Programme 1: City Life
Why Cell City?
Viewed from outer space, it is hard to see how human
life is organised. It is only when we get closer that we
start to see the structure of individual communities.
Closer still, and we see that these communities are
highly organised, with a constant flow of people and
materials adapting to an ever-changing environment.
In a city, for example, we probably take most of the
bustle of daily life for granted, but we can learn a lot
from looking at the complex ways in which cities have
grown, and continue to survive.
Although it may seem rather a large jump from a
working city to a living cell…
The organisation of different activities into different
areas, the transport of raw materials, the building of
new structures, the production of energy, the
removal of waste, and effective communication
systems are all just as important for a cell as they are
for a city.
The Cell
All living beings are made up of tiny, microscopic
units called cells. Some, like primitive organisms
found in ponds, consist simply of a single cell.
Others, like humans, start life as a single cell but grow
into a vast complex of many different types of cell,
with each cell performing a specialised function.
Equally, towns and cities vary greatly in size.
Nevertheless, whatever countries they are in, they all
have many similar features.
Likewise, despite the great variety of life forms and
cell types, different shapes and sizes, all cells have
many features in common. In this series we
concentrate on animal cells but most of the same
basic structures and organising principles apply
equally well to cells in plants, for example, as they do
to cells in humans
The Outer Boundary
The city has a clearly defined edge. In medieval times
this was a hard defensive wall; today the boundary is
less distinct and more likely to be a ring road, but the
principle is the same. Healthy and productive life can
carry on within the city, in the shelter offered by the
boundary.
In the cell, an outer membrane - called the plasma
membrane - performs a similar function to the city
wall or city ring road. It completely surrounds the cell
and provides the boundary between one cell and the
next. It is constructed, not from bricks and mortar, but
from molecules called lipids. These lipids come backto-back, and form a thin, flexible, but strong covering.
The outer boundary of the city is not, of course,
continuous. For the city to survive materials, and
people, have both to enter and to leave. In some
cases, such as very heavy lorries, it is sensible to
prevent them from entering at all.
The cell membrane too must allow materials into and
out of the cell. It is important that fuel, raw materials
and some signalling molecules can cross the
membrane and those waste products and some
manufactured goods can leave.
Other materials must be prevented from crossing the
membrane at all. Therefore, within the membrane
there are small groups of proteins that form gates.
There are different types of gate designed to allow the
free passage of different types of molecules.
Compartments
Different activities in the city go on in different
buildings or places. The cell too has its own
specialised areas. Just as in a city you would not
want a restaurant in the same building as a sewage
works, so in the cell different functions are separated
into different compartments.
All these compartments can then coexist quite happily
within the outer boundary of the cell. The membranes
of these internal compartments are made up of the
same material as the outer membrane.
The Nucleus
At the centre of the cell lies the nucleus. It is at the
same time the command centre, the planning
department and the central library - roughly
equivalent to City Hall or a civic forum.
Inside the nucleus lie the chromosomes that carry the
genetic instructions. They are a sort of master plan
that specifies how the cell should develop. It would
many millions of such files to write down all the
instructions for even one cell, so a special code is
used, based on very long chain molecules of DNA deoxyribonucleic acid.
A DNA molecule is rather like a necklace of two
entwined strands. On each strand are strung four
different kinds of beads, which are called bases. It is
the sequence of these bases that makes up the
genetic code. The DNA molecules are organised into
units or libraries called chromosomes.
For the information in the DNA to be used by the
cell, portions of it first have to be copied. Short
single stranded copies of one of the two DNA strands
in the nucleus are made. These copies represent
detailed messages that can be transported out of the
nucleus, through special holes or pores, into the rest
of the cell. Each message is like a bar-code, only
longer, and each contains the information to
manufacture to protein.
Programme 2: City Works
The Power Station
Constant production of energy is necessary to keep
the city alive. Energy arrives stored in fuel (such as
coal, gas, or atomic rods). It is converted, in a power
station, into a more useable form - electricity - that is
then distributed to where it is needed throughout the
city.
Like cities, cells are active, energetic beings. They too
need a constant supply of energy and they produce it
in exactly the same way: by converting fuel.
The power stations of the cell are called
mitochondria, and the most common fuel that they
consume is sugar (glucose). In this case the energy
generated is passed on, not as electricity, but as
small universal molecules called ATP.
The Factory
New goods and products are continually being
manufactured from raw materials. In cities this takes
place in workshops and factories. Raw materials are
transformed, usually in a sequence of steps on a
production line, into finished products. The process is
governed by a clear set of instructions or
specifications. In some cases the final products are
for immediate or local use, in others they are
packaged for export.
In the cell too there are production lines, in this case
manufacturing new proteins of many different sorts.
The messages from the cell's DNA act as a blueprint.
This blueprint is interpreted by machines on a
production line, which are called ribosomes, to
assemble the correct sequence of amino acids.
Amino acids are the building blocks used to make
proteins. As in cities where products made in factories
are used within the city, so the completed proteins are
often for home consumption, but others may be
exported outside the cell for use elsewhere.
The Framework
Everything knows where it belongs and where it is
going. In the city, letters, for example, carry a
postcode or zip code to help the postman ensure
that they reach their destination.
Proteins in cells have their own postcodes too. 10
thousand million proteins, 10 thousand different kinds,
and yet they all end up in the right place.
Also, the sites of the various activities are not just
randomly distributed, but tend to be organised:
houses in residential areas, factories in industrial
estates, shops in the City Centre, and so on. Roads
form an effective network of communications between
these areas and give an overall shape and structure
to the city. They are the most important route for
moving people and materials around.
Within a cell there is a complex set of structures that
define the centre, distinguish one end of the cell from
the other, and provide routes for transportation.
These structures are collectively called the cell
skeleton - or cytoskeleton - and consist of a series of
fibrous proteins - micro-filaments, intermediate
filaments and microtubules that help organise,
structure and orient the cell.
In animal cells, but not plant cells, there is a major
organising centre for microtubules known as the
centrosome. On one level, they can be seen as a
city ‘meeting place’.
Waste Disposal
All manufacturing creates waste product, which need
to be disposed of.
A city generates waste. Some waste is transported
away, for example to land-fill sites, whereas other
waste is disposed of inside the city. Nowadays, of
course, environmental and economic issues are
important, and so as much waste as possible is
broken down and recycled.
A cell also generates waste. Carbon dioxide and urea
- the by-products of energy production - are expelled
and disposed of elsewhere. Many components of the
cell eventually wear out, and need to be broken down
and the parts recycled.
This activity takes place inside the cell in specialized
compartments called lysosomes. A mitochondrian,
for example, that has passed its sell-by date, will be
engulfed, disassembled and reused by the cell.
The beauty of the cell is that the waste is recycled.
Examples in the city are water recycling and car scrap
metal.
Cell Division
Sir Paul Nurse, Director-General, Cancer Research
UK:
“I can imagine a city as a good metaphor, in thinking
of a large city that is growing, and then establishes
suburbs on its fringes.
Because what you imagine as the city grows, and
then, when it gets to a certain size, you need to
establish centres, shopping malls, town halls and the
like. And these are built in the periphery of the major
city. And are like, sort of cell division. You are
establishing a new, a new suburb.
And you could also imagine that the plans that we
used in the middle of the city, to be transferred to the
outer parts of the city, to help develop the malls and
the like.
This would be like the genes, the coding book of the
genetic information in this, the town planners, and the
plans that they have.
So I could imagine that metaphor being useful for
thinking about the cell cycle process.
The metaphor of the city does however break down,
because there is something very special about
biological reproduction… It’s a reproductive process
of a particular type, the genes encoded based on a
division of the original cell. “
Programme 3: Inter-City
Communications
A city will rapidly grind to a halt unless there are
effective communication systems between all the
component parts, and particularly between the city
and the rest of the world.
Cells depend crucially on communication and
signalling. The ability of a cell to interact with its
neighbours – external or international
communications - is as important as local
communication.
Cells constantly exchange information in the form of
chemical message or electrical signals. These signals
are used, not simply to tell their neighbours where
they are, but to help them grow and survive.
The cells in our bodies are in touch with their nearest
neighbours and with cells in distant parts. The brain in
our head registers a toe twitching; our hand knows
where to scratch an itchy back. There are elaborate
systems for conveying these messages between
individual cells around the body.
Matters of Life and Death
Looking at various features of a city can help us to
understand the problems that cells face, and how to
solve them.
Like all comparisons, however, they are not perfect:
cells in fact are more complex and versatile than even
the largest city.
If we count protein molecules, for example, as
inhabitants, the average cell would have a population
of about 10 thousand million - that's a thousand times
the population of London.
And yet cells can duplicate their entire contents and
divide into two daughter cells in only a few hours.
A fertilised egg becomes two cells, each of these
divides in turn and before long there are millions of
cells as they form the young embryo.
In making a new living being, not all of the cells are
used. Very large numbers of cells die, carefully
committing suicide at the right place and at the right
time, to ensure the correct patterning and
organisation of the remainder.
It is hard to make parallels between this sort of
activity and events within a city, although cities do of
course die: for example Pompeii in Italy and the ghost
mining towns in the Rocky Mountains.
One good city analogy however is with the use of
scaffolding to construct buildings which is then taken
away once completed – just like the webbing between
the webbing between our fingers as they develop
within the womb. Both are somewhat curiously
examples of cell death.
Cells constantly produce molecules that ensure the
survival of their neighbours. Most cells only die when
they begin to lose this support.
This concept of programmed cell death - or
apoptosis as it is called - remains one of the most
exciting new concepts in modern biological research.
‘Mother Nature’ - ‘Father Time’
Father and son, with a little help from a specialist on
the ageing process, discuss why we inevitably
change in appearance as we grow older? And can
science do anything to stop the ravages of time?
City analogies? We consider the breakdown of
infrastructure such as sewage pipes within a city.
And the cell has to decide how much to spend on
maintaining itself – what’s it willing to pay… Like the
maintenance of a city, it all boils down to budgets.
What does the future hold?
“Unless we understand how cells work, we can never
understand why they go wrong, and that means we’ll
never properly understand disease. We now have the
opportunity to really understand how cells work and it
gets us also nicely into the secret of life.”
Sir Paul Nurse
“I think the next 20, 30 years, in biology, are going to
be as exciting as biology has ever been, as a science.
It is more probably like the golden era of high energy
physics was, back in the 50s, when they were just
beginning to learn how to smash apart atoms. It is a
revolution.”
Julie Theriot, Stanford University, California
And turning it on its head…
Perhaps we should turn the metaphor on its head,
and ask if there are things we can learn about our
cities by looking at living cells?
The ‘politics’ of a cell are incredibly robust. Cells have
survived the tough selection pressures of millions of
years of biological evolution.
Cells are highly organised and exceptionally efficient;
they are neither too big nor too small. They have
evolved effective ways to interact with their
neighbours.
There is a wonderful sense of co-operation within a
cell. Every part of the cell works towards the healthy
functioning of the whole – a sort of cellular ‘mutual
aid’.
Cells pass on knowledge from one generation to the
next. Accessible, useable information is the key to
evolutionary success.
Perhaps when we next walk around our cities, we will
see them just a little differently.