The molecules with a low relative potential are considered to be strong oxidizing agents, and those with a high relative potential are considered to be strong reducing agents. It is now known that light reaction I can use light of a slightly longer wavelength than red (λ = 680 nm), while light reaction II requires light with a wavelength of 680 nm or shorter. From photosystem II, the excited electron travels along a series of proteins. Photosystem II The light absorption processes associated with photosynthesis take place in large protein complexes known as photosystems. The energized electrons are replaced by oxidizing water to form hydrogen ions and molecular oxygen. Where do the electrons entering photosystem II come from? The general features of a widely accepted mechanism for photoelectron transfer, in which two light reactions (light reaction I and light reaction II) occur during the transfer of electrons from water to carbon dioxide, were proposed by Robert Hill and Fay Bendall in 1960. As the electron passes along these proteins, energy from the electron fuels membrane pumps that actively move hydrogen ions against their concentration gradient from the stroma into the thylakoid space. ATP is produced by other electron acceptors in the electron transport chain. a) electron transport chain b) light c) water d) chlorophyll molecules e) ATP. c) the light reactions by linear electron flow. a) stroma b) thylakoids c) granum d) stomata e) mitochorndrion. a) the calvin cycle incorporates each CO2 molecule, one at a time, by attaching it to a 5-carbon sugar (RuBP). Where do the electrons entering photosystem II come from? Which of the following processes occurs during the second phase, the reduction phase, of the Calvin cycle? B. The light reactions by linear electrons flow see more 10. G3P is used in which of the following processes? Photosystem II is the first step of photosynthesis, where the chlorophyll molecule uses light energy to take an electron from a water molecule. The one known as Photosystem II contains the same kind of chlorophyll a as Photosystem I but in a different protein environment with an absorption peak at 680 nm. ¡ They come from chemical reactions with splitting water. The source of the oxygen produced by photosynthesis has been identified through experiments using radioactive tracers. Metabolic pathways are typically redox processes. c) ATP and NADPH produced in the light reactions provide the energy for the production of sugars in the Calvin Cycle. Order now and Get 10% Discount! In light reaction II, the first acceptor may be pheophytin, which is a molecule similar to chlorophyll that also has a strong reducing potential and quickly transfers electrons to the next acceptor. The most important role of pigments in photosynthesis is to __________. These electrons are used in several ways. Each time a P680 or P700 molecule gives up an electron, it returns to its ground (unexcited) state, but with a positive charge due to the loss of the electron. Movement of H+ through a membrane. Why? Light energy (indicated by wavy arrows) absorbed by photosystem II causes the formation of high-energy electrons, which are transferred along a series of acceptor molecules in an electron transport chain to photosystem I. Photosystem II obtains replacement electrons from water molecules, resulting in their split into hydrogen ions (H+) and oxygen atoms. Where do the electrons entering photosystem II come from? 6. chain. (Concept 10.1) Your Answer: It helps produce ATP from the light reactions.Correct Answer: It forms NADPH to be used in the Calvin cycle. During photosynthesis in chloroplasts, O 2 is produced from _____ via a series of reactions associated with: Term. H2O and photosystem II: Definition. This process is called cyclic electron flow. Treatment of lamellar fragments with neutral detergents releases these particles, designated photosystem I and photosystem II, respectively. C) water. In light reaction I, P700+ recovers electrons from plastocyanin, which in turn receives them from intermediate carriers, including the plastoquinone pool and cytochrome b and cytochrome f molecules. The light reactions of photosynthesis supply the Calvin cycle with __________. After the photon hits, photosystem II transfers the free electron to the first in a series of proteins inside the thylakoid membrane called the electron transport chain. The light reactions also produce __________ and __________. Water 9. Since two electrons were removed from P700, they have to be replaced. Oxygen evolution was measured in each case. What structure is formed by the reaction center, light-harvesting complexes, and primary electron acceptors that cluster, and is located in … As discussed previously, lamellae can be disrupted mechanically into fragments that absorb light energy and break the bonds of water molecules (i.e., oxidize water) to produce oxygen, hydrogen ions, and electrons. The primary function of the photosystem I is in NADPH synthesis, where it receives the electrons from PS II. There is good evidence that two or more manganese atoms complexed with protein are involved in this catalysis, taking four electrons from two water molecules (with release of four hydrogen ions). These first acceptors also are strong reducing agents and rapidly pass electrons to more stable carriers. 25. During the Calvin cycle, what happens during the carbon fixation phase? Water. _____ _____. Flow of electrons during the light reaction stage of photosynthesis. (It is designated P680). c) water. What is the role of NADP + in photosynthesis? During which process is molecular oxygen produced in photosynthesis? Just like photosystem I, photosystem II absorbs the energy of photon and that energy is transferred around the chlorophyll molecules until it … But it was too late, the name stuck. The light reactions of photosynthesis generate high-energy electrons, which end up in __________. 11. Photosystem II occurs with two series of enzymes followed by Photosystem I in order to create energy for a plant1. a. Many lines of evidence support the concept of electron flow via two light reactions. Use Code "Newclient" Carbon dioxide and oxygen enter and exit a leaf by diffusion. The reactions of the Calvin cycle are not directly dependent on light, but they usually do not occur at night. 8. ¡ The photosystems are named for the dates of discovery, not for which one goes first or second. b) they are the ultimate sources of organic compounds for all nonautotrophic. The Calvin cycle could not occur without the light reactions. Photosystem II is the first link in the chain of photosynthesis. The pool of intermediate carriers may receive electrons from water via light reaction II and the quinones. c) water is oxidized and carbon dioxide is reduced. A pigment molecule in photosystem I accepts the electron. Where do the electrons entering photosystem II come from? With both red and blue light together, the amount of oxygen evolved far exceeded the sum of that seen with blue and red light alone. water 18 During photosynthesis in chloroplasts, O2 is produced from _____ via a series of reactions associated with _____. Special quinones are next in the series. Electrons first travel through photosystem II and then photosystem I. Where does the Calvin cycle of photosynthesis occur? The soluble molecule plastoquinone, or PQ, acts as the electron carrier from photosystem II to the cytochrome b6f complex. water. This electron transport system uses the energy from the electron to pump hydrogen ions into the interior of the thylakoid. H2O... photosystem II~When photosystem II absorbs light, the electron that is excited to a higher energy level in the special chlorophyll leaves a "hole" that is filled when an enzyme extracts electrons from the hydrogen of water, releasing oxygen. During which process is molecular oxygen produced in photosynthesis? Where do the electrons entering photosystem II come from? When treated with certain detergents, lamellae can be broken down into smaller particles capable of carrying out single light reactions. The excited chlorophyll electrons are transferred from the electron acceptors of photosystem II to photosystem I through an electron transport chain. Where do the electrons entering photosystem II come from? The _____ is given off as waste. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Of the following, which occurs during the Calvin cycle? A) the electron carrier, plastocyanin. This mechanism is based on the relative potential (in volts) of various cofactors of the electron-transfer chain to be oxidized or reduced. a. The primary function of the photosystem II is in the hydrolysis of water and ATP synthesis. Which of the following statements correctly describes the difference in carbon fixation between C3 and C4 plants? The energy used to produce ATP in the light reactions of photosynthesis comes from __________. The electrons of photosystem II are excited and transferred to electron carriers. b) the Calvin cycle requires products only produced when the photosystems are illuminated. A photosystem consists of which of the following structures? The P 680 + of light reaction II is capable of taking electrons from water in the presence of appropriate catalysts. Where does the Calvin cycle of photosynthesis occur? Why does Photosystem II occur before Photosystem I??? Molecules that in their oxidized form have the strongest affinity for electrons (i.e., are strong oxidizing agents) have a low relative potential. Where do the electrons entering photosystem II come from? In light reaction I, electrons are passed on to iron-sulfur proteins in the lamellar membrane, after which the electrons flow to ferredoxin, a small water-soluble iron-sulfur protein. Which of the following statements correctly describes the relationship between the light reactions and the Calvin cycle? the light reactions of photosynthesis. B) photosystem I. 11. By signing up for this email, you are agreeing to news, offers, and information from Encyclopaedia Britannica. Generating an Energy Carrier: ATP Which of the following statements describes why this is the case? The light reactions of photosynthesis use chemiosmosis to produce ATP that will be used in the Calvin cycle. Where do the electrons entering photosystem II come from? This splits the water molecule, generating oxygen and hydrogen ions. It was substantial with blue light alone but not with red light alone. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. Photosystem I was discovered first. Molecular oxygen is produced during. In the presence of electron donors, such as a reduced dye, a second type of lamellar particle can absorb light and transfer electrons from the electron donor to ferredoxin (light reaction I). The Electron Transport Chain. transport electrons from photosystem II to photosystem I: Definition. Arrows pointing upward represent light reactions that increase the chemical potential, and arrows slanting downward represent the flow of electrons via carriers in the membrane. Transfer of electrons from water to ferredoxin via the two light reactions and intermediate carriers is called noncyclic electron flow. How is photosynthesis similar in C 4 plants and CAM plants? There is good evidence that two or more manganese atoms complexed with protein are involved in this catalysis, taking four electrons from two water molecules (with release of … During photosynthesis in a eukaryotic cell, an electrochemical gradient is formed across the _____.-thylakoid membrane 7. All of the listed structures are parts of a photosystem. One type of particle can absorb light energy, oxidize water, and produce oxygen (light reaction II), but a special dye molecule must be supplied to accept the electrons. The manganese-protein complex gives up these electrons one at a time via an unidentified carrier to P680+, reducing it to P680. Alternatively, electrons may be transferred only by light reaction I, in which case they are recycled from ferredoxin back to the intermediate carriers. The light reactions produce carbon dioxide, ATP, and NADPH, all of which are used in the Calvin cycle. Later, photosystem II was discovered and found to be earlier in the electron transport chain. While at photosystem II and I, the electrons gather energy from sunlight. The subsequent addition of certain reduced dyes (i.e., electron donors) restores the light reduction of NADP+ but without oxygen production, suggesting that light reaction I but not light reaction II is functioning. No. It is now known that DCMU blocks the transfer of electrons between the first quinone and the plastoquinone pool in light reaction II. The P680+ of light reaction II is capable of taking electrons from water in the presence of appropriate catalysts. Within the photosystem, enzymes capture photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce plastoquinone to plastoquinol. These molecules are similar to plastoquinone; they receive electrons from pheophytin and pass them to the intermediate electron carriers, which include the plastoquinone pool and the cytochromes b and f associated in a complex with an iron-sulfur protein. water 18 During photosynthesis in chloroplasts, O2 is produced from _____ via a series of reactions associated with _____. The electron transport chain provides energy to turn _____ into _____ ... Where do the electrons that enter Photosystem II come from? When photosystem II absorbs light, electrons in the reaction-center chlorophyll are excited to a higher energy level and are trapped by the primary electron acceptors. The electrochemical gradient that drives this chemiosmosis is formed across which structure(s)? During photosynthesis, an electron transport chain is used to _____. "Looking for a Similar Assignment? These experimental data pointed to the existence of two types of light reactions that, when operating in tandem, would yield the highest rate of oxygen evolution. a. It captures photons and uses the energy to extract electrons from water molecules. Photoexcited electrons travel through the cytochrome b6f complex to photosystem I via an electron transport chain set in the thylakoid membrane . Which structure(s) on a leaf allow(s) this process to happen. The electrons and hydrogen ions are used to power the creation of ATP, and ultimately carbohydrates, in later stages of photosynthesis. 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