Optimal disk packing of chloroplasts in plant cells.

Photosynthesis is essential for ecosystem survival, but while plants require light, excessive exposure can damage cells. Chloroplasts, photosynthetic organelles, respond via self-organized motion within cells to optimize light absorption. These disk-shaped organelles must balance two competing needs: dense packing to enhance absorption under dim light and rapid spatial rearrangement to avoid damage from excess light. Using microscopy, we show that plant cell shape and chloroplast size achieve both goals: dense monolayer packing for optimal absorption in low light and sidewall packing for light avoidance. We present a theoretical model using random close packing simulations of polydispersed hard disks in rectangular boxes and find optimal cell shapes that match plant cell measurements. Our findings highlight how particle packing principles under confinement enable light adaptation in plants, offering insights into organelle organization under confinement, a physical challenge relevant across biological systems.