$Bienertia\; cycloptera$ belongs to a diverse set of plants, recently discovered to perform C$4$ photosynthesis within individual mesophyll cells. How these plants accomplish high photosynthetic efficiency without adopting Kranz anatomy remains unanswered. By modelling the processes of diffusion, capture, and release of carbon dioxide and oxygen inside a typical $Bienertia$ mesophyll cell geometry, we show that a spatial separation as low as 10 μm between the primary and the secondary carboxylases, can, on its own, provide enough diffusive resistance to sustain a viable C$4$ pathway at 20 °C, with a CO$2$ leakage <35%. This critical separation corresponds to a cell diameter of 50 μm, consistent with the observed range where $Bienertia$’s mesophyll cells start to develop their characteristic mature anatomy. Our results are robust to significant alterations in model assumptions and environmental conditions, their applicability extending even to aquatic plants.