Write Short Notes on the Following: Z-Scheme and Light Harvesting Systems in Microbes
(a) Z-Scheme
The Z-scheme is a model that illustrates the energy flow of electrons during the light reactions of photosynthesis in plants, algae, and cyanobacteria. Named for its characteristic “Z” shape, it describes the oxidation-reduction reactions in Photosystem II (PSII) and Photosystem I (PSI).
- Light excites electrons in PSII, which are transferred through a chain of carriers to PSI via plastoquinone, cytochrome b₆f, and plastocyanin.
- Electrons from PSII are replaced by water splitting (photolysis), releasing O₂.
- PSI also absorbs light, further exciting electrons that are then used to reduce NADP⁺ to NADPH.
The Z-scheme highlights the energy increase at each photosystem and how light energy is captured and used to generate reducing power (NADPH) and ATP, essential for the Calvin cycle.
(b) Light Harvesting Systems in Microbes
Microbial light harvesting systems are specialized pigment-protein complexes that capture solar energy for photosynthesis. These systems vary significantly among microbial groups.
- Cyanobacteria and Algae: Use phycobilisomes, complexes composed of pigments like phycocyanin and allophycocyanin, to absorb light and funnel energy to PSII and PSI.
- Green Sulfur Bacteria: Contain chlorosomes that absorb low-intensity light, enabling photosynthesis in deep or murky environments.
- Purple Bacteria: Utilize membrane-bound light harvesting complexes (LH1, LH2) that encircle the reaction center and assist in energy capture.
These adaptations allow microbes to thrive in diverse environments by optimizing light absorption across different wavelengths and intensities.