Question

When electrons are passed along the electron transport chain during noncylic photophosphorylation, in photosystem I, this results in the production of (A) NADPH (B) ATP (C) cytochromes (D) water (E) glucose

Short answer

(A) NADPH

Step-by-step solution

Understanding the Noncylic Photophosphorylation Process

Noncyclic photophosphorylation involves two photosystems, Photosystem I (PSI) and Photosystem II (PSII). In Photosystem I, light energy is used to oxidize P700, a primary electron donor, which then triggers an electron transfer to an electron acceptor based on their redox potential.

Identifying the End Product

As a result of this process in Photosystem I, an electron is transferred to NADP+ to reduce it to NADPH. This NADPH is then used in the Calvin cycle of photosynthesis.

Selection of the Correct Answer

By understanding that NADPH is the end product of the electron transfer in Photosystem I, we can identify that the correct answer is (A) NADPH.

Key concepts

Noncyclic Photophosphorylation

Noncyclic photophosphorylation is a crucial part of the light-dependent reactions in photosynthesis occurring within the chloroplasts. It is termed "noncyclic" because the flow of electrons does not cycle back to the original photosystem. Instead, the electrons that are excited and leave the chlorophyll molecule in Photosystem II (PSII) are ultimately transferred to NADP+, forming NADPH.

During this process, two photosystems work in tandem: PSII and PSI. Light energy is absorbed by chlorophyll pigments, which excites the electrons. These high-energy electrons travel through an electron transport chain, leading to ATP and NADPH production, which are essential for the Calvin Cycle.

• PSII: Absorbs light, which energizes electrons; these electrons come from the splitting of water molecules.
• PSI: Uses light energy to further boost electrons for NADPH formation.

Noncyclic photophosphorylation also results in the generation of a proton gradient that contributes to ATP synthesis via chemiosmosis. The overall products of this pathway are NADPH, ATP, and oxygen.

Electron Transport Chain

The electron transport chain (ETC) is a series of protein complexes embedded in the thylakoid membrane of chloroplasts. It plays a critical role in the conversion of light energy into chemical energy during photosynthesis. When electrons are excited in Photosystem II, they enter the ETC.

The movement of these electrons down the chain releases energy, which is used to pump protons from the stroma into the thylakoid lumen, creating a proton gradient.

• High-energy electrons move through the chain, losing energy progressively.
• Energy from these electrons is harnessed to pump protons across the membrane.
• The gradient generated is essential for ATP synthesis through ATP synthase.

This transport chain also connects Photosystem II to Photosystem I. As electrons lose energy while moving down the chain, they are eventually passed to Photosystem I, where they are re-energized by another photon of light.

Photosystem I

Photosystem I (PSI) is one of two photosystems involved in the light-dependent reactions of photosynthesis. It is primarily responsible for the production of NADPH. The reaction center of PSI contains chlorophyll a, which absorbs light most effectively at a wavelength of 700 nm, hence its reference as P700.

When PSI absorbs light, P700 becomes excited and loses an electron. This electron is passed along a different part of the electron transport chain before eventually reducing NADP+ to form NADPH.

• PSI boosts the energy level of electrons coming from Photosystem II.
• These electrons are then used to reduce NADP+ to NADPH, a crucial step for subsequent photosynthesis reactions.
• PSI operates following PSII, receiving its electrons.

Without Photosystem I, plants would not be able to produce NADPH, which is essential for the Calvin Cycle, where carbon fixation occurs.

NADPH Production

The production of NADPH is fundamental in the photosynthetic process, serving as an electron carrier with a critical role in the reduction reactions during the Calvin Cycle. This production occurs in the final stages of the noncyclic photophosphorylation process, largely facilitated by Photosystem I.

As electrons are excited and passed through PSI, they eventually reduce NADP+ to NADPH. This reaction involves the transfer of two electrons and one proton, enabling NADP+ to hold on to the extra electron as NADPH:

• NADP+ + 2e- + H+ → NADPH

NADPH acts as a reducing power in the Calvin Cycle, where it facilitates the conversion of 3-phosphoglycerate to glyceraldehyde-3-phosphate, an important intermediate in glucose synthesis.

• NADPH provides the necessary protons and electrons for reduction reactions.
• It helps in the synthesis of carbohydrates from carbon dioxide and water.

Understanding this process underlines its importance in sustaining plant life and in turn, life on Earth, by driving the production of vital carbohydrates through photosynthesis.