Representative of another class of activities in molecular biology is the examination of passive ion flux across axon membranes. This work is being done by Goldman at the National Naval Medical Center. The question of stimulus transmission by nerve tissue is far from simple, and the ion concentrations associated with nerve membranes is a significant part of the answer. Because the space environment may very well produce alterations in these ion potentials, an investigation of their natures and significance becomes extremely important. A working theory is now being developed as a result of this study. Vital cell processes, chemical transformations, and mechanisms that provide energy for cell maintenance and activity have been studied by Kiesow (refs. [ref.157] and [ref.158]) at the Naval Medical Research Institute. The common objective of all phases of this project is the elucidation of reaction steps in which energy and matter are transformed in living systems. Compared with _photo_synthetic organisms, _chemo_synthetic bacteria offer distinct advantages for the study of energy assimilation. These studies have led to the following experimental findings. With the energy from oxidation of nitrite, NO₂— to nitrate, NO₃— as an _inorganic_ source, and with added _organic_ chemical energy from the hydrolysis of adenosinetriphosphate (ATP) to adenosinediphosphate (ADP) and inorganic phosphate, chemosynthetic bacteria are capable of reducing diphosphopyridinenucleotide (DPN⁺) to DPNH, in a coupled oxidoreduction-dephosphorylation. Thus, in the crucial step of chemosynthesis, _ATP is consumed, not produced_. However, in simultaneously proceeding cell respiration, the energy donor, DPNH, is oxidized and generates more ATP than is required for DPN⁺ reduction. This "breeder cycle" for DPNH—with different ratios of cell respiration and biosynthesis—results in a net production of either DPNH, or ATP, or both. Production of DPNH in the cycle leads immediately to the assimilation of C¹⁴ from HC¹⁴O₃—. These observations explain the bacteria’s energy source without the classical hypotheses of either direct phosphorylation or direct CO₂ reduction by inorganic chemical or electromagnetic energy. The cycle transforms the free energy of nitrite oxidation into the free energy of the organic compounds. Cell respiration and elementary biosynthesis proceed through structure-bound enzyme systems in the same fraction of subcellular particles. Three components, two cytochromes and one flavoprotein, have been identified. A thermodynamic analysis of the DPNH "breeder cycle" appears to be attainable by measurements of redox potentials and calorimetric determinations of heats of reaction.