Fundamentals of Molecular Biology. Part 1: Molecular Cell Biology - A. N. Ogurtsov 2011

Molecular mechanisms of photosynthesis
Regulation of the relative activity of photosystems PS1 and PS2

For Photosystems PSII and PSI to function as sequential elements in a single (linear) Electron Transport Chain, the light irradiation intensity of these systems must be adjusted so that they produce an equal number of electrons. If this balance is not maintained, PSI initiates a Cyclic electron transport process, while PSII becomes less active.

METABOLISM/14.html">Chloroplasts feature a regulatory mechanism controlling the relative intensity of linear and cyclic electron flow, which relies on the reversible phosphorylation and dephosphorylation of Proteins within the Light-Harvesting Complexes associated with PSII.

In this case, a membrane protein kinase Senses the relative activity of the Two Photosystems based on the overall Redox Potential of the entire plastoquinone pool in the thylakoid membrane, which shuttles electrons from PSII to cytochrome bf en route to PSI.

When the photoexcitation of PSI and PSII is balanced, the Linear electron transport chain operates (Figure 186(a)).

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Figure 186 - Distribution of multi-Structure/178.html">Protein Complexes in the thylakoid membrane: a - under high light intensity, the linear electron transport chain is active; b - under unbalanced illumination, light-harvesting complexes (LHCII) reassociate with PSI, and the cyclic electron pathway becomes dominant

However, when an excess of plastoquinone is reduced (due to overactivity of PSII relative to PSI), the protein kinase is activated and begins phosphorylating specific light-harvesting complexes LHCII.

Phosphorylated LHCII complexes dissociate from PSII (RC), which are predominantly located in the grana, and diffuse to the non-appressed Regions of the thylakoid membrane, where they participate in activating PSI (PCI) (Figure 186(b)).

This redistribution of LHCII complexes—which decreases the light-harvesting antenna size of PSII while increasing it for PSI—is accompanied by the migration of cytochrome bf complexes from the regions where PSII is primarily localized (grana) to the areas enriched in PSI (non-appressed regions). As a result, cyclic Electron transport is intensified.

This Organization of photosystems in plant thylakoids allows photosynthetic processes to be redirected toward either ATP synthesis (Figure 186(b)) or the synthesis of both NADPH and ATP (Figure 186(a)).

NADPH and ATP are essential for converting CO₂ into sucrose and starch during the Fourth Stage of Photosynthesis.

Review Questions

1. What are the key differences between the photosynthetic systems of plants and purple Bacteria?

2. What is the function of the "special pair" of chlorophylls in the reaction center of a photosystem?

3. What is the Q-cycle, and how does it contribute to the Generation of the proton-motive force?

4. What is the functional difference between the linear and cyclic Electron Transport Chains in the Photosystem of purple bacteria?

5. Why do excited chlorophylls in the reaction centers of purple bacteria photosystems not use Water molecules as electron Donors?

6. In what processes do purple bacteria utilize the proton-motive force generated by cyclic electron transport in their photosystems?

7. What are the structural and Functional differences between the Two photosystems in plant chloroplasts?

8. What is the functional difference between linear and cyclic electron transport in the photosynthetic systems of plant chloroplasts?

9. Which processes in plant photosystems increase the proton-motive force across the chloroplast thylakoid membrane?

10. What are the Functions of ferredoxin and plastocyanin proteins in plant chloroplasts?

11. Why can the photochemically oxidized chlorophyll P₆₈₀ in the PSII reaction center of plant chloroplasts utilize water molecules as an electron donor?

12. What is the function of manganese ions in the water-splitting complex of photosystem PSII?

13. How is the relative intensity of linear and cyclic electron transport regulated in plant chloroplasts?

14. What is the function of membrane protein kinase in the chloroplast thylakoid?

15. How do LHCII light-harvesting complexes redistribute under changing light conditions?