Complexity_ A Guided Tour - Melanie Mitchell [84]
Just as lymphocytes affect immune system dynamics by releasing cytokines, and as ants affect foraging behavior by releasing pheromones, chemical reactions that occur along a metabolic pathway continually change the speed of and resources given to that particular pathway.
In general, metabolic pathways are complex sequences of chemical reactions, controlled by self-regulating feedback. Glycolysis is one example of a metabolic pathway that occurs in all life forms—it is a multistep process in which glucose is transformed into the chemical pryruvate, which is then used by the metabolic pathway called the citric acid cycle to produce, among other things, the molecule called ATP (adenosine triphosphate), which is the principal source of usable energy in a cell.
At any given time, hundreds of such pathways are being followed, some independent, some interdependent. The pathways result in new molecules, initiation of other metabolic pathways, and the regulation of themselves or other metabolic pathways.
Similar to the regulation mechanisms I described above for the immune system and ant colonies, metabolic regulation mechanisms are based on feedback. Glycolysis is a great example of this. One of the main purposes of glycolysis is to provide chemicals necessary for the creation of ATP. If there is a large amount of ATP in the cell, this slows down the rate of glycolysis and thus decreases the rate of new ATP production. Conversely, when the cell is lacking in ATP, the rate of glycolysis goes up. In general, the speed of a metabolic pathway is often regulated by the chemicals that are produced by that pathway.
Information Processing in These Systems
Let me now attempt to answer the questions about information processing I posed at the beginning of this chapter:
What plays the role of “information” in these systems?
How is it communicated and processed?
How does this information acquire meaning? And to whom?
WHAT PLAYS THE ROLE OF INFORMATION?
As was the case for cellular automata, when I talk about information processing in these systems I am referring not to the actions of individual components such as cells, ants, or enzymes, but to the collective actions of large groups of these components. Framed in this way, information is not, as in a traditional computer, precisely or statically located in any particular place in the system. Instead, it takes the form of statistics and dynamics of patterns over the system’s components.
In the immune system the spatial distribution and temporal dynamics of lymphocytes can be interpreted as a dynamic representation of information about the continually changing population of pathogens in the body. Similarly, the spatial distribution and dynamics of cytokine concentrations encode large-scale information about the immune system’s success in killing pathogens and avoiding harm to the body.
In ant colonies, information about the colony’s food environment is represented, in a dynamic way, by the statistical distribution of ants on various trails. The colony’s overall state is represented by the dynamic distribution of ants performing different tasks.
In cellular metabolism information about the current state and needs of the cell are continually reflected in the spatial concentrations and dynamics of different kinds of molecules.
HOW IS INFORMATION COMMUNICATED AND PROCESSED?
Communication via Sampling
One consequence of encoding information as statistical and time-varying patterns of low-level components is that no individual component of the system can perceive or communicate the “big picture” of the state of the system. Instead, information must be communicated via spatial and temporal sampling.
In the immune system, for example, lymphocytes sample their environment via receptors for both antigens and signals from other immune system cells in the form of cytokines. It is the results of the lymphocytes’ samples of the spatial and temporal concentration