Question

how an inhibitor of electron passage through pheophytin would affect electron transfer through (a) photosystem II and (b) photosystem I. Explain your reasoning.

Short answer

Inhibiting pheophytin stops electron flow and ATP/NADPH production in PS II; PS I runs out of electrons.

Step-by-step solution

Introduction to the Problem

We are asked to understand how an inhibitor of electron passage through pheophytin would affect the electron transfer process in both photosystem II and photosystem I. Pheophytin plays a role in the electron transport chain, specifically within Photosystem II, whereas Photosystem I does not directly involve pheophytin.

Identify the Role of Pheophytin in Photosystem II

In Photosystem II, pheophytin acts as the first electron acceptor from the primary donor chlorophyll molecule, P680. Once P680 is excited by light energy, it transfers electrons to pheophytin, which subsequently transfers them to plastoquinone. This is a crucial step in the linear electron flow.

Predict the Impact on Photosystem II

If an inhibitor blocks electron passage through pheophytin, then the electron transport chain in Photosystem II will be disrupted. Electrons will not proceed further downstream, effectively halting the reduction of plastoquinone. This will stop the usual flow of electrons and prevent ATP and NADPH formation, as well as stop oxygen evolution from water splitting.

Impact on Photosystem I

Since Photosystem I does not directly involve pheophytin, the immediate electron transfer processes within it won't be directly impacted by the inhibitor. Photosystem I relies on electrons supplied from the plastoquinone pool and the interconnected chain that begins at Photosystem II. If the upstream electron flow from Photosystem II is halted, Photosystem I will eventually run out of electrons to reduce its primary acceptor P700, leading to indirect effects.

Summarize Effects on Overall Photosynthesis

Blocking electron passage through pheophytin affects the entire electron transport chain. Photosystem II's blockade will reduce the production of ATP and NADPH, which are essential for the Calvin cycle. Photosystem I will eventually be affected due to a lack of electron supply from Photosystem II.

Key concepts

Pheophytin

Pheophytin is an important component in the photosynthetic process, particularly in Photosystem II (PSII). It acts as the initial electron acceptor following the energization of chlorophyll molecules by light. When light strikes PSII, it excites P680, the primary chlorophyll donor. This excitation causes P680 to release electrons, which are immediately accepted by pheophytin.
Pheophytin is essentially a modified chlorophyll molecule, lacking a central magnesium ion, which makes it an ideal electron carrier. It is specifically positioned to transport these electrons to plastoquinone, another component in the electron transport chain.
If pheophytin is inhibited, this crucial step is disrupted, leading to a blockage in the electron flow within PSII. This disruption would halt further processes dependent on this initial electron transfer, affecting energy production within the plant.

Electron Transport Chain

The electron transport chain (ETC) is a series of protein complexes and other molecules that pass electrons through a membrane, thereby facilitating the formation of vital molecules like ATP and NADPH. Within photosynthesis, the ETC plays a crucial role in converting light energy into chemical energy, a fundamental process within all green plants.
In PSII, the ETC begins with the transfer of electrons from chlorophyll to pheophytin, and then to plastoquinone, through cytochrome b6f, plastocyanin, and finally to Photosystem I (PSI). Each step is meticulously arranged to ensure efficient energy transformation.

• Pheophytin acts as a primary electron acceptor.
• Plastoquinone carries electrons and pumps protons, aiding in creating a proton gradient.
• Ultimately, this leads to the synthesis of ATP and NADPH, the energy carriers required by the plant.

When pheophytin is inhibited, the entire chain within PSII experiences a logjam, preventing the downstream processes, including the operation of PSI and energy currency production.

Photosynthesis

Photosynthesis is the complex biochemical process through which plants, algae, and some bacteria convert light energy, usually from the sun, into chemical energy stored in glucose. This process is crucial for life on Earth, providing the primary energy source for almost all ecosystems.
The process occurs in two main stages: the light-dependent reactions and the Calvin cycle.

• Light-dependent reactions use light to produce ATP and NADPH.
• The Calvin cycle does not require light and synthesizes sugars from carbon dioxide, using the ATP and NADPH generated in the light reactions.

Photosystems II and I are integral parts of the light-dependent reactions. They capture solar energy, facilitating electron movement through the ETC. Any disruption in this electron flow, such as from an inhibitor acting on pheophytin, can severely impair the photosynthetic efficiency and overall energy production, affecting the growth and survival of plants.

Photosystem I

Photosystem I (PSI) is one part of the dual-system structure in plants that captures light and converts it into energy during photosynthesis. PSI primarily functions towards the climax of the light-dependent reactions. It is located after PSII in the sequence of the electron transport chain, where it uses the energy from light to transfer electrons ultimately to NADP+, forming NADPH.
PSI consists of chlorophyll and bound proteins that respond differently to light than those in PSII. The key chlorophyll component in PSI is P700, which becomes energized and passes its electrons down the ETC.
If PSII is blocked by an inhibitor affecting pheophytin, PSI eventually feels the impact due to decreased electrons coming from the earlier processes. Although PSI is not directly involved with pheophytin, its operation relies on the electron flow initiated by PSII. Hence, disruptions upstream can lead to diminished activity in PSI, affecting the plant's ability to generate energy.