The Lifelike Nature Arising from the Coupling Between “Fluctuations” and “Structure” in Cells

The interior of a cell, the fundamental unit of life, is a nonequilibrium environment in which energy consumption and material transport occur continuously. Under such conditions, not only thermal fluctuations (thermal noise), but also non-thermal “nonequilibrium fluctuations” originating from metabolic activity are persistently generated.

Within the cell, various structures exist, including biomolecular machines such as enzymes and motor proteins, organelles, and droplet-like microstructures. These entities dynamically flow, deform, and interact with one another across nano- to micrometer scales (the mesoscale), forming complex and dynamic “structures.” These structures are constantly changing in space and time, reminiscent of turbulence or chaos, and it is within this ever-evolving environment that life processes take place.

Such a nonequilibrium state, in which “fluctuations” and “structure” are tightly coupled at the mesoscale, underlies the characteristic dynamism of living systems. It represents an essential feature of the “living” state that is extremely difficult to reproduce artificially. This perspective provides important insight into the fundamental question of what it means for matter to be alive.

Life as a Complex System and Its Integration with Soft Matter Physics

Understanding life as a “complex system” is one of the central themes in modern physics. Living systems are dynamic entities in which individual components (molecules and cells) and the whole (organisms and tissues) mutually influence each other, giving rise to phenomena such as replication, adaptation, development, and evolution.

It has become increasingly clear that such phenomena cannot be fully understood through traditional reductionist approaches that simply decompose systems into their constituent elements. Instead, a perspective is required that treats life as a whole in terms of structure and dynamics—namely, complex systems biology.

In addition, living organisms are largely composed of soft matter—such as gels, colloids, emulsions, and glassy materials—which are assemblies of flexible molecules. Within cells, these materials coexist under nonequilibrium conditions, mixing and competing with one another. To understand the characteristic physical properties and behaviors of living cells, it is essential to ground complex systems biology in the physics of biomaterials, that is, soft matter physics.

Experimental Approaches to Mesoscale Properties and Their Development

Traditionally, questions such as “What is life?” and “Where does lifelike behavior originate?” have been discussed primarily at a conceptual level, with limited experimental validation. However, recent advances in nonequilibrium and nonlinear physics, together with cutting-edge techniques such as molecular manipulation and genetic engineering at the mesoscale, are making quantitative approaches to these questions increasingly feasible.

In our research group, we actively employ these advanced techniques while also developing our own original measurement methods. By investigating the physical properties of soft matter within living cells and the nonequilibrium dynamics driven by metabolism from nano- to microscale perspectives, we aim to uncover the fundamental principles of life.

In particular, we have recently been constructing model cellular systems in which metabolic activity can be artificially controlled. Using these systems, we seek to elucidate the metabolic regulation mechanisms underlying physical phenomena such as sol–gel transitions (fluid-to-gel transitions), liquid–glass transitions, and phase separation (e.g., droplet formation).

Through the integration of soft matter physics and complex systems biology, we aim to reveal the unique physical properties of living systems and the fundamental nature of what it means to be “alive.” For further details, please refer to the “Recent Research” page and additional explanatory materials.