Lesson Explainer: The Light-Dependent Reactions Biology - Nagwa

Example 1: Recalling How Electrons Are Supplied to the Electron Transport Chain in the Light-Dependent Stage of Photosynthesis

The diagram provided shows a basic outline of the light-dependent reactions. As electrons move from photosystem II to photosystem I, how are they replaced?

1. By the movement of H+ ions
2. By the photolysis of water
3. By the absorption of light energy
4. By the reduction of NADP+

Answer

The light-dependent stage of photosynthesis is the first stage, and it involves a series of reactions that rely on the movement of electrons down an electron transport chain. The initial step is for the pigments in photosystem II to absorb light energy, which excites the electrons contained within the chlorophyll, so they move to a higher energy level. After this, they are passed down proteins and enzymes in the electron transport chain. This means the electrons that were transported from photosystem II need to be replaced.

As we can see from the diagram, the first photosystem in the chain (which is named photosystem II) absorbs light energy. However, this does not provide the photosystem with electrons but instead provides the energy needed to excite electrons. We can also note from the diagram that NADP+ is not involved in the electron transport chain until after the electrons have passed through photosystem I, so NADP+ would not be able to replace these first electrons. The movement of hydrogen (H+) ions will not replace electrons, as electrons are negatively charged particles, and we can see hydrogen ions are positively charged. However, we can see from the diagram that at photosystem II, water (HO2) is split into its components, oxygen and hydrogen. This process, known as photolysis, also produces two electrons. These electrons are used to replace the electrons that left photosystem II to be moved along the electron transport chain.

Therefore, electrons are replaced by the photolysis of water.