Biological Information Processing
Cells and computers process information quite differently. Yet computers are the basic unit of the digital world just as cells are the basic unit of life.
A National Science Foundation workshop report points out that, “ A series of discoveries over the past fifty years have illuminated the extraordinary capabilities of living cells to store and process information. We have learned that genes encoded digitally as nucleotide sequences serve as a kind of instruction manual for the chemical processes within the cell and constitute the hereditary information that is passed from parents to their offspring. Information storage and processing within the cell is more efficient by many orders of magnitude than electronic digital computation, with respect to both information density and energy consumption.” Computing professionals would do well to understand the parallels too.
All living organisms, from single cells in pond water to humans,
survive by
constantly processing information about threats and opportunities in
the
world around them. For example, single-cell E-coli bacteria have a
sophisticated chemical sensor patch on one end (orange molecules in
image below) that detect several
different aspects of its environment. The sensor's output biases the
cell's movement toward attractant
and away from repellent chemicals. Each individual sensor has a dynamic
range of about 102, far less than the range encountered in
their environment. However the sensor system is cleverly coupled such
that, jointly, the sensors have a dynamic range more than 105.
Information processing machinery within single cells involves a complex network of tens or hundreds of thousands of protein mechanisms, genes and gene-expression control pathways that dynamically adapt the cell’s function to its environment.
We cannot directly compare the information processing “power” of a cell to that of a computer. Size, power usage, robustness, and computational density clearly favor the cell whereas repeatability, precision and linear computational speed clearly favor the computer. Cells use a highly parallel architecture whereas computers use a serial architecture. Cells exploit randomness whereas computers do everything possible to suppress it. And cells apply themselves to quite different tasks than computers do. Single cell organisms are self organizing, gather their own energy, reproduce themselves, and defend themselves. All of those tasks require substantial information processing, In contrast, computers rely on their human "masters" for most of those functions. So we know, or fancy that we know, the totality of the information processing done in a computer and are still largely ignorant of the information processing done in a cell.
We may attempt to deduce something about the information processing
capacity of a cell from how much "code" it contains. A cell's DNA
plays a role roughly analogous to program code in a computer. Although
some of
the DNA has been labeled as "junk," new research
casts
doubt
on
that
notion, A human
cell contains about 3.5 billion bases. Since each base is one of four
possibilities
(A, T, C, G), each encodes 2 bits of information. So the human genome
contains about
7 billion bits, or roughly a Gbyte of "code." E-coli bacteria are
relatively simple by comparison, with
about 5 million bases. Other single cell organisms can at least appear
to be much more complex than a human. An
Amoeba (one of the most complex single cell organisms) contains a
hundred
times
more
DNA
than a human cell. This likely is a consequence of the fact that single
cell organisms cannot
specialize the way cells in
multicellular organisms do. The Amoeba must support "code" (DNA) for
all eventualities.
Cells and computers face different tasks and have quite different
capabilities. Progress has
recently been reported in computer simulations of the cellular
lifecycle
processes of a very simple bacterium. Yet at this time even the world's
most
powerful computer
cannot simulate the folding of a single complex protein
molecule
into its working shape in anything like real-time. Nor can
even
the most complex cell simulate what a mundane four function
calculator can do, especially in terms of precision and
reproducibility.
Their differences not withstanding, individual computers and single
cells play similar
roles in the large-scale
sweep of evolution. Just as computers are the initial unit of
computation,
cells are the initial unit of life. And the challenges of
communication and collaboration between networked computers are similar
to those between cells in a multicellular organism. (more in this pdf presentation)
Contact: sburbeck at mindspring.com
Last revised 7/22/2012