Which of the following statements best describes the relationship between photosystem i and photosystem ii?

a. they are interchangeable systems allowing plant cells to use one or the other.
b. they operate independently but simultaneously to maximize light absorption.
c. they operate in sequence, with photosystem i supplying electrons to photosystem ii.
d. they operate in sequence, with photosystem ii supplying electrons to photosystem i.

D. They operate in sequence, with photosystem II supplying electrons to photosystem I.

The correct answer is d. They operate in sequence, with photosystem II supplying electrons to photosystem I.

Photosystem I or pigment system I consists of chlorophyll a and chlorophyll b. The reaction centre of photosystem I is P700, is a chlorophyll a molecule whose absorption spectrum peaks at 700 nm. Whereas, photosystem II or pigment system II consists of chlorophyll b and chlorophyll a. The reaction centre of photosystem II is P680, is a chlorophyll a molecule whose absorption spectrum peaks at 680 nm.  

Photosystem II uses light energy to splits (photolysis) water and extracts electrons. The electrons form photosystem II are passed to photosystem I through to cytochromes. The electrons at photosystem I reduce NADP+ to NADPH.