3 The Cell as a Complex System
Cells exhibit multiple properties indicative of complex systems:
- Non-bijectivity and nonlinearity in genotype–phenotype mapping (e.g., pleiotropy, polygenicity).
- Connectivity and sensitivity to initial conditions via gene regulatory networks.
- Multiple equilibria, self-organization, quasi-determinism, and thermodynamic openness.
These features create a cellular landscape where small perturbations can trigger large-scale transitions, especially in disease states like cancer.
All life is cellular in nature and additionally the cell is a unitary whole, a unit of life so to say (Morowitz 1992; Harold 2001). For these reasons, the cell provides a useful and clear designation of a system. Within the cell, we observe a number of system properties (Harold 2001; Waliszewski and Konarski 2001). We begin with the relationship between the cell’s genotype to its phenotype. We characterize this correspondence as non-bijective in that one gene will typically contribute to more than one phenotypic trait (pleiotropy), and furthermore major phenotypic traits are the products of several genes (polygenic inheritance). For example, it is estimated that the formation of the compound eye in Drosophila involves approximately 20% of the genome, or close to two-thousand-five-hundred genes (Adams, Celniker et al. 2000). The non-bijective relationship between the genome, on the one hand, and function and structure, on the other, implies mathematically nonlinearity in that there is unproportional response between gene and trait. This non-bijective relationship is actualized via a hierarchical network of multiple cross-interacting elements (genes and their products) sensitive to initial conditions (as presented by the genome and the cellular environment) and possessing multiple equilibria. The connectivity governing the network of genes and their products shows complex dynamic relationships that enable the emergence of the system’s (i.e., the cell’s) global features. For this reason, we say the cell possesses complexity, as its whole is greater than the sum of the parts. Additionally, this genetic network is quasi-deterministic, as only some events are deterministically driven, while other events are stochastic. Other important system properties of the cell are its self-organization, self-regulation, and self-replication. The cell likewise is catalytic, dynamic, thermodynamic (isothermal and open), and operates on the principle of complementarity.
In the Table below, we list these system properties of the cell.
| Property |
|---|
| Non-bijectivity |
| Nonlinearity |
| Connectivity |
| Sensitivity to initial conditions |
| Multiple equilibria |
| Complexity |
| Quasi-deterministic |
| Self-organization |
| Self-regulation |
| Self-replication |
| Catalytic |
| Dynamic |
| Thermodynamic (isothermal and open) |
| Complementarity |