Question

In both photosystem I and photosystem II, light energy is used to excite electrons. The two photosystems were named in the order of their discovery, and not in the order of their work in the light-dependent reactions. Photosystem II comes first in the sequence of events that moves electrons from water molecules to their destination, NADP+. Hydrogen ions travel with the electrons, and they combine with NADP+ to form NADPH. The process is summarized in this diagram.

In the diagram, an arrow leads from the splitting of water, to the two photosystems, and then to NADP+. What process is represented by this arrow?
A.
the formation of high-energy sugars
B.
the cycling of light energy within the photosystems
C.
the movement of electrons along the electron transport chain
D.
the conversion of electrons into sunlight

Answer 1

The arrow represents the movement of electrons along the electron transport chain, which starts with the splitting of water at photosystem II. This movement drives ATP production and ultimately leads to the reduction of NADP+ to NADPH.

Step-by-step explanation:

The process represented by the arrow leading from the splitting of water, to the two photosystems, and then to NADP+ in the diagram is the movement of electrons along the electron transport chain. This chain is a series of proteins that transfer electrons from photosystem II to photosystem I, driving the creation of an electrochemical gradient across the thylakoid membrane.

This gradient is harnessed to produce ATP through chemiosmosis, with ATP synthase mediating the conversion of ADP to ATP. Concurrently, the electrons reach photosystem I, which uses light energy to re-energize them, before they are ultimately used to reduce NADP+ into NADPH, a key energy carrier in photosynthesis.

The initial source of electrons for the chloroplast electron transport chain comes from the splitting of water within photosystem II. This is a critical step because it provides not only electrons but also protons and oxygen as a byproduct. The subsequent flow of electrons and hydrogen ions contributes to the synthesis of ATP and the reduction of NADP+ to NADPH, which are vital for the Calvin cycle, the second stage of photosynthesis.