In all photosynthetic organisms, the light is being captured by membrane bound structures named Thylakoid membranes. Thylakoid membranes of ancient photosynthetic prokaryotes, the cyanobacteria, form stacks of parallel sheets close to the cell membrane, while the photosynthetic membranes of eukaryotes are arranged in a thylakoid that is made of those membranes that surround a thylakoid lumen. The light, required for photosynthesis, is captured by those membranes and starts a series of reactions that basically generate a movement of electrons from high-energy donor to low-energy acceptor. In this process, ATP and NADPH are formed and use as the energy to reduce CO2 to carbohydrate in the Calvin-Benson cycle. The major protein complexes that facilitate this process are: Photosystem II (PSII), cytochromes, Photosystem I (PSI), and ATP synthase (ATPase). This is universal to all the organisms that preform oxygenic photosynthesis. However, it is well established that there are major differences in the function and structure of those membranes in the many oxygenic photoautotrophs known to us. In this project, my goal is to understand the structural and functional aspects of thylakoid membranes from various photosynthetic organism. This could provide information on: 1. The basic photo-physiology of the cell and how does it adjust and respond to its environment, and 2. The evolutionary history of photosynthesis. To achieve that I am using combination of tomograms obtained by state-of-the-art Cryo-Electron microscopy (CryoET), biophysics, advanced protein analysis, bioinformatics, and spectroscopy.