Thylakoids are important membrane proteins in the chloroplast that are responsible for harvesting light during the first stage of photosynthesis where light-dependent reactions occur. These thylakoids found in the stroma frequently form stacks of discs in an alternating pattern of dark and light bands called grana that function as a single compartment.
Thylakoid membranes enclose thylakoid lumens that are responsible for phosphorylation during photosynthesis. The thylakoid membrane is responsible for pumping protons across the thylakoid lumen making the structure acidic with a pH 4.