When light energy excites electrons in photosystem where do the electrons to replace them come from

Light excites the electrons in chlorophyll a molecules of photosystem 1. As these electrons move to another primary electron receptor, electrons from photosystem 2 replace them.
Therefore the answer is FROM PHOTOSYSTEM 2.

Both prokaryotic and eukaryotic cells have ribosomes and cell membranes

The name of the instrument that the scientist uses is called a microscope.a microscope is used to view objects which is not visible to the naked eyes