Life on earth depends on photosynthesis, the source of all of our food, oxygen and most of our energy. The early steps of photosynthesis involve trapping of solar energy by electron transfer reactions in the photosynthetic membrane. Our recent studies have revealed an unexpected role for the membrane protein photosystem II (PSII) supercomplexes in mediating the stacking of chloroplast thylakoid membranes. Membrane stacking instigates the spatial segregation of the slow excitation energy trap PSII from the faster trap photosystem I (PSI), and promotes energy transfer among PSII units both of which are crucial for the efficiency of photosynthesis. For the first time we have biochemically-isolated a unique stacked form of the PSII supercomplex which will allow us to investigate this critical feature of the process. This project will make use of the latest advances in structural and functional microscopies to characterise this PSII supercomplex and understand how and why thylakoid membranes stack in molecular detail.