars or the presence of glucose.
Ans: The appearance of a brick-red precipitate in
Benedict’s test shows the presence of reducing sugars
only and glucose is only one example of reducing sugar
3. Only fat or oil can form a translucent spot on a piece of filter
paper.
Ans: Fat-containing substances can also form a
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translucent spot
on a piece of filter paper