Photosystem I and II — Revision Notes

⚡ 30-Second Revision

Photosystem II (PSII) — P680 reaction center, absorbs $680,\text{nm}$.
Function of PSII — Water splitting ($2H_2O \rightarrow 4H^+ + 4e^- + O_2$), initiates non-cyclic electron flow.
Location of PSII — Grana lamellae (stacked thylakoids).
Photosystem I (PSI) — P700 reaction center, absorbs $700,\text{nm}$.
Function of PSI — Reduces NADP+ to NADPH, involved in cyclic and non-cyclic flow.
Location of PSI — Stromal lamellae (unstacked thylakoids) and grana edges.
Non-cyclic Photophosphorylation (Z-scheme) — Involves both PSII & PSI. Products: ATP, NADPH, $O_2$.
Electron flow (Non-cyclic) — $H_2O \rightarrow$ PSII $ightarrow$ Pheophytin $ightarrow$ Plastoquinone $ightarrow$ Cyt $b_6f$ $ightarrow$ Plastocyanin $ightarrow$ PSI $ightarrow$ Ferredoxin $ightarrow$ NADP+ Reductase $ightarrow$ NADPH.
Cyclic Photophosphorylation — Involves only PSI. Products: ATP only. No $O_2$, no NADPH.
Proton Gradient — Formed by $H^+$ from water splitting (lumen) and $H^+$ pumping by Cyt $b_6f$ (stroma to lumen). Drives ATP synthesis.

2-Minute Revision

Photosystems I (PSI) and II (PSII) are the light-capturing units in the thylakoid membranes of chloroplasts. PSII, with its P680 reaction center (max absorption $680,\text{nm}$), acts first in non-cyclic electron flow.

It absorbs light, excites electrons, and crucially splits water ($H_2O$) to replace lost electrons, releasing oxygen ($O_2$), protons ($H^+$), and electrons. These electrons then move through an electron transport chain, generating a proton gradient for ATP synthesis.

PSI, with its P700 reaction center (max absorption $700,\text{nm}$), receives electrons from PSII via plastocyanin. It also absorbs light, excites its own electrons, which are then used to reduce NADP+ to NADPH.

Non-cyclic photophosphorylation, involving both PSII and PSI, produces ATP, NADPH, and oxygen. Cyclic photophosphorylation, involving only PSI, produces only ATP, without NADPH or oxygen release, helping balance the ATP:NADPH ratio for the Calvin cycle.

Remember, PSII is mainly in grana, PSI in stromal lamellae.

5-Minute Revision

Photosystems I (PSI) and II (PSII) are integral membrane protein complexes vital for the light-dependent reactions of photosynthesis. Each photosystem consists of an antenna complex, which captures light energy from various pigments (chlorophylls, carotenoids) and funnels it to a reaction center. The reaction center contains a special pair of chlorophyll 'a' molecules capable of photo-oxidation.

Photosystem II (PSII):

Reaction Center: — P680 (absorbs maximally at $680,\text{nm}$).
Location: — Primarily in the grana lamellae (stacked thylakoids).
Function: — Initiates non-cyclic electron flow. Upon light absorption, P680 loses an electron. This electron is replaced by the splitting of water molecules ($H_2O$) by the oxygen-evolving complex (OEC) associated with PSII. This photolysis releases electrons ($e^-$), protons ($H^+$) into the thylakoid lumen, and molecular oxygen ($O_2$) as a byproduct. The excited electrons from PSII pass through an electron transport chain (pheophytin $ightarrow$ plastoquinone $ightarrow$ cytochrome $b_6f$ complex $ightarrow$ plastocyanin), contributing to the proton gradient across the thylakoid membrane, which drives ATP synthesis.

Photosystem I (PSI):

Reaction Center: — P700 (absorbs maximally at $700,\text{nm}$).
Location: — Predominantly in the stromal lamellae (unstacked thylakoids) and at the edges of the grana.
Function: — Receives electrons from PSII via plastocyanin. Upon light absorption, P700 also loses an electron. These electrons are then passed through another short electron transport chain (ferredoxin) and ultimately used by the enzyme NADP+ reductase to reduce NADP+ to NADPH, utilizing protons from the stroma. NADPH is a crucial reducing agent for the Calvin cycle.

Photophosphorylation:

Non-cyclic (Z-scheme): — Involves both PSII and PSI. Electrons flow unidirectionally from water to NADP+. Produces ATP, NADPH, and $O_2$. This is the primary pathway for generating both energy carriers.
Cyclic: — Involves only PSI. Electrons from PSI are cycled back to the cytochrome $b_6f$ complex and then to PSI, bypassing NADP+ reductase. This pathway only produces ATP, without generating NADPH or releasing $O_2$. It helps balance the ATP:NADPH ratio when more ATP is required by the cell.

Key Takeaway: Both photosystems work in concert to convert light energy into chemical energy (ATP and NADPH), which fuels the synthesis of sugars in the Calvin cycle. PSII is the source of electrons and oxygen, while PSI is the primary site for NADPH production.

Prelims Revision Notes

1
Photosystems Overview — Functional units in thylakoid membranes for light absorption and electron transfer. Each has an antenna complex (light-harvesting pigments) and a reaction center (special chlorophyll 'a' pair).
2
Photosystem II (PSII)

* Reaction Center: P680 (max absorption at $680,\text{nm}$). * Location: Primarily in grana lamellae (stacked thylakoids). * Electron Source: Water ($H_2O$) via photolysis. * Photolysis: $2H_2O \rightarrow 4H^+ + 4e^- + O_2$.

Occurs in thylakoid lumen. Releases $O_2$ (atmospheric source), $H^+$ (contributes to proton gradient), and $e^-$ (replenish P680). * Primary Electron Acceptor: Pheophytin. * Role: Initiates non-cyclic electron flow, generates $O_2$, contributes to proton gradient for ATP synthesis.

1
Photosystem I (PSI)

* Reaction Center: P700 (max absorption at $700,\text{nm}$). * Location: Primarily in stromal lamellae (unstacked thylakoids) and grana edges. * Electron Source: Photosystem II (via plastocyanin) in non-cyclic flow; its own recycled electrons in cyclic flow. * Primary Electron Acceptor: Modified chlorophyll ($A_0$). * Role: Reduces NADP+ to NADPH in non-cyclic flow; involved in cyclic photophosphorylation for ATP production.

1
Non-cyclic Photophosphorylation (Z-scheme)

* Involves: Both PSII and PSI. * Electron Flow: Unidirectional from $H_2O \rightarrow$ PSII $ightarrow$ ETC $ightarrow$ PSI $ightarrow$ NADP+ reductase $ightarrow$ NADPH. * Products: ATP, NADPH, $O_2$. * Significance: Produces both energy carriers (ATP, NADPH) for Calvin cycle and releases oxygen.

1
Cyclic Photophosphorylation

* Involves: Only PSI. * Electron Flow: Electrons from PSI are cycled back to the cytochrome $b_6f$ complex and then to PSI via plastocyanin. * Products: ATP only. * Significance: Supplements ATP production when the ATP:NADPH ratio needs to be adjusted (e.g., higher ATP demand for Calvin cycle or other cellular processes). No $O_2$ release, no NADPH formation.

1
Electron Transport Chain (ETC) Components — Pheophytin, Plastoquinone (PQ), Cytochrome $b_6f$ complex, Plastocyanin (PC), Ferredoxin (Fd), NADP+ Reductase.
2
Proton Gradient — Formed by $H^+$ from water splitting in lumen and $H^+$ pumped from stroma to lumen by plastoquinone/cytochrome $b_6f$ complex. Drives ATP synthesis via ATP synthase (chemiosmosis).

Vyyuha Quick Recall

To remember the functional order and key features:

Photosystem Second Is Water Splitter, Oxygen Releaser, ATP Generator.

Photosystem First Is NADPH Reducer, Cyclic Flow Enabler.