<s>
Membrane	B-Application
computing	I-Application
(	O
or	O
MC	O
)	O
is	O
an	O
area	O
within	O
computer	B-General_Concept
science	I-General_Concept
that	O
seeks	O
to	O
discover	O
new	O
computational	B-Application
models	I-Application
from	O
the	O
study	O
of	O
biological	O
cells	O
,	O
particularly	O
of	O
the	O
cellular	O
membranes	O
.	O
</s>
<s>
Membrane	B-Application
computing	I-Application
deals	O
with	O
distributed	O
and	O
parallel	O
computing	O
models	O
,	O
processing	O
multisets	O
of	O
symbol	O
objects	O
in	O
a	O
localized	O
manner	O
.	O
</s>
<s>
The	O
various	O
types	O
of	O
membrane	O
systems	O
are	O
known	O
as	O
P	B-Application
systems	I-Application
after	O
Gheorghe	O
Păun	O
who	O
first	O
conceived	O
the	O
model	O
in	O
1998	O
.	O
</s>
<s>
An	O
essential	O
ingredient	O
of	O
a	O
P	B-Application
system	I-Application
is	O
its	O
membrane	O
structure	O
,	O
which	O
can	O
be	O
a	O
hierarchical	O
arrangement	O
of	O
membranes	O
,	O
as	O
in	O
a	O
cell	O
,	O
or	O
a	O
net	O
of	O
membranes	O
(	O
placed	O
in	O
the	O
nodes	O
of	O
a	O
graph	O
)	O
,	O
as	O
in	O
a	O
tissue	O
or	O
a	O
neural	O
net	O
.	O
</s>
<s>
P	B-Application
systems	I-Application
are	O
often	O
depicted	O
graphically	O
with	O
drawings	O
.	O
</s>
<s>
Indeed	O
,	O
the	O
membrane	B-Application
computing	I-Application
literature	O
contains	O
a	O
very	O
large	O
number	O
of	O
models	O
.	O
</s>
<s>
The	O
region	O
,	O
which	O
is	O
defined	O
by	O
a	O
membrane	O
,	O
can	O
contain	O
other	O
symbols	O
or	O
strings	O
(	O
collectively	O
referred	O
to	O
as	O
objects	O
)	O
or	O
other	O
membranes	O
,	O
so	O
that	O
a	O
P	B-Application
system	I-Application
has	O
exactly	O
one	O
outer	O
membrane	O
,	O
called	O
the	O
skin	O
membrane	O
,	O
and	O
a	O
hierarchical	O
relationship	O
governing	O
all	O
its	O
membranes	O
under	O
the	O
skin	O
membrane	O
.	O
</s>
<s>
Many	O
variant	O
models	O
have	O
been	O
studied	O
,	O
and	O
interest	O
has	O
focused	O
on	O
proving	O
computational	O
universality	O
for	O
systems	O
with	O
a	O
small	O
number	O
of	O
membranes	O
,	O
for	O
the	O
purpose	O
of	O
solving	O
NP-complete	O
problems	O
such	O
as	O
Boolean	B-Algorithm
satisfiability	I-Algorithm
(	O
SAT	O
)	O
problems	O
and	O
the	O
traveling	B-Algorithm
salesman	I-Algorithm
problem	I-Algorithm
(	O
TSP	O
)	O
.	O
</s>
<s>
The	O
P	B-Application
systems	I-Application
may	O
trade	O
space	O
and	O
time	O
complexities	O
and	O
less	O
often	O
use	O
models	O
to	O
explain	O
natural	O
processes	O
in	O
living	O
cells	O
.	O
</s>
<s>
To	O
date	O
,	O
the	O
P	B-Application
systems	I-Application
are	O
nearly	O
all	O
theoretical	O
models	O
that	O
have	O
never	O
been	O
reduced	O
to	O
practice	O
,	O
although	O
a	O
practical	O
system	O
is	O
given	O
in	O
.	O
</s>
