Biomolecules | Zoology Optional Notes for UPSC PDF Download

 Page 1


There is a wide diversity in living organisms in our biosphere. Now a
question that arises in our minds is: Are all living organisms made of the
same chemicals, i.e., elements and compounds? You have learnt in
chemistry how elemental analysis is performed. If we perform such an
analysis on a plant tissue, animal tissue or a microbial paste, we obtain a
list of elements like carbon, hydrogen, oxygen and several others and
their respective content per unit mass of a living tissue. If the same analysis
is performed on a piece of earth’s crust as an example of non-living matter,
we obtain a similar list. What are the differences between the two lists? In
absolute terms, no such differences could be made out. All the elements
present in a sample of earth’s crust are also present in a sample of living
tissue. However, a closer examination reveals that the relative abundance
of carbon and hydrogen with respect to other elements is higher in any
living organism than in earth’s crust (Table 9.1).
9.1 HOW TO ANALYSE CHEMICAL COMPOSITION?
We can continue asking in the same way, what type of organic compounds
are found in living organisms? How does one go about finding the answer?
To get an answer, one has to perform a chemical analysis. We can take any
living tissue (a vegetable or a piece of liver, etc.) and grind it in trichloroacetic
acid (Cl
3
CCOOH) using a mortar and a pestle. We obtain a thick slurry. If
we were to strain this through a cheesecloth or cotton we would obtain two
fractions. One is called the filtrate or more technically, the acid-soluble
pool, and the second, the retentate or the acid-insoluble fraction. Scientists
have found thousands of organic compounds in the acid-soluble pool.
BIOMOLECULES
  
Page 2


There is a wide diversity in living organisms in our biosphere. Now a
question that arises in our minds is: Are all living organisms made of the
same chemicals, i.e., elements and compounds? You have learnt in
chemistry how elemental analysis is performed. If we perform such an
analysis on a plant tissue, animal tissue or a microbial paste, we obtain a
list of elements like carbon, hydrogen, oxygen and several others and
their respective content per unit mass of a living tissue. If the same analysis
is performed on a piece of earth’s crust as an example of non-living matter,
we obtain a similar list. What are the differences between the two lists? In
absolute terms, no such differences could be made out. All the elements
present in a sample of earth’s crust are also present in a sample of living
tissue. However, a closer examination reveals that the relative abundance
of carbon and hydrogen with respect to other elements is higher in any
living organism than in earth’s crust (Table 9.1).
9.1 HOW TO ANALYSE CHEMICAL COMPOSITION?
We can continue asking in the same way, what type of organic compounds
are found in living organisms? How does one go about finding the answer?
To get an answer, one has to perform a chemical analysis. We can take any
living tissue (a vegetable or a piece of liver, etc.) and grind it in trichloroacetic
acid (Cl
3
CCOOH) using a mortar and a pestle. We obtain a thick slurry. If
we were to strain this through a cheesecloth or cotton we would obtain two
fractions. One is called the filtrate or more technically, the acid-soluble
pool, and the second, the retentate or the acid-insoluble fraction. Scientists
have found thousands of organic compounds in the acid-soluble pool.
BIOMOLECULES
  
In higher classes you will learn about how
to analyse a living tissue sample and identify a
particular organic compound. It will suffice to
say here that one extracts the compounds, then
subjects the extract to various separation
techniques till one has separated a compound
from all other compounds. In other words, one
isolates and purifies a compound. Analytical
techniques, when applied to the compound give
us an idea of the molecular formula and the
probable structure of the compound. All the
carbon compounds that we get from living
tissues can be called ‘biomolecules’. However,
living organisms have also got inorganic
elements and compounds in them. How do we
know this? A slightly different but destructive
experiment has to be done. One weighs a small
amount of a living tissue (say a leaf or liver and
this is called wet weight) and dry it. All the water,
evaporates. The remaining material gives dry
weight. Now if the tissue is fully burnt, all the
carbon compounds are oxidised to gaseous
form (CO
2
, water vapour) and are removed. What
is remaining is called ‘ash’. This ash contains
inorganic elements (like calcium, magnesium
etc). Inorganic compounds like sulphate,
phosphate, etc., are also seen in the acid-soluble
fraction. Therefore elemental analysis gives
elemental composition of living tissues in the
form of hydrogen, oxygen, chlorine, carbon etc.
while analysis for compounds gives an idea of
Element % Weight of
  Earth’s crust Human body
Hydrogen (H) 0.14 0.5
Carbon  (C) 0.03 18.5
Oxygen (O) 46.6 65.0
Nitrogen (N) very little 3.3
Sulphur (S) 0.03 0.3
Sodium (Na) 2.8 0.2
Calcium (Ca) 3.6 1.5
Magnesium (Mg) 2.1 0.1
Silicon (Si) 27.7 negligible
* Adapted from CNR Rao, Understanding Chemistry,
Universities Press, Hyderabad.
TABLE 9.1 A Comparison of Elements Present
in Non-living and Living Matter*
Component Formula
Sodium Na
+
Potassium K
+
Calcium Ca
++
Magnesium Mg
++
Water H
2
O
Compounds NaCl, CaCO
3
,
PO SO
4
3
4
2 - -
,
TABLE 9.2 A List of Representative Inorganic
Constituents of Living Tissues
the kind of organic (Figure 9.1) and inorganic constituents (Table 9.2)
present in living tissues. From a chemistry point of view, one can identify
functional groups like aldehydes, ketones, aromatic compounds, etc. But
from a biological point of view, we shall classify them into amino acids,
nucleotide bases, fatty acids etc.
Amino acids are organic compounds containing an amino group and
an acidic group as substituents on the same carbon i.e., the a-carbon.
Hence, they are called a-amino acids. They are substituted methanes. There
are four substituent groups occupying the four valency positions. These
are hydrogen, carboxyl group, amino group and a variable group
designated as R group. Based on the nature of R group there are many
amino acids. However, those which occur in proteins are only of twenty
Page 3


There is a wide diversity in living organisms in our biosphere. Now a
question that arises in our minds is: Are all living organisms made of the
same chemicals, i.e., elements and compounds? You have learnt in
chemistry how elemental analysis is performed. If we perform such an
analysis on a plant tissue, animal tissue or a microbial paste, we obtain a
list of elements like carbon, hydrogen, oxygen and several others and
their respective content per unit mass of a living tissue. If the same analysis
is performed on a piece of earth’s crust as an example of non-living matter,
we obtain a similar list. What are the differences between the two lists? In
absolute terms, no such differences could be made out. All the elements
present in a sample of earth’s crust are also present in a sample of living
tissue. However, a closer examination reveals that the relative abundance
of carbon and hydrogen with respect to other elements is higher in any
living organism than in earth’s crust (Table 9.1).
9.1 HOW TO ANALYSE CHEMICAL COMPOSITION?
We can continue asking in the same way, what type of organic compounds
are found in living organisms? How does one go about finding the answer?
To get an answer, one has to perform a chemical analysis. We can take any
living tissue (a vegetable or a piece of liver, etc.) and grind it in trichloroacetic
acid (Cl
3
CCOOH) using a mortar and a pestle. We obtain a thick slurry. If
we were to strain this through a cheesecloth or cotton we would obtain two
fractions. One is called the filtrate or more technically, the acid-soluble
pool, and the second, the retentate or the acid-insoluble fraction. Scientists
have found thousands of organic compounds in the acid-soluble pool.
BIOMOLECULES
  
In higher classes you will learn about how
to analyse a living tissue sample and identify a
particular organic compound. It will suffice to
say here that one extracts the compounds, then
subjects the extract to various separation
techniques till one has separated a compound
from all other compounds. In other words, one
isolates and purifies a compound. Analytical
techniques, when applied to the compound give
us an idea of the molecular formula and the
probable structure of the compound. All the
carbon compounds that we get from living
tissues can be called ‘biomolecules’. However,
living organisms have also got inorganic
elements and compounds in them. How do we
know this? A slightly different but destructive
experiment has to be done. One weighs a small
amount of a living tissue (say a leaf or liver and
this is called wet weight) and dry it. All the water,
evaporates. The remaining material gives dry
weight. Now if the tissue is fully burnt, all the
carbon compounds are oxidised to gaseous
form (CO
2
, water vapour) and are removed. What
is remaining is called ‘ash’. This ash contains
inorganic elements (like calcium, magnesium
etc). Inorganic compounds like sulphate,
phosphate, etc., are also seen in the acid-soluble
fraction. Therefore elemental analysis gives
elemental composition of living tissues in the
form of hydrogen, oxygen, chlorine, carbon etc.
while analysis for compounds gives an idea of
Element % Weight of
  Earth’s crust Human body
Hydrogen (H) 0.14 0.5
Carbon  (C) 0.03 18.5
Oxygen (O) 46.6 65.0
Nitrogen (N) very little 3.3
Sulphur (S) 0.03 0.3
Sodium (Na) 2.8 0.2
Calcium (Ca) 3.6 1.5
Magnesium (Mg) 2.1 0.1
Silicon (Si) 27.7 negligible
* Adapted from CNR Rao, Understanding Chemistry,
Universities Press, Hyderabad.
TABLE 9.1 A Comparison of Elements Present
in Non-living and Living Matter*
Component Formula
Sodium Na
+
Potassium K
+
Calcium Ca
++
Magnesium Mg
++
Water H
2
O
Compounds NaCl, CaCO
3
,
PO SO
4
3
4
2 - -
,
TABLE 9.2 A List of Representative Inorganic
Constituents of Living Tissues
the kind of organic (Figure 9.1) and inorganic constituents (Table 9.2)
present in living tissues. From a chemistry point of view, one can identify
functional groups like aldehydes, ketones, aromatic compounds, etc. But
from a biological point of view, we shall classify them into amino acids,
nucleotide bases, fatty acids etc.
Amino acids are organic compounds containing an amino group and
an acidic group as substituents on the same carbon i.e., the a-carbon.
Hence, they are called a-amino acids. They are substituted methanes. There
are four substituent groups occupying the four valency positions. These
are hydrogen, carboxyl group, amino group and a variable group
designated as R group. Based on the nature of R group there are many
amino acids. However, those which occur in proteins are only of twenty
types. The R group in these proteinaceous amino acids could be a hydrogen
(the amino acid is called glycine), a methyl group (alanine), hydroxy methyl
(serine), etc. Three of the twenty are shown in Figure 9.1.
The chemical and physical properties of amino acids are essentially
of the amino, carboxyl and the R functional groups. Based on number of
amino and carboxyl groups, there are acidic (e.g., glutamic acid), basic
(lysine) and neutral (valine) amino acids. Similarly, there are aromatic
amino acids (tyrosine, phenylalanine, tryptophan). A particular property
of amino acids is the ionizable nature of –NH
2
 and –COOH groups. Hence
in solutions of different pH, the structure of amino acids changes.
B is called zwitterionic form.
Lipids are generally water insoluble. They could be simple fatty acids.
A fatty acid has a carboxyl group attached to an R group. The R group
could be a methyl (–CH
3
), or ethyl  (–C
2
H
5
) or higher number of –CH
2
groups (1 carbon to 19 carbons). For example, palmitic acid has 16
carbons including carboxyl carbon. Arachidonic acid has 20 carbon
atoms including the carboxyl carbon. Fatty acids could be saturated
(without double bond) or unsaturated (with one or more C=C double
bonds). Another simple lipid is glycerol which is trihydroxy propane. Many
lipids have both glycerol and fatty acids. Here the fatty acids are found
esterified with glycerol. They can be then monoglycerides, diglycerides
and triglycerides. These are also called fats and oils based on melting
point. Oils have lower melting point (e.g., gingelly oil) and hence remain
as oil in winters. Can you identify a fat from the market? Some lipids
have phosphorous and a phosphorylated organic compound in them.
These are phospholipids. They are found in cell membrane. Lecithin is
one example. Some tissues especially the neural tissues have lipids with
more complex structures.
Living organisms have a number of carbon compounds in which
heterocyclic rings can be found. Some of these are nitrogen bases –
adenine, guanine, cytosine, uracil, and thymine. When found attached to
a sugar, they are called nucleosides. If a phosphate group is also found
esterified to the sugar they are called nucleotides. Adenosine, guanosine,
thymidine, uridine and  cytidine are nucleosides. Adenylic acid, thymidylic
acid, guanylic acid, uridylic acid and cytidylic acid are nucleotides. Nucleic
acids like DNA and RNA consist of nucleotides only. DNA and RNA function
as genetic material.
Page 4


There is a wide diversity in living organisms in our biosphere. Now a
question that arises in our minds is: Are all living organisms made of the
same chemicals, i.e., elements and compounds? You have learnt in
chemistry how elemental analysis is performed. If we perform such an
analysis on a plant tissue, animal tissue or a microbial paste, we obtain a
list of elements like carbon, hydrogen, oxygen and several others and
their respective content per unit mass of a living tissue. If the same analysis
is performed on a piece of earth’s crust as an example of non-living matter,
we obtain a similar list. What are the differences between the two lists? In
absolute terms, no such differences could be made out. All the elements
present in a sample of earth’s crust are also present in a sample of living
tissue. However, a closer examination reveals that the relative abundance
of carbon and hydrogen with respect to other elements is higher in any
living organism than in earth’s crust (Table 9.1).
9.1 HOW TO ANALYSE CHEMICAL COMPOSITION?
We can continue asking in the same way, what type of organic compounds
are found in living organisms? How does one go about finding the answer?
To get an answer, one has to perform a chemical analysis. We can take any
living tissue (a vegetable or a piece of liver, etc.) and grind it in trichloroacetic
acid (Cl
3
CCOOH) using a mortar and a pestle. We obtain a thick slurry. If
we were to strain this through a cheesecloth or cotton we would obtain two
fractions. One is called the filtrate or more technically, the acid-soluble
pool, and the second, the retentate or the acid-insoluble fraction. Scientists
have found thousands of organic compounds in the acid-soluble pool.
BIOMOLECULES
  
In higher classes you will learn about how
to analyse a living tissue sample and identify a
particular organic compound. It will suffice to
say here that one extracts the compounds, then
subjects the extract to various separation
techniques till one has separated a compound
from all other compounds. In other words, one
isolates and purifies a compound. Analytical
techniques, when applied to the compound give
us an idea of the molecular formula and the
probable structure of the compound. All the
carbon compounds that we get from living
tissues can be called ‘biomolecules’. However,
living organisms have also got inorganic
elements and compounds in them. How do we
know this? A slightly different but destructive
experiment has to be done. One weighs a small
amount of a living tissue (say a leaf or liver and
this is called wet weight) and dry it. All the water,
evaporates. The remaining material gives dry
weight. Now if the tissue is fully burnt, all the
carbon compounds are oxidised to gaseous
form (CO
2
, water vapour) and are removed. What
is remaining is called ‘ash’. This ash contains
inorganic elements (like calcium, magnesium
etc). Inorganic compounds like sulphate,
phosphate, etc., are also seen in the acid-soluble
fraction. Therefore elemental analysis gives
elemental composition of living tissues in the
form of hydrogen, oxygen, chlorine, carbon etc.
while analysis for compounds gives an idea of
Element % Weight of
  Earth’s crust Human body
Hydrogen (H) 0.14 0.5
Carbon  (C) 0.03 18.5
Oxygen (O) 46.6 65.0
Nitrogen (N) very little 3.3
Sulphur (S) 0.03 0.3
Sodium (Na) 2.8 0.2
Calcium (Ca) 3.6 1.5
Magnesium (Mg) 2.1 0.1
Silicon (Si) 27.7 negligible
* Adapted from CNR Rao, Understanding Chemistry,
Universities Press, Hyderabad.
TABLE 9.1 A Comparison of Elements Present
in Non-living and Living Matter*
Component Formula
Sodium Na
+
Potassium K
+
Calcium Ca
++
Magnesium Mg
++
Water H
2
O
Compounds NaCl, CaCO
3
,
PO SO
4
3
4
2 - -
,
TABLE 9.2 A List of Representative Inorganic
Constituents of Living Tissues
the kind of organic (Figure 9.1) and inorganic constituents (Table 9.2)
present in living tissues. From a chemistry point of view, one can identify
functional groups like aldehydes, ketones, aromatic compounds, etc. But
from a biological point of view, we shall classify them into amino acids,
nucleotide bases, fatty acids etc.
Amino acids are organic compounds containing an amino group and
an acidic group as substituents on the same carbon i.e., the a-carbon.
Hence, they are called a-amino acids. They are substituted methanes. There
are four substituent groups occupying the four valency positions. These
are hydrogen, carboxyl group, amino group and a variable group
designated as R group. Based on the nature of R group there are many
amino acids. However, those which occur in proteins are only of twenty
types. The R group in these proteinaceous amino acids could be a hydrogen
(the amino acid is called glycine), a methyl group (alanine), hydroxy methyl
(serine), etc. Three of the twenty are shown in Figure 9.1.
The chemical and physical properties of amino acids are essentially
of the amino, carboxyl and the R functional groups. Based on number of
amino and carboxyl groups, there are acidic (e.g., glutamic acid), basic
(lysine) and neutral (valine) amino acids. Similarly, there are aromatic
amino acids (tyrosine, phenylalanine, tryptophan). A particular property
of amino acids is the ionizable nature of –NH
2
 and –COOH groups. Hence
in solutions of different pH, the structure of amino acids changes.
B is called zwitterionic form.
Lipids are generally water insoluble. They could be simple fatty acids.
A fatty acid has a carboxyl group attached to an R group. The R group
could be a methyl (–CH
3
), or ethyl  (–C
2
H
5
) or higher number of –CH
2
groups (1 carbon to 19 carbons). For example, palmitic acid has 16
carbons including carboxyl carbon. Arachidonic acid has 20 carbon
atoms including the carboxyl carbon. Fatty acids could be saturated
(without double bond) or unsaturated (with one or more C=C double
bonds). Another simple lipid is glycerol which is trihydroxy propane. Many
lipids have both glycerol and fatty acids. Here the fatty acids are found
esterified with glycerol. They can be then monoglycerides, diglycerides
and triglycerides. These are also called fats and oils based on melting
point. Oils have lower melting point (e.g., gingelly oil) and hence remain
as oil in winters. Can you identify a fat from the market? Some lipids
have phosphorous and a phosphorylated organic compound in them.
These are phospholipids. They are found in cell membrane. Lecithin is
one example. Some tissues especially the neural tissues have lipids with
more complex structures.
Living organisms have a number of carbon compounds in which
heterocyclic rings can be found. Some of these are nitrogen bases –
adenine, guanine, cytosine, uracil, and thymine. When found attached to
a sugar, they are called nucleosides. If a phosphate group is also found
esterified to the sugar they are called nucleotides. Adenosine, guanosine,
thymidine, uridine and  cytidine are nucleosides. Adenylic acid, thymidylic
acid, guanylic acid, uridylic acid and cytidylic acid are nucleotides. Nucleic
acids like DNA and RNA consist of nucleotides only. DNA and RNA function
as genetic material.
Cholesterol
Phospholipid (Lecithin)
Fats and oils (lipids)
(CH )
2 14
CH
3
COOH
Fatty acid
(Palmitic acid)
Glycerol
Triglyceride (R
1
, R
2
and R
3
 are fatty acids)
Nitrogen bases
OH OH
Adenine
O
OCH
2
P HO
OH
O
Adenylic acid
Nucleotide
OH
OH
HOCH
2
Adenine O
OH OH
HOCH
2
Uracil
O
Adenosine
Uridine
Nucleosides
OH
OH OH
HOCH
2
O
OH
OH
HO OH
CH OH
2
O
C
6
H
12
O
6 
(Glucose) C
5
H
10
O
5 
(Ribose)
Sugars (Carbohydrates)
Serine Glycine
Amino acids
Alanine
Figure 9.1 Diagrammatic representation of small molecular weight organic
compounds in living tissues
O
O
HN
N
H
Adenine (Purine)
Uracil (Pyrimidine)
Page 5


There is a wide diversity in living organisms in our biosphere. Now a
question that arises in our minds is: Are all living organisms made of the
same chemicals, i.e., elements and compounds? You have learnt in
chemistry how elemental analysis is performed. If we perform such an
analysis on a plant tissue, animal tissue or a microbial paste, we obtain a
list of elements like carbon, hydrogen, oxygen and several others and
their respective content per unit mass of a living tissue. If the same analysis
is performed on a piece of earth’s crust as an example of non-living matter,
we obtain a similar list. What are the differences between the two lists? In
absolute terms, no such differences could be made out. All the elements
present in a sample of earth’s crust are also present in a sample of living
tissue. However, a closer examination reveals that the relative abundance
of carbon and hydrogen with respect to other elements is higher in any
living organism than in earth’s crust (Table 9.1).
9.1 HOW TO ANALYSE CHEMICAL COMPOSITION?
We can continue asking in the same way, what type of organic compounds
are found in living organisms? How does one go about finding the answer?
To get an answer, one has to perform a chemical analysis. We can take any
living tissue (a vegetable or a piece of liver, etc.) and grind it in trichloroacetic
acid (Cl
3
CCOOH) using a mortar and a pestle. We obtain a thick slurry. If
we were to strain this through a cheesecloth or cotton we would obtain two
fractions. One is called the filtrate or more technically, the acid-soluble
pool, and the second, the retentate or the acid-insoluble fraction. Scientists
have found thousands of organic compounds in the acid-soluble pool.
BIOMOLECULES
  
In higher classes you will learn about how
to analyse a living tissue sample and identify a
particular organic compound. It will suffice to
say here that one extracts the compounds, then
subjects the extract to various separation
techniques till one has separated a compound
from all other compounds. In other words, one
isolates and purifies a compound. Analytical
techniques, when applied to the compound give
us an idea of the molecular formula and the
probable structure of the compound. All the
carbon compounds that we get from living
tissues can be called ‘biomolecules’. However,
living organisms have also got inorganic
elements and compounds in them. How do we
know this? A slightly different but destructive
experiment has to be done. One weighs a small
amount of a living tissue (say a leaf or liver and
this is called wet weight) and dry it. All the water,
evaporates. The remaining material gives dry
weight. Now if the tissue is fully burnt, all the
carbon compounds are oxidised to gaseous
form (CO
2
, water vapour) and are removed. What
is remaining is called ‘ash’. This ash contains
inorganic elements (like calcium, magnesium
etc). Inorganic compounds like sulphate,
phosphate, etc., are also seen in the acid-soluble
fraction. Therefore elemental analysis gives
elemental composition of living tissues in the
form of hydrogen, oxygen, chlorine, carbon etc.
while analysis for compounds gives an idea of
Element % Weight of
  Earth’s crust Human body
Hydrogen (H) 0.14 0.5
Carbon  (C) 0.03 18.5
Oxygen (O) 46.6 65.0
Nitrogen (N) very little 3.3
Sulphur (S) 0.03 0.3
Sodium (Na) 2.8 0.2
Calcium (Ca) 3.6 1.5
Magnesium (Mg) 2.1 0.1
Silicon (Si) 27.7 negligible
* Adapted from CNR Rao, Understanding Chemistry,
Universities Press, Hyderabad.
TABLE 9.1 A Comparison of Elements Present
in Non-living and Living Matter*
Component Formula
Sodium Na
+
Potassium K
+
Calcium Ca
++
Magnesium Mg
++
Water H
2
O
Compounds NaCl, CaCO
3
,
PO SO
4
3
4
2 - -
,
TABLE 9.2 A List of Representative Inorganic
Constituents of Living Tissues
the kind of organic (Figure 9.1) and inorganic constituents (Table 9.2)
present in living tissues. From a chemistry point of view, one can identify
functional groups like aldehydes, ketones, aromatic compounds, etc. But
from a biological point of view, we shall classify them into amino acids,
nucleotide bases, fatty acids etc.
Amino acids are organic compounds containing an amino group and
an acidic group as substituents on the same carbon i.e., the a-carbon.
Hence, they are called a-amino acids. They are substituted methanes. There
are four substituent groups occupying the four valency positions. These
are hydrogen, carboxyl group, amino group and a variable group
designated as R group. Based on the nature of R group there are many
amino acids. However, those which occur in proteins are only of twenty
types. The R group in these proteinaceous amino acids could be a hydrogen
(the amino acid is called glycine), a methyl group (alanine), hydroxy methyl
(serine), etc. Three of the twenty are shown in Figure 9.1.
The chemical and physical properties of amino acids are essentially
of the amino, carboxyl and the R functional groups. Based on number of
amino and carboxyl groups, there are acidic (e.g., glutamic acid), basic
(lysine) and neutral (valine) amino acids. Similarly, there are aromatic
amino acids (tyrosine, phenylalanine, tryptophan). A particular property
of amino acids is the ionizable nature of –NH
2
 and –COOH groups. Hence
in solutions of different pH, the structure of amino acids changes.
B is called zwitterionic form.
Lipids are generally water insoluble. They could be simple fatty acids.
A fatty acid has a carboxyl group attached to an R group. The R group
could be a methyl (–CH
3
), or ethyl  (–C
2
H
5
) or higher number of –CH
2
groups (1 carbon to 19 carbons). For example, palmitic acid has 16
carbons including carboxyl carbon. Arachidonic acid has 20 carbon
atoms including the carboxyl carbon. Fatty acids could be saturated
(without double bond) or unsaturated (with one or more C=C double
bonds). Another simple lipid is glycerol which is trihydroxy propane. Many
lipids have both glycerol and fatty acids. Here the fatty acids are found
esterified with glycerol. They can be then monoglycerides, diglycerides
and triglycerides. These are also called fats and oils based on melting
point. Oils have lower melting point (e.g., gingelly oil) and hence remain
as oil in winters. Can you identify a fat from the market? Some lipids
have phosphorous and a phosphorylated organic compound in them.
These are phospholipids. They are found in cell membrane. Lecithin is
one example. Some tissues especially the neural tissues have lipids with
more complex structures.
Living organisms have a number of carbon compounds in which
heterocyclic rings can be found. Some of these are nitrogen bases –
adenine, guanine, cytosine, uracil, and thymine. When found attached to
a sugar, they are called nucleosides. If a phosphate group is also found
esterified to the sugar they are called nucleotides. Adenosine, guanosine,
thymidine, uridine and  cytidine are nucleosides. Adenylic acid, thymidylic
acid, guanylic acid, uridylic acid and cytidylic acid are nucleotides. Nucleic
acids like DNA and RNA consist of nucleotides only. DNA and RNA function
as genetic material.
Cholesterol
Phospholipid (Lecithin)
Fats and oils (lipids)
(CH )
2 14
CH
3
COOH
Fatty acid
(Palmitic acid)
Glycerol
Triglyceride (R
1
, R
2
and R
3
 are fatty acids)
Nitrogen bases
OH OH
Adenine
O
OCH
2
P HO
OH
O
Adenylic acid
Nucleotide
OH
OH
HOCH
2
Adenine O
OH OH
HOCH
2
Uracil
O
Adenosine
Uridine
Nucleosides
OH
OH OH
HOCH
2
O
OH
OH
HO OH
CH OH
2
O
C
6
H
12
O
6 
(Glucose) C
5
H
10
O
5 
(Ribose)
Sugars (Carbohydrates)
Serine Glycine
Amino acids
Alanine
Figure 9.1 Diagrammatic representation of small molecular weight organic
compounds in living tissues
O
O
HN
N
H
Adenine (Purine)
Uracil (Pyrimidine)
9.2 PRIMARY AND SECONDARY METABOLITES
The most exciting aspect of chemistry deals with isolating thousands of
compounds, small and big, from living organisms, determining their
structure and if possible synthesising them.
If one were to make a list of biomolecules, such a list would have
thousands of organic compounds including amino acids, sugars, etc.
For reasons that are given in section 9.10, we can call these biomolecules
as ‘metabolites’. In animal tissues, one notices the presence of all such
categories of compounds shown in Figure 9.1. These are called primary
metabolites. However, when one analyses plant, fungal and microbial cells,
one would see thousands of compounds other than these called primary
metabolites, e.g. alkaloids, flavonoids, rubber, essential oils, antibiotics,
coloured pigments, scents, gums, spices. These
are called secondary metabolites (Table 9.3).
While primary metabolites have identifiable
functions and play known roles in normal
physiologial processes, we do not at the moment,
understand the role or functions of all the
‘secondary metabolites’ in host organisms.
However, many of them are useful to ‘human
welfare’ (e.g., rubber, drugs, spices, scents and
pigments). Some secondary metabolites have
ecological importance. In the later chapters and
years you will learn more about this.
9.3 BIOMACROMOLECULES
There is one feature common to all those compounds found in the acid
soluble pool. They have molecular weights ranging from 18 to around
800 daltons (Da) approximately.
The acid insoluble fraction, has only four types of organic compounds
i.e., proteins, nucleic acids, polysaccharides and lipids. These classes of
compounds with the exception of lipids, have molecular weights in the
range of ten thousand daltons and above. For this very reason,
biomolecules, i.e., chemical compounds found in living organisms are of
two types. One, those which have molecular weights less than one
thousand dalton and are usually referred to as micromolecules or simply
biomolecules while those which are found in the acid insoluble fraction
are called macromolecules or biomacromolecules.
The molecules in the insoluble fraction with the exception of lipids
are polymeric substances. Then why do lipids, whose molecular weights
do not exceed 800 Da, come under acid insoluble fraction, i.e.,
macromolecular fraction? Lipids are indeed small molecular weight
Pigments Carotenoids, Anthocyanins,
etc.
Alkaloids Morphine, Codeine, etc.
Terpenoides Monoterpenes, Diterpenes etc.
Essential oils Lemon grass oil, etc.
Toxins Abrin, Ricin
Lectins Concanavalin A
Drugs Vinblastin, curcumin, etc.
Polymeric Rubber, gums, cellulose
substances
TABLE 9.3  Some Secondary Metabolites