Why is it difficult to understand an organism by studying each of its parts separately?

In systems biology, it is difficult to understand an organism by studying its parts in isolation because biological entities are complex, integrated systems where function arises from the collective behavior of many components.12

Key reasons for this difficulty include:

• Emergent Properties: Many critical biological functions are "emergent properties," meaning they result from the interactions of numerous parts rather than the properties of any single component. For example, a memory in the human brain cannot be understood by studying a single neuron; it is a collective property of many neurons acting together.2
• Complexity of Interactions: Biological organisms involve a vast number of interactions that must be inferred after the fact from the system's behavior. Unlike engineered systems, which are built from known components according to specific plans, biologists must perform "systems reverse engineering" to identify how individual components and their relationships create observable biological behaviors.12
• Information Context: Biological processes are governed by information stored and transmitted through molecular relationships. The function of a specific part—such as a gene—is often dependent on the broader context, including metabolic states and external signals, making it insufficient to study parts in a vacuum.23

Ultimately, life is defined by these relationships and interactions rather than the properties of any one molecule.2