LIGHT REACTION IN CYANOBACTERIA
- 1. LIGHT REACTION IN CYANOBACTERIA Ms Saajida Sultaana Mahusook
- 2. Oxygenic photosynthesis is used by cyanobacteria, algae, and plants. Cyanobacteria contain chlorophyll while other forms of bacteria contain bacteriochlorophyll. The chlorophyll in PSI is called P700, and the chlorophyll in PSII is called P680, for the wavelengths of light each absorbs most efficiently. Electron flow happens through two different electron transport chains that are connected; together, these electron transport chains are called the Z scheme. The compounds used to carry electrons include pheophytin (chlorophyll without the magnesium ion (Mg2+) center), quinones, cytochromes, plastocyanins (copper-containing proteins), nonheme iron sulfur proteins, ferredoxin, and flavoproteins.
- 5. Oxygenic Photosynthesis in Bacteria: In cyanobacteria (and in all phototrophic eukaryotes), there are two distinct, interconnected photosystems: photosystem I and photosystem II. Photosystem I absorbs longer wavelength of light and funnels its energy to a special reaction centre chlorophyll ‘a’ molecule called P700. The P700 signifies that this reaction centre chlorophyll ‘a’ absorbs light at a wavelength of 700 nm most effectively. Photosystem II absorbs light at shorter wavelengths and transfer its energy to the reaction centre chlorophyll molecules called P680.
- 7. Cyclic Photophosphorylation: When the PS I antenna chlorophylls funnel light energy to the reaction centre chlorophyll P700, the latter gets excited and its reduction potential becomes very negative. The excited or high-energy electron of P700 is captured by a special chlorophyll ‘a’ molecule (A) or an iron sulphur protein (FeS). The electron is eventually transferred to ferredoxin. The latter transfers electron to a cyclic route through a series of electron carriers (cytochrome b563 → plastaquinone → cytochrome b6 → cytochrome f → plastocyanin) back to oxidized P700. Since the electrons travel in a cyclic pathway (i.e. they originate from P700 and come back to the P700), the process is called cyclic photophosphorylation in which only PS I is involved. During cyclic phosphorylation, ATP is generated in the region of cytochrome b6.
- 8. Non-Cyclic Photophosphorylation In this process, both PS I and II are involved. The reduction potential of P680 chlorophyll ‘a’ molecule of PS II is very electropositive, slightly more positive than that of the H2O/O2 couple. This facilitates the first step in oxygenic electron flow, the splitting of water (photolysis) into oxygen atoms (1/2O2) and hydrogen ions (2H+). Photolysis donates an electron to the oxidized P680 molecule following the absorption of a quantum of light near 680 nm. The P680 molecule is now excited and reduces pheophytin ‘a’ which is chlorophyll ‘a’ without the magnesium atom.
- 9. Electrons subsequently travel through quinone, plastaquinone, cytochrome b6 (ATP is generated in the region of cytochrome b6), cytochrome f and plastocyanin; the latter donates electrons to photosystem I. The electron is accepted by the oxidized reaction centre chlorophyll ‘a’ of PS I (P700) which has previously absorbed light quanta and begun the steps to lead the reduction of NADP+ into NADPH.
- 11. Thank You