Exam (elaborations)
CHAPTER 9 BIOMOLECULES
CHAPTER 9 BIOMOLECULES
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142 BIOLOGYC HAPTER 9
B IOMOLECULES
9.1 How to Analyse There is a wide diversity in living organisms in our biosphere. Now a
Chemical question that arises in our minds is: Are all living organisms made of the
Composition? same chemicals, i.e., elements and compounds? You have learnt in
9.2 Primary and chemistry how elemental analysis is performed. If we perform such an
Secondary analysis on a plant tissue, animal tissue or a microbial paste, we obtain a
Metabolites list of elements like carbon, hydrogen, oxygen and several others and
9.3 Biomacromolecules their respective content per unit mass of a living tissue. If the same analysis
9.4 Proteins
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
9.5 Polysaccharides
absolute terms, no such differences could be made out. All the elements
9.6 Nucleic Acids present in a sample of earth’s crust are also present in a sample of living
9.7 Structure of tissue. However, a closer examination reveals that the relative abundance
Proteins of carbon and hydrogen with respect to other elements is higher in any
9.8 Nature of Bond living organism than in earth’s crust (Table 9.1).
Linking Monomers
in a Polymer 9.1 HO W TO ANALYSE CHEMICAL COMPOSITION?
9.9 Dynamic State of
We can continue asking in the same way, what type of organic compounds
Body Constituents
- Concept of
are found in living organisms? How does one go about finding the answer?
Metabolism 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
9.10 Metabolic Basis for
acid (Cl3CCOOH) using a mortar and a pestle. We obtain a thick slurry. If
Living
we were to strain this through a cheesecloth or cotton we would obtain two
9.11 The Living State
fractions. One is called the filtrate or more technically, the acid-soluble
9.12 Enzymes pool, and the second, the retentate or the acid-insoluble fraction. Scientists
have found thousands of organic compounds in the acid-soluble pool.
2021-22
, BIOMOLECULES 143
In higher classes you will learn about how TABLE 9.1 A Comparison of Elements Present
to analyse a living tissue sample and identify a in Non-living and Living Matter*
particular organic compound. It will suffice to Element % Weight of
say here that one extracts the compounds, then Earth’s crust Human body
subjects the extract to various separation Hydrogen (H) 0.14 0.5
techniques till one has separated a compound Carbon (C) 0.03 18.5
from all other compounds. In other words, one Oxygen (O) 46.6 65.0
Nitrogen (N) very little 3.3
isolates and purifies a compound. Analytical
Sulphur (S) 0.03 0.3
techniques, when applied to the compound give Sodium (Na) 2.8 0.2
us an idea of the molecular formula and the Calcium (Ca) 3.6 1.5
probable structure of the compound. All the Magnesium (Mg) 2.1 0.1
carbon compounds that we get from living Silicon (Si) 27.7 negligible
tissues can be called ‘biomolecules’. However, * Adapted from CNR Rao, Understanding Chemistry,
Universities Press, Hyderabad.
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
TABLE 9.2 A List of Representative Inorganic
weight. Now if the tissue is fully burnt, all the Constituents of Living Tissues
carbon compounds are oxidised to gaseous
Component Formula
form (CO2, water vapour) and are removed. What
is remaining is called ‘ash’. This ash contains Sodium Na+
inorganic elements (like calcium, magnesium Potassium K+
etc). Inorganic compounds like sulphate, Calcium Ca++
phosphate, etc., are also seen in the acid-soluble Magnesium Mg++
fraction. Therefore elemental analysis gives Water H2O
elemental composition of living tissues in the Compounds NaCl, CaCO3,
form of hydrogen, oxygen, chlorine, carbon etc.
PO34 − , SO24 −
while analysis for compounds gives an idea of
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 α-carbon.
Hence, they are called α-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
2021-22
, 144 BIOLOGY
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 –NH2 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 (–CH3), or ethyl (–C2H5) or higher number of –CH2
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.
2021-22
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