From cyanobacteria to higher plants, photosynthetic membranes are composed of two galactolipids, mono- and digalactosyldiacylglycerol (MGDG and DGDG, respectively), and two negatively charged lipids, sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG). In many environments, plants and algae grow in a shortage of nutrients, leading to the development of nutrient-saving mechanisms. For example, at the cellular level, in phosphate starvation, these mechanisms include conversion of phospholipids into phosphorus-free lipids. In photosynthetic membranes, PG is supposed to be replaced by SQDG in phosphate starvation whereas the opposite occurs in sulfur deprivation. All biological data confirm a complementary relationship between SQDG and PG and suggest the importance of maintaining the total amount of anionic lipids in photosynthetic membranes. Using neutron diffraction on reconstituted SQDG or PG lipid membranes, we demonstrate that, despite chemically different headgroups, PG and SQDG have similar physicochemical properties. With an equivalent diacylglycerol backbone, PG and SQDG membranes have a similar bilayer thickness and bending rigidity. They also have essentially the same response to hydration in terms of repulsion and interaction forces. The results presented here establish that SQDG and PG are good substitutes to each other in nutrient starvation conditions to maintain the chloroplast functional organization and its photosynthesis activity.