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4.5 explain the significance of the difference in function of photosystems I and II
explain the significance of the difference in function of photosystems I and II
- In the 1950s Emerson showed a sharp decrease in oxygen production in the red part of the spectrum at wavelengths above 700nm when observing the action spectrum of the green alga Chlorella sp. This has been called the “red drop”. When Emerson exposed the alga to wavelengths of light at both 680nm and 700nm, oxygen evolution was greater than the sum of each wavelength, the “Emerson enhancement effect”. Emerson interpreted these results to mean there were two photosystems, one using light of 700nm and the other using light of 680 nm or less.
- Photosystem I is defined as containing reaction centre chlorophylls with red light absorption at 700nm. It is not involved in O2(g) production but provides NADP+ which accepts an electron from water and the hydrogen atom split in Photosystem II. NADPH is formed which is later involved in photophosphorylation. (See later in this Option 9.9 Biochemistry: 6.Using isotopes to prove carbon dioxide is used in the light independent reaction. also 9.9 Biochemistry: 7. Structure and function of ATP.)
- Photosystem II uses reaction centres that absorb red light at 680 nm. It splits water, producing oxygen and feeds the electrons to an electron transport chain that couples photosystem II to photosystem I. Electron transfer between photosystems I and II provides electron flow and a proton gradient for the production of ATP from ADP and inorganic phosphate (Pi). The reaction occurring is
2H2O + 4 photons of light → 4H+ + 4e- + O2