Question

this diagram provides an overview of major events in photosynthesis. which of the following statements are accurate? select all that apply.

sunlight has to strike enzymes in the calvin cycle for the reactions to proceed normally.

electron carriers are essential components because they shuttle electrons from one molecular machine or enzyme to another.

if you trace the sequence of redox reactions starting with light hitting psii, water is oxidized and then electron acceptors and electron carriers are alternately reduced and oxidized as they gain and then release electrons, with the energy released being used to generate atp and nadph.

the arrows that represent electrons essentially describe a circuit that starts with h₂o and ends with nadph, though these electrons get passed to the calvin cycle and eventually end up in sugars.

this diagram is missing some of the elements in the summary reaction for photosynthesis: co₂ + h₂o + sunlight → glucose + o₂

Explanation:

• Statement 1: The Calvin cycle is a light - independent reaction. It does not require sunlight to strike its enzymes. So, this statement is incorrect.
• Statement 2: Electron carriers (such as plastoquinone, plastocyanin) are crucial as they transfer electrons between different molecular complexes (like from PSII to the electron transport chain (ETC) and then to PSI). This shuttling of electrons is essential for the flow of energy in the light - dependent reactions. So, this statement is correct.
• Statement 3: In the light - dependent reactions, when light hits PSII, water is oxidized (\(2H_2O

ightarrow4H^{+}+O_2 + 4e^{-}\)). The electron acceptors (e.g., pheophytin) accept electrons (get reduced) and then pass them to electron carriers. As electrons move through the ETC, energy is released which is used to make ATP. At PSI, further electron transfer leads to the formation of NADPH. So, electron acceptors and carriers are alternately reduced and oxidized. This statement is correct.

• Statement 4: The electrons start from water (\(H_2O\)) in PSII. They are transferred through the ETC and PSI to form NADPH. These electrons are then used in the Calvin cycle to reduce carbon dioxide and eventually end up in sugars (as part of the organic molecules synthesized). So, this statement is correct.
• Statement 5: The summary reaction for photosynthesis is \(6CO_2+6H_2O + sunlight

ightarrow C_6H_{12}O_6+6O_2\). The diagram shows \(CO_2\) entering the Calvin cycle, \(H_2O\) being split in PSII, sunlight hitting PSII and PSI, glucose being formed from 3 - carbon sugar, and \(O_2\) being released. So, the diagram is not missing elements of the summary reaction. This statement is incorrect.

Answer:

• B. Electron carriers are essential components because they shuttle electrons from one molecular machine or enzyme to another.
• C. If you trace the sequence of redox reactions starting with light hitting PSII, water is oxidized and then electron acceptors and electron carriers are alternately reduced and oxidized as they gain and then release electrons, with the energy released being used to generate ATP and NADPH.
• D. The arrows that represent electrons essentially describe a circuit that starts with \(H_2O\) and ends with NADPH, though these electrons get passed to the Calvin cycle and eventually end up in sugars.