Information Technology Reference
In-Depth Information
“ports” controlling the input/output flux of molecules, protocells maintain a stable
flux of in-coming and out-going matter, in order to ensure that internal chemical
species are kept within certain specific quantitative ranges. However, in this kind of
protocell, two essential requirements of life are missing: growth and reproduction.
When protocells exhibit some primitive form of these features, then they are close
to cells.
The basic problem of protocell realization is: keeping the introduction-trans-
formation-expulsion ability, and keeping the membrane structure separating the ex-
ternal environment from the interior space. It is important to realize that even these
easy actions are not as simple as they may appear at a first glance. In fact, they
are performed without any intervention of external agents. This is not so trivial if
only molecules are available for performing all these tasks. In fact, some kinds of
molecules have to be arranged in order to construct membranes, some have to con-
trol selectively the input/output (ensuring that only certain molecules can enter and
only certain others can exit), and some molecules have to direct the chemical trans-
formations. The problem becomes even more challenging if we consider that any
kind of matter aggregation, the more complex it is, the more easily it is subjected to
degradation, therefore, after some time (depending on the kind of aggregation), it is
naturally destined to be destroyed, or to lose its functionality.
1.2
Sequences and Polymers
Sets
or
classes
(in this context we use these terms as synonyms) are collections of
distinct objects. A set is completely specified by the elements which belong to it,
and if it has a finite number of elements, then it is completely described by a list of
them. In this list, the appearance order of the elements is not relevant, and moreover,
each element has to appear once.
Consider the set of decimal digits
{
,
,
,
,
,
,
,
,
,
}
0
1
2
3
4
5
6
7
8
9
. If we arrange them in
the following way:
314159265358979
we have the first 15 digits (decimal dot is omitted) of the decimal representation
of
(the ratio between any circle and its diameter). Here, two aspects are different
with respect to sets: some symbols occur many times and the order of symbol oc-
currences is relevant. Structures with a linear arrangement of components, possibly
occurring many times, where the order and the number of occurrences is relevant
are called
sequences
. They are expressed in many ways, for example, by putting the
occurrences between parentheses and by separating them by commas:
π
(
3
,
1
,
4
,
1
,
5
,
9
,
2
,
6
,
5
,
3
,
5
,
8
,
9
,
7
,
9
)
however, what is essential is a clear indication of the order and the number of oc-
currences. The total number of the elements occurring in a sequence, counting the
repetitions, constitutes its
length
. In general, sequences are referred to a preliminary
fixed set. For example, the sequence above is a sequence over the set of decimal digits.
