"The Bioenergetic Costs of a Gene," reported in the Proceedings of the National Academy of Sciences, describes for the first time how much total energy is needed to build and maintain a cell, and how this scales with cell size.

The first step, reported in the paper, describes the total energy costs of individual genes, as incurred at the DNA, RNA and protein levels.

The unit of energy used in the study was ATP, or adenosine nucleoside triphosphate, a simple molecule that serves as the major carrier of cellular energy in cells through the breakage of phosphate bonds. In terms of energy, a single ATP molecule produces about ten quintillionth of a joule when one of these bonds is broken.

The simplest cells in the study, bacteria, needed about 100 million molecules of ATP energy over the course of their life. The most complex cells in the study, eukaryotic ciliates, needed more than 1,000 times that amount across their lifespan.

In eukaryotes, ATP is mostly produced by the mitochondrion, which is the cellular structure responsible for the majority of energy utilization in complex eukaryotic cells.

Because they possess a genome independent of their host cells, mitochondria are known to have started out as an ancient bacteria absorbed into early single-celled organisms about 2 billion years ago.

Symbiogenesis is the term used to describe the concept that cells without nuclei or organelles -- prokaryotes -- gave rise to more complex cells with nuclei and organelles -- eukaryotes -- after a single-celled organism absorbed a bacterial ancestor of the mitochondrion. It is regarded as one of the most important events in evolutionary history.

Surprisingly, however, the IU scientists found that energy consumption grew linearly with cell volume, not exponentially or stepwise, as cells grew more complex. So while eukaryotic cells require more energy for cellular reproduction, they didn’t increase their energy needs at a higher rate relative to cell volume compared to prokaryotic cells.

"The enhanced ability to generate energy made possible by the mitochondrion is often seen as a prerequisite for the evolution of eukaryotic cell features such as increased gene number, protein length, protein folds, protein-protein interactions and regulatory elements," author said. "But our observations suggest that the energy boost associated with the mitochondrion is not a precondition for genome complexity."

http://news.indiana.edu/releases/iu/2015/11/cells-lifetime-energy-requirements.shtml