I love April Fool’s Day. As a practical joker, it’s the one time of the year my mischievous acts are officially sanctioned (perhaps tolerated is the more appropriate word).
But sometimes the joke is on me. That was the case when I took a course in metabolism as a graduate student. And it was no laughing matter!
Metabolism refers to the myriad chemical reactions that occur in organisms necessary to sustain life. Metabolic activity makes it possible for life forms to extract energy from the environment and construct life’s components. These processes allow organisms to grow, reproduce, maintain biological structures, and respond to changes in the environment. Metabolic reactions include the production and breakdown of proteins and RNA molecules, DNA replication, and the assembly of cell membranes and cell walls.
Additionally, metabolism involves reactions of small molecules. A significant number of metabolic reactions produce small molecules used by the cell’s machinery as building blocks to assemble proteins, DNA, the RNAs and cell membrane bilayers. On the other hand, some metabolic activities breakdown compounds like glucose and other sugar molecules into smaller molecules to provide energy for the cell’s operations. Some metabolic activities prepare materials the cell no longer needs (cellular waste) for elimination. Other reactions detoxify materials harmful to the cell.
Within the cell’s interior, metabolic processes are often organized into pathways comprised of a series of chemical reactions that transform a starting compound into a final product via a series of small, stepwise chemical changes. Each step in a metabolic route is mediated by a protein (called an enzyme) that assists in the chemical transformation. These pathways can be linear, branched, or circular. The chemical components that form part of a particular metabolic sequence sometimes take part in other metabolic pathways. These shared compounds cause metabolic pathways to be interconnected and networked together.
The sum total of metabolic processes represents a complex, reticulated web of chemical reactions, each one catalyzed by an enzyme. Poor biochemistry students are expected to know each chemical transformation (and the enzymes that catalyzes them) for each pathway in the cell—not to mention how the various pathways interconnect! (For a sample of what biochemistry students are expected to master go here) When informed of that fact on the first day of class, I hoped that it was the professor’s idea of a bad April Fool’s Day joke—but it wasn’t. Not fun. Not fun, at all!
Given the vast complexity of the cell’s metabolism, it’s easy to envision how evolutionary processes could have kludged the pathways together bit by bit over a vast period of time. But recent scientific advances suggest otherwise. It turns out metabolic pathways appear to be highly robust and optimized to withstand error cascades that could potentially result when a single enzyme fails. New research indicates that metabolic systems are also designed to be robust in the face of changes in metabolite concentrations within the cell.1
Given the dynamic environment of the cell, fluctuations in the levels of metabolites are bound to happen. When these unintended variations occur, they will travel throughout the networks. Some processes in the cell are sensitive to metabolite concentrations and will be negatively affected as a result. To combat these affects, metabolic systems have regulatory systems in place (based on engineering principles) that dampen concentration bounces, keeping them within tight bounds. In other words, metabolic pathways are optimized to withstand inevitable concentration changes of metabolites.
In The Cell’s Design, I argue that the salient characteristics of biochemical systems, such as their optimization, are identical to features we would immediately recognize as evidence for the work of a human designer. The similarities between biochemical systems and manmade machines logically compels the conclusion that life’s most fundamental processes and structures stem from the work of an intelligent Agent. And that is no joke.
- Guy Shinar and Martin Feinberg, “Structural Sources of Robustness in Biochemical Reaction Networks,” Science 327 (2010): 1389–91.