Biology Reference
In-Depth Information
Fig. 4.3
hierarchy of complexity, and this process is continued down through decreasing
levels of complexity. In other words, it is much more useful to describe how a
motor car works in terms of the interactions between the cylinders, petrol tank,
carburettor etc., than in terms of fundamental particles like electrons and quarks,
even though it is the properties of fundamental particles that ultimately account for
everything.
If we apply this reductionist approach to organisms, we discover that organ-
isms are composed of several different levels of complexity, from organs to tissue,
from tissues to cells, and from cells to subcellular organelles. But the ultimate
components - the parts that determine the interactions at all the higher levels of
organisation - are protein molecules. This simple fact enables us to construct the set
of four principles shown in Fig. 4.3 that form the essence of molecular biology.
Principle 1:
All the properties of living organisms result from the properties of the
proteins they contain.
Proteins are the action molecules of all organisms, that is, they carry out most of
the processes necessary to sustain the living state. Proteins function by presenting
binding sites on their surface for chemicals in their environment, including other
proteins - proteins are chemicals too! These sites are able to recognise and bind a
huge range of chemicals in a highly specific fashion. Specificity means that each
type of protein binds just one type of chemical, so cells contain many different
proteins because cells are composed of many different chemicals. Among the many
functions of proteins is to act as enzymes. Enzymes are defined as specific catalysts,
that is, each type of enzyme catalyses one type of chemical reaction, which would
proceed very slowly, or not at all, in its absence. The word “enzyme” originates from
