In a polypeptide or a protein, amino acids are linked by a peptide
bond which is formed when the carboxyl (-COOH) group of one amino
acid reacts with the amino (-NH2
) group of the next amino acid with
the elimination of a water moiety (the process is called dehydration).
In a polysaccharide the individual monosaccharides are linked by a
glycosidic bond. This bond is also formed by dehydration. This bond
is formed between two carbon atoms of two adjacent monosaccharides.
In a nucleic acid a phosphate moiety links the 3’-carbon of one sugar
of one nucleotide to the 5’-carbon of the sugar of the succeeding
nucleotide. The bond between the phosphate and hydroxyl group of
sugar is an ester bond. As there is one such ester bond on either side,
it is called phosphodiester bond (Figure 9.5).
Nucleic acids exhibit a wide variety of secondary structures. For
example, one of the secondary structures exhibited by DNA is the
famous Watson-Crick model. This model says that DNA exists as a
double helix. The two strands of polynucleotides are antiparallel i.e.,
run in the opposite direction. The backbone is formed by the sugar-
phosphate-sugar chain. The nitrogen bases are projected more or less
perpendicular to this backbone but face inside. A and G of one strand
compulsorily base pairs with T and C, respectively, on the other strand.There are two hydrogen bonds between A and T and three hydrogen
bonds between G and C. Each strand appears like a helical staircase.
Each step of ascent is represented by a pair of bases. At each step of
ascent, the strand turns 36°. One full turn of the helical strand would
involve ten steps or ten base pairs. Attempt drawing a line diagram.
The pitch would be 34Å. The rise per base pair would be 3.4Å. This
form of DNA with the above mentioned salient features is called B-
DNA. In higher classes, you will be told that there are more than a
dozen forms of DNA named after English alphabets with unique
structural features.
9.9 DYNAMIC STATE OF BODY CONSTITUENTS – CONCEPT OF
METABOLISM
What we have learnt till now is that living organisms, be it a simple bacterial
cell, a protozoan, a plant or an animal, contain thousands of organic
compounds. These compounds or biomolecules are present in certain
concentrations (expressed as mols/cell or mols/litre etc.). One of the greatest
discoveries ever made was the observation that all these biomolecules have
a turn over. This means that they are constantly being changed into some
other biomolecules and also made from some other biomolecules. This
breaking and making is through chemical reactions constantly occuring
in living organisms. Together all these chemical reactions are called
metabolism. Each of the metabolic reactions results in the transformation
of biomolecules. A few examples for such metabolic transformations are:
removal of CO2
from amino acids making an amino acid into an amine,
removal of amino group in a nucleotide base; hydrolysis of a glycosidic
bond in a disaccharide, etc. We can list tens and thousands of such
examples. Majority of these metabolic reactions do not occur in isolation
but are always linked to some other reactions. In other words, metabolites
are converted into each other in a series of linked reactions called metabolic
pathways. These metabolic pathways are similar to the automobile traffic
in a city. These pathways are either linear or circular. These pathways criss-
cross each other, i.e., there are traffic junctions. Flow of metabolites through
metabolic pathway has a definite rate and direction like automobile traffic.
This metabolite flow is called the dynamic state of body constituents. What
is most important is that this interlinked metabolic traffic is very smooth
and without a single reported mishap for healthy conditions. Another feature
of these metabolic reactions is that every chemical reaction is a catalysed
reaction. There is no uncatalysed metabolic conversion in living systems.
Even CO2
dissolving in water, a physical process, is a catalysed reaction in
living systems. The catalysts which hasten the rate of a given metabolic
conversation are also proteins. These proteins with catalytic power are
named enzymes.
