**Berkley J. Walker 1,2,\*, David M. Kramer 1,3, Nicholas Fisher <sup>1</sup> and Xinyu Fu <sup>1</sup>**


Received: 17 January 2020; Accepted: 15 February 2020; Published: 1 March 2020

**Abstract:** Given their ability to harness chemical energy from the sun and generate the organic compounds necessary for life, photosynthetic organisms have the unique capacity to act simultaneously as their own power and manufacturing plant. This dual capacity presents many unique challenges, chiefly that energy supply must be perfectly balanced with energy demand to prevent photodamage and allow for optimal growth. From this perspective, we discuss the energy balancing network using recent studies and a quantitative framework for calculating metabolic ATP and NAD(P)H demand using measured leaf gas exchange and assumptions of metabolic demand. We focus on exploring how the energy balancing network itself is structured to allow safe and flexible energy supply. We discuss when the energy balancing network appears to operate optimally and when it favors high capacity instead. We also present the hypothesis that the energy balancing network itself can adapt over longer time scales to a given metabolic demand and how metabolism itself may participate in this energy balancing.

**Keywords:** energy balancing; cyclic electron flux; malate valve; photorespiration; photosynthesis; C3 cycle
