Cyclic and Noncyclic Photophosphorylation

Cyclic and noncyclic photophosphorylation: In chloroplasts, phosphorylation occurs in two ways – noncyclic photophosphorylation and cyclic photophosphorylation. Noncyclic photophosphorylation: When the molecules in the PS I are excited the electrons are released. So, an electron deficiency or a hole is made in the PS I. This electron is now transferred to ferredoxin to reduce NADP+. When the molecules in the PS II get excited, electrons are released. They are transferred to fill the hole in PS I through plastoquinone, cytochrome b6, cytochrome f and plastocyanin. When the electron is transported between plastoquinone and cytochrome f, ADP is phosphorylated to ATP. The ‘hole’ in the PS I has been filled by the electron from PS II. Then the electrons are transferred from PS I to NADP+ for reduction. Therefore, this electron transport is called noncyclic electron transport and the accompanying phosphorylation as noncyclic photophosphorylation. The noncyclic electron transport takes place in the form of ‘Z’. Hence, it is also called Z-scheme. Cyclic photophosphorylation : Under the conditions of (i) PS I only remains active (ii) photolysis of water does not take place (iii) requirement of ATP is more and (iv) nonavailability of NADP+ the cyclic photophosphorylation takes place. When the molecule in the PS I is excited, the electrons are released. The electrons are captured by ferredoxin through ferredoxin reducing substrate (FRS). Due to non-availability of NADP+, electrons from ferredoxin fall back to the molecules of PS I through the electron carriers - cytochrome b6, cytochrome f and plastocyanin. These electron carriers facilitate the down hill transport of electrons from FRS to PS I. During this transport of electrons, two phosphorylations take place - one between ferredoxin and cytochrome b6 and the other between cytochrome b6 and cytochrome f. Thus, two ATP molecules are produced in this cycle. Difference between cyclic and noncyclic electron transport and photophosphorylation: Cyclic photophosphorylation: 1. It is associated with PS I 2. The electron expelled from chlorophyll molecule is cycled back 3. Photolysis of water and evolution of oxygen do not take place. 4. Photophosphorylation takes place at two places. 5. NADP is not reduced. Noncyclic photophosphorylation: 1. It is associated with both PS I and PS II. 2. The electrons are not cycled back but compensated by the electrons from photolysis of water. 3. Photolysis of water and evolution of oxygen take place. 4. Photophosphorylation takes place only at one place. 5. NADP+ is reduced to NADPH2. carbondioxide in plants during photosynthesis occurs in three stages – fixation, reduction and regeneration of RuBP. Disclaimer This channel does not promote or encourage any illegal activities. All contents provided by this channel for general and education purpose only. Copyright disclaimer under section 107 of the copyright act 1976,allowance is made for "fair use policy" for purposes such as criticism, comment, news reporting,teaching,scholarship and research. Fair use is a use permitted by copyright statute that might otherwise be infringing. Non-profit, educational or personal use tips the balance in favor of fair use.