Which of the following is not a step in the light-dependent reactions?

a. high energy electrons move through the electron transport chain.
b. Pigments in the photocenters absorb light.
c. ATP synthase allows H+ ions to pass through the thylakoid membrane
d. ATP and NADPH are used to produce high energy sugars

To identify which of the following is not a step in the light-dependent reactions, we need to understand what happens during these reactions.

The light-dependent reactions are part of the photosynthesis process in plants and algae, which occur in the thylakoid membranes of the chloroplasts. These reactions use light energy to convert water molecules into ATP (adenosine triphosphate), NADPH (nicotinamide adenine dinucleotide phosphate), and oxygen.

Let's examine each option and its relevance to the light-dependent reactions:

a. High energy electrons move through the electron transport chain.
During the light-dependent reactions, light energy is absorbed by pigments in the photocenters. This energy excites electrons, which are then transferred through an electron transport chain. These electrons are eventually used to produce ATP and NADPH. Therefore, this step is indeed a part of the light-dependent reactions.

b. Pigments in the photocenters absorb light.
This statement accurately represents one of the critical steps in the light-dependent reactions. When pigments in photocenters absorb light energy, they transfer it to the reaction center, initiating the entire process.

c. ATP synthase allows H+ ions to pass through the thylakoid membrane.
During the light-dependent reactions, a concentration of H+ ions (protons) is built up within the thylakoid space. As a result, there is a higher concentration of H+ ions inside the thylakoid than in the stroma, leading to a proton motive force. ATP synthase uses this proton motive force to generate ATP by allowing the H+ ions to pass through the thylakoid membrane. So, this statement accurately describes a step in the light-dependent reactions.

d. ATP and NADPH are used to produce high-energy sugars.
This statement refers to the light-independent reactions, also known as the Calvin cycle or dark reactions. In this phase, ATP and NADPH, which are products of the light-dependent reactions, are used to fix carbon dioxide and produce high-energy sugars, such as glucose. Therefore, this step is part of the light-independent reactions and not the light-dependent reactions.

In conclusion, the option that is not a step in the light-dependent reactions is d. ATP and NADPH are used to produce high-energy sugars.