What restricts an individual cell from becoming more complex in its functions?

The complexity of an individual cell's functions is primarily restricted by the limited number of biomolecular machines it possesses. Biomolecular machines, which include proteins, enzymes, and other molecular structures, play critical roles in various cellular processes, such as metabolism, signal transduction, and cellular transport.

When a cell has a restricted set of these biomolecular machines, its ability to perform a wide range of functions is limited. For instance, some cellular processes require specific enzymes or protein complexes to carry out complex biochemical reactions. If a cell lacks the necessary biomolecular machines, it cannot effectively execute these functions, thereby capping its complexity and capabilities.

In contrast, while factors like genetic diversity, ability to reproduce, and energy supply are important for the overall health and functioning of an organism, they do not directly limit the inherent complexities of an individual cell's functions to the same extent as the absence or limitation of biomolecular machines does.