22.4 Molecular Mechanism of Recognition and Actuation

307

only electrons are involved, in biochemical signalling the molecules in the chain of

transmission change their identities, but if it can be assumed that one molecule (or

some fixed multiple thereof) conveys one bit of information according to its con-

text, the notion of amplification is legitimately applicable. In many nonbiological

contexts, the physical nature of the embodiment of the information changes. For

example, in heliography (in the sense of sending messages by flashes of sunlight)

the information is conveyed by light rays, but the recipient (after mental processing

in the brain) may write the received information down in pencil on a piece of paper.

22.3

Signalling Channel Capacities

The functional effectiveness of many communications systems is critically depen-

dent on the informational capacities of the channels connecting the various nodes

of the system. Relatively little work seems to have been carried out for molecular

communications systems in biology. ⁷ Ultimately the capacity of the channels con-

necting the external receptors to the gene expression machinery limits the ability of

an organism to survive in a changing environment. Ashby’s law of requisite vari-

ety is universally applicable. ⁸ Most experiments typically explore the consequences

of a single environmental change; ⁹ Such work should be extended to encompass

multiple environmental changes; for example, principal nutrient source could be

switched between two alternatives and the dependence of phenotypic outcomes on

the frequency of switching determined.

Eckford and Thomas have carried out important work on determining the capacity

of some actual biological signal transduction channels, ¹⁰ obtaining results general-

izable to many types of molecular signalling. Further progress has been made by

Tsuruyama (2018), who developed a quantitative analysis of information transfer

using the Kullback–Leibler divergence.

22.4

Molecular Mechanism of Recognition and Actuation

Protein molecules are large and have the possibility of forming many, individually

weak, secondary intramolecular bonds; together strong enough to stabilize a par-

ticular conformation, but weak enough to allow conformational transitions. Hence

proteins are kinetically not in equilibrium; they have rigid memory on multiple

7 The blowfly photoreceptor was analysed along these lines by Abshire and Andreou in 2001.

8 Ashby (1958).

9 E.g., Ramsden and Vohradský (1998).

10 Thomas and Eckford (2016), Eckford and Thomas (2018); see also Tkaˇcik and Bialek (2016).