118 REAL GENES, REAL INTELLIGENCESIGNALING ENVIRONMENTAL CHANGEFixed DNA codes cannot register the changing statistical patterns needed
to survive in rapidly changing environments. Th at can only be done
through the intelligent computations among components mentioned by
Bruggerman and colleagues. Th ey are not trivial, even in single cells.
Making sense of the environment through constantly novel arrays of
signals is not easy. Cells are receiving hundreds of thousands of envi-
ronmental cues every second— a relentless dynamic storm of information.
But they are acutely sensitive to changes and to structure within it.
Even single- cell creatures like bacteria pick up tiny changes in their
environments. Th ese include concentrations of chemical attractant and
repellent substances; physical stimuli, such as light or heat; or mechani-
cal bumps and shocks. For example, the common gut bacterium Esche-
richia coli can detect fewer than ten molecules of nutrient like an amino
acid in a volume of fl uid about equal to that of its own size— the equiva-
lent of detecting a few drops of foreign liquid in a bathtub of water.
Th is intense monitoring of the environment, in all single cells, is done
through rich signaling receptors on the cell surface (fi gure 4.4). Th ese re-
ceptors are specialized molecules embedded in the cell membrane. Th ey
react to physical and chemical changes in their surroundings and pass the
information to internal systems. So impor tant is this function that the
production of relevant signal- receptor proteins is itself a major activity of
the cell involving over a quarter of its genes.
Th e receptors need to do their work, though, not as in de pen dent units
but as coordinated teams. Th ey are sensitive to external chemical signals,
or ligands, but not as isolated cues. A single sugar molecule hitting a cell
membrane receptor in E. coli, for example, gives no information about
direction of source or how to pursue it: its track cannot be visualized.
Instead, directions have to be computed from an integration of a series of
such signals over time and space on a number of receptors— just as bats
use their ears in echolocation; or you use your eyes in fast traffi c: in other
words, through the detection of statistical patterns in time and space.
Registering the statistical pattern is crucial for predicting where
the change is leading and therefore, for the guidance of responses,
such as motion, development, and metabolism. It depends on the four-
This content downloaded from 139.184.14.159 on Tue, 17 Oct 2017 13:52:50 UTChttp://www.ebook3000.com