Imagine fifty people trying to build a house, only none of
them can speak to each other or converse in any other way. The fifty people are
all working towards a common purpose, but without any communication, their
endeavour is bound to fail!
Instead, the workforce must cooperate in a highly organised
manner. Firstly, they should be divided into groups, each with a particular
purpose: laying the bricks, making the measurements, collecting and
transporting the necessary resources, and so on. Yet even between these groups
there must be communication: the brick-laying team could run out of cement, and
would need to call on the resource-collecting group to obtain or make some
more. With a clearly defined organisational system, it can now be hoped that
the task in hand will be completed.
The very same is true of the cells in our bodies. Our cells
are accumulated into tissues; several types of tissue make up one organ; and
multiple organs that work together to ensure one aspect of the body’s
functioning make up an organ system. Organ systems include the circulatory
system, the nervous system and the digestive system.
Like the workers in the house analogy, cells must converse all
the time, and they do so in a language consisting largely of proteins and
lipids. These chemical “words” must be picked up and recognised without fail,
and they must specify a particular response – though this response may be
different in different cell types. Our cells have a very impressive vocabulary
and, between them, can understand and respond to many thousands of chemical
messengers. How does it work?
The entire process is based on two simple but absolutely
fundamental principles in biology. The first is that two molecules will only
piece together if their shapes allow one to fit into another, like a key into a
lock. The second is that proteins, enormous molecules which regulate pretty
much every process in our bodies, are very flexible in structure and the
binding or unbinding of one substance will cause it to change its shape.
Take, for example, what is known as a protein kinase cascade. A messenger from one cell may fit perfectly
into a receptor on another. The receptor, a protein, will then change shape,
allowing it to bind other proteins and add a chemical group to them, so that
these proteins in turn will change shape, and so on and so on. An important
feature of this cascade is that the message is amplified at every stage, in the
sense that every protein, once activated, will activate many more. It is like
when a rumour is spread – one person tells several people, each of whom tells
several more, and before long, everyone knows!
The message takes its effect when a final activated protein
binds to DNA in the cell, thus altering the way the cell behaves in some way.
For instance, the message could cause the cell to divide rapidly, creating many
clones of the same cell. This is often the case when our immune system responds
to an infection: the bacteria or viruses which cause the illness carry
particular proteins which may enter our own cells, and hence be recognised by
white blood cells because of their specific shape. The white blood cells
divide, kill the bacteria or viruses, and send out signals which are recognised
by other types of white blood cells. This second class of white blood cells,
once activated, divide into many copies and begin making antibodies, which fit
perfectly into the proteins on the bacteria or viruses and hence allow them to
be destroyed.
The field of cell signalling is concerned with understanding
exactly how cells make sense of the multitude of chemical messengers around
them, and exactly how these chemicals cause the specific responses they are
intended to trigger. It is an expanding field and one which I find fascinating.
I hope you will also be interested and eager to learn more!
References:
Life: the Science of Biology – Purves, W.K; Sadava, J;
Orians, G.H; Heller, H.C
The Biochemistry of Cell Signalling – Helmreich, E.J.M.
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