Thylakoid membrane is not a cell. Is inside the chloroplast (organelle) double membrane.
When cyanobacteria got engulfed by some cell and had 2 membranes and became plant
cell. Sacs are called thylakoid. Embedded in tthylakoid are chlorophyll molecules. They
funnel energy down to one chlorophyll molecule. Photosystem I is sensitive to around
700 wl light (primitive situation). Light will zap chemicals and and hit one atom in the
center. When light (or other energy) hits them they go to higher energy level. They don’t
like this and want to go to rest position. The atom puts energy in packets (photons) and
gives it off (light). Ferredoxin (Fe protein with iron) can handle the high energy electrons.