These are the two types of photophosphorylation and they occur in the light-dependent stage of photosynthesis. We will start with non-cyclic photophosphorylation as this is the step we expect to see when the plant is sufficiently supplied with water and sunlight. Non-cyclic photophosphorylation involves both photosystems. Water will first be broken down into 4H+ ions and 4e- (electrons) and oxygen. This will happen in the thylakoid space - remember that the thylakoids are the ones that stack up to make the granum? The electrons produced will pass down the electron transport that stems from photosystem II first and then they will reach photosystem I. They then pass down another electron transport chain. At the end, the electrons are reduced to reduce NADP to NADPH. As the electrons pass down these chains of transport they release energy. This energy is used to pump H+ ions into the thylakoid space. The hydrogen ions then pass through the ATP synthase enzyme to form ATP from ADP. So in non-cyclic photophosphorylation, you make oxygen, from splitting the water molecule, you make ATP using the H+ ions and you make NADPH. In cyclic photophosphorylation, you only use photosystem I. There is no splitting of water - the electrons only come from the light harvesting complex. When the electrons reach the end of the chain, they return back to the start of the chain instead of helping to make NADPH as there is no NADP to accept them. They electrons passing down the chain means H+ ions are still pumped into the thylakoid membrane so we still make ATP but this is the only product of cyclic photophosphorylation. Show
The difference between cyclic and noncyclic photophosphorylation is mainly due to the following factors: Movement of electrons: During cyclic and non-cyclic photophosphorylation, the electrons move in a cyclic and straight path, respectively. Content: Cyclic Vs Noncyclic Photophosphorylation
Comparison Chart
Definition of Cyclic PhotophosphorylationIt refers to the light reaction of the photosynthetic process, which prevalently occurs in the prokaryotes or photosynthetic bacteria. Cyclic photophosphorylation only involves photosystem I (P700). It is termed cyclic because electrons return to the photosystem I after releasing the chemical energy, i.e. ATP. Thus, the electrons move to and fro within the photosystem by following a cyclic path. The organisms undergoing cyclic photosynthesis restore electron supply, due to which reduction of NADP does not occur. It does not require water hydrolysis to replenish electron loss. Definition of Noncyclic PhotophosphorylationIt refers to the light reaction of the photosynthetic process, which predominately occurs in green plants and algae. Noncyclic photophosphorylation involves both the photosystems, i.e. PS-I (P700) and PS-II (P680). It is termed noncyclic due to the zig-zag flow of electrons. The organisms undergoing noncyclic photosynthesis synthesize ATP via PS-II. Concurrently, PS-I releases electrons that the PS-I captures to reduce NADP into NADPH. It requires water hydrolysis to replenish electron loss by the PS-II. Oxygen is produced as a byproduct through the oxidation of water. Cyclic Photophosphorylation StepsIt involves photosystem-I that absorbs a photon of wavelength (725-1035 nm). Green sulfur and nonsulfur bacteria, purple bacteria, acidobacteria etc., carry out cyclic photophosphorylation. Bacteria undergo cyclic photosynthesis via four bacteriochlorophylls. Two bacteriochlorophylls participate in the photon absorption, and the other two remains inactive. Thus, the reaction centre excites electrons to the next state via photon absorption by the special pair of bacteriochlorophyll. As the high energy electrons leave the reaction centre, it makes the bacteriochlorophyll positively charged. Then, the excited electrons pass through many cofactors and protein complexes in the ETS and finally return to the final electron acceptor, i.e. photosystem-I. As photosystem II is not used during cyclic photophosphorylation, no oxygen is produced. Noncyclic Photophosphorylation StepsIt requires both PS-I and PS-II. Generally, noncyclic photophosphorylation predominantly occurs in all green plants, algae, and cyanobacteria. PS-II (P680) captures photons from the light source and later transfers them to the RC chlorophyll. The RC chlorophyll excites electrons from a ground state to a higher energy level. Then, the primary electron acceptor receives the e–. Afterwards, excited electrons travel through the cytochrome b6f complex to PS-I (P700) via ETS. During this transfer of electrons, photosynthetic cells utilize energy to transport proton molecules (H+) across the thylakoid membrane into the thylakoid lumen. Then, plastoquinone accepts protons. The movement of proton ions across the membrane creates a potential difference between the thylakoid lumen and the chloroplast stroma. The potential difference generates a proton-motive force, which a cell uses to drive free energy, i.e. ATP via an enzyme complex (ATP synthase). Photosynthetic cells use ATP to transport H+ ions from the lumen to the stroma. Then, the electrons reach PS-I through plastocyanin to fulfil the electron deficiency of the RC chlorophyll. Electrons pass through the cofactor ferredoxin to the enzyme complex NADP+ reductase. The electrons and H+ ions pass the NADP+ complex to form NADPH. Then, the NADPH (reducing agent) and ATP are transported to the Calvin cycle to fix carbon dioxide into triose sugars. Both react with glycerate 3-phosphate to form a triose sugar glyceraldehyde 3-phosphate. G 3-P is the basic building block from which plants can make a variety of substances. Noncyclic photosynthetic liberates molecular oxygen along with the energy molecules (ATP and NADPH).
Similarities
ConclusionTherefore, we can conclude that cyclic and noncyclic photophosphorylation is the light-dependent photosynthetic pathways that carry out phosphorylation to produce ATP. The ATP is then used by the photosynthetic cells to perform various activities for their growth and survival. |