Question

Put the following steps of the light reactions in the correct order:

1. Light excites an electron from photosystem I.

2. Light excites an electron from photosystem II.

3. Electrons reduce NADP+ to NADPH.

4. Electrons pass through an electron transport chain, which generates an H+ gradient used to make ATP.

Answer 1

In order:
2. Light excites an electron from Photosystem  II
4.Electrons pass through an electron transport chain, which generates an H+ gradient used to make ATP.
1. Light excites an electron from PS I
3. Electrons reduce NADP+ to NADPH.
Explanation:
Light is absorbed by a pigment from PSII and it is transferred to reaction center, energy is passed to P680 where the electron goes into high energy state and splitting of water takes place to replace this electron .Down the electron transport chain, we find high energy electrons travelling , losing energy as it goes.The released energy helps in pumping of H+ from the stroma into the thylakoid interior, that builds a concentration gradient and these H+ ions pass through ATP synthase driving ATP production.
and electrons later arrive at PSI, electron on getting high energy is transferred to acceptor molecule and is replaced by incoming electrons from PSII , At the end of chain electron is passed to NADP+ reducing it to NADPH.