Inner World of the Cell

Definition of Life
Size of Cells and the Components
Structure in a Cell (Update 2022)
Molecular Tracks, (Microtubule, Actin, DNA and mRNA as Molecular Tracks)
Molecular Motors, (Kinesin, Myosin, Replication Motors, Ribosome, Molecular Pumps)
Effective Theories
Novel Properties at Nano-scale, (Free Energy, Confluence of Energies , Cooperative Dynamics,
Entropic Force, Walking Mechanism of the Molecular Motors)
Effect of Mechanical Force

Definition of Life

There is no unequivocal definition of life. A working definition usually considers something to be living if it exhibits all or most of the following attributes : 1. Homeostasis (maintaining a stable state), 2. Organization (assembly from parts), 3. Metabolism (energy consumption), 4. Growth (to mature size), 5. Adaptation (to environmental change), 6. Reproduction (capable of producing next generation), and 7. Response to stimuli.

Figure 01 Non-life vs Life [view large image]

Since all these attributes are processes, an entity would be easier to recognize, if it is living, by checking out its running processes than by its physical appearance.

Figure 01 shows the innards of a watch and a macrophage (a kind of immunity cell inside our body, the image is taken by an Atomic Force Microscope). While the macrophage is obviously alive; the watch is not because it does not grow, cannot adapt, and would not reproduce. On the other hand a virus is on the border line between non-life and life because it misses only the attribute of metabolism, it lives at the expense of the host cell. The smallest size of a living unit is the cell. It is the intricate processes within that makes it alive. The components inside were indiscernible until the advent of the atomic force microscope (AFM, the popular name is electron microscope) in 1986. It can be used to probe at objects of nano-scale - the parts inside the cell.

Advocates of Intelligent Design used to ridicule evolution theory by the analogy of "tornado in a junkyard" that assembles an airplane (a watch for that matter) from scraps at one fell swoop. Since the man-made objects such as airplane or watch is assembled for a purpose by a creator (the manufacturer or craftsman/woman) and life is more complicated than an airplane or watch, they argue that there must be a creator for life, which is created for a purpose. This is not a good analogy since the process of creating life is quite different. Its assembly is incremental and without a purpose. It takes billion years of trial and error to assemble something which is fit to live in certain environment. It was created without any purpose, it just happens to be here and there only because it is compatible with the environment (see more in Darwinian evolution).