New Word List Word List. Save This Word! We could talk until we're blue in the face about this quiz on words for the color "blue," but we think you should take the quiz and find out if you're a whiz at these colorful terms. Words nearby dark reaction dark mineral , dark money , dark nebula , darkness , dark of the moon , dark reaction , darkroom , dark slide , darksome , dark star , dark tourism. How to use dark reaction in a sentence The Calvin cycle is sometimes also called the dark reaction because none of its steps require light.
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Merriam-Webster's Words of the Week - Nov. Cyclic electron flow involves the recycling of electrons from ferredoxin to plastoquinone, with the result that there is no net production of NADPH; however, since protons are still transferred into the lumen by oxidation of plastoquinol by cyt b 6 f , ATP can still be formed.
Photosynthesis begins with the absorption of light by pigments molecules located in the thylakoid membrane. The most well-known of these is chlorophyll, but there are also carotenoids and, in cyanobacteria and some algae, bilins. The chemical structures of the chlorophyll and carotenoid pigments present in the thylakoid membrane. Note the presence in each of a conjugated system of carbon—carbon double bonds that is responsible for light absorption.
Light, or electromagnetic radiation, has the properties of both a wave and a stream of particles light quanta. Each quantum of light contains a discrete amount of energy that can be calculated by multiplying Planck's constant, h 6. Photons with slightly different energies colours excite each of the vibrational substates of each excited state as shown by variation in the size and colour of the arrows.
Upon excitation, the electron in the S 2 state quickly undergoes losses of energy as heat through molecular vibration and undergoes conversion into the energy of the S 1 state by a process called internal conversion. The energy of a blue photon is thus rapidly degraded to that of a red photon. Excitation of the molecule with a red photon would lead to promotion of an electron to the S 1 state directly. The energy of the excited electron in the S 1 state can have one of several fates: it could return to the ground state S 0 by emission of the energy as a photon of light fluorescence , or it could be lost as heat due to internal conversion between S 1 and S 0.
Alternatively, if another chlorophyll is nearby, a process known as excitation energy transfer EET can result in the non-radiative exchange of energy between the two molecules Figure 9. Two chlorophyll molecules with resonant S 1 states undergo a mirror transition resulting in the non-radiative transfer of excitation energy between them.
In photosynthetic systems, chlorophylls and carotenoids are found attached to membrane-embedded proteins known as light-harvesting complexes LHCs. Through careful binding and orientation of the pigment molecules, absorbed energy can be transferred among them by EET. A photosystem consists of numerous LHCs that form an antenna of hundreds of pigment molecules.
Light energy is captured by the antenna pigments and transferred to the special pair of RC chlorophylls which undergo a redox reaction leading to reduction of an acceptor molecule. The oxidized special pair is regenerated by an electron donor. It is worth asking why photosynthetic organisms bother to have a large antenna of pigments serving an RC rather than more numerous RCs.
The answer lies in the fact that the special pair of chlorophylls alone have a rather small spatial and spectral cross-section, meaning that there is a limit to the amount of light they can efficiently absorb.
The amount of light they can practically absorb is around two orders of magnitude smaller than their maximum possible turnover rate, Thus LHCs act to increase the spatial hundreds of pigments and spectral several types of pigments with different light absorption characteristics cross-section of the RC special pair ensuring that its turnover rate runs much closer to capacity.
PSII is a light-driven water—plastoquinone oxidoreductase and is the only enzyme in Nature that is capable of performing the difficult chemistry of splitting water into protons, electrons and oxygen Figure PSII uses light energy to excite a special pair of chlorophylls, known as P due to their nm absorption peak in the red part of the spectrum.
Nonetheless, since water splitting involves four electron chemistry and charge separation only involves transfer of one electron, four separate charge separations turnovers of PSII are required to drive formation of one molecule of O 2 from two molecules of water. Progressive extraction of electrons from the manganese cluster is driven by the oxidation of P within PSII by light and is known as the S-state cycle Figure After the fourth turnover of P, sufficient positive charge is built up in the manganese cluster to permit the splitting of water into electrons, which regenerate the original state of the manganese cluster, protons, which are released into the lumen and contribute to the proton gradient used for ATP synthesis, and the by-product O 2.
Thus charge separation at P provides the thermodynamic driving force, whereas the manganese cluster acts as a catalyst for the water-splitting reaction. The organization of PSII and its light-harvesting antenna. Protein is shown in grey, with chlorophylls in green and carotenoids in orange. Progressive extraction of electrons from the manganese cluster is driven by the oxidation of P within PSII by light. Each of the electrons given up by the cluster is eventually repaid at the S 4 to S 0 transition when molecular oxygen O 2 is formed.
The protons extracted from water during the process are deposited into the lumen and contribute to the protonmotive force. Plastoquinone reduction to plastoquinol requires two electrons and thus two molecules of plastoquinol are formed per O 2 molecule evolved by PSII. Two protons are also taken up upon formation of plastoquinol and these are derived from the stroma. PSI is a light-driven plastocyanin—ferredoxin oxidoreductase Figure In PSI, the special pair of chlorophylls are known as P due to their nm absorption peak in the red part of the spectrum.
The organization of PSI and its light-harvesting antenna. Plastoquinone is a small lipophilic electron carrier molecule that resides within the thylakoid membrane and carries two electrons and two protons from PSII to the cyt b 6 f complex.
It has a very similar structure to that of the molecule ubiquinone coenzyme Q 10 in the mitochondrial inner membrane. The cyt b 6 f complex is a plastoquinol—plastocyanin oxidoreductase and possess a similar structure to that of the cytochrome bc 1 complex complex III in mitochondria Figure 14 A. As with Complex III, cyt b 6 f exists as a dimer in the membrane and carries out both the oxidation and reduction of quinones via the so-called Q-cycle. The Q-cycle Figure 14 B involves oxidation of one plastoquinol molecule at the Qp site of the complex, both protons from this molecule are deposited in the lumen and contribute to the proton gradient for ATP synthesis.
The two electrons, however, have different fates. The first is transferred via an iron—sulfur cluster and a haem cofactor to the soluble electron carrier plastocyanin see below.
The second electron derived from plastoquinol is passed via two separate haem cofactors to another molecule of plastoquinone bound to a separate site Qn on the complex, thus reducing it to a semiquinone. When a second plastoquinol molecule is oxidized at Qp, a second molecule of plastocyanin is reduced and two further protons are deposited in the lumen.
The second electron reduces the semiquinone at the Qn site which, concomitant with uptake of two protons from the stroma, causes its reduction to plastoquinol. Thus for each pair of plastoquinol molecules oxidized by the complex, one is regenerated, yet all four protons are deposited into the lumen. The Q-cycle thus doubles the number of protons transferred from the stroma to the lumen per plastoquinol molecule oxidized.
B The protonmotive Q-cycle showing how electrons from plastoquinol are passed to both plastocyanin and plastoquinone, doubling the protons deposited in the lumen for every plastoquinol molecule oxidized by the complex. Plastocyanin is a small soluble electron carrier protein that resides in the thylakoid lumen. Ferredoxin is a small soluble electron carrier protein that resides in the chloroplast stroma.
The FNR complex is found in both soluble and thylakoid membrane-bound forms. According to the structure, 4. The enzyme is a rotary motor which contains two domains: the membrane-spanning F O portion which conducts protons from the lumen to the stroma, and the F 1 catalytic domain that couples this exergonic proton movement to ATP synthesis.
The cyt b 6 f complex, in contrast, is evenly distributed throughout the grana and stromal lamellae. Another possible advantage of membrane stacking in thylakoids may be the segregation of the linear and cyclic electron transfer pathways, which might otherwise compete to reduce plastoquinone. The cyclic electron transfer pathway recycles electrons from ferredoxin back to plastoquinone and thus allows protonmotive force generation and ATP synthesis without net NADPH production. Cyclic electron transfer thereby provides the additional ATP required for the Calvin—Benson cycle see below.
A Electron micrograph of the thylakoid membrane showing stacked grana and unstacked stromal lamellae regions. B Model showing the distribution of the major complexes of photosynthetic electron and proton transfer between the stacked grana and unstacked stromal lamellae regions. The reaction forms an unstable 6C intermediate that immediately splits into two molecules of 3-phosphoglycerate.
For every three CO 2 molecules initially combined with ribulose 1,5-bisphopshate, six molecules of GAP are produced by the subsequent steps. However only one of these six molecules can be considered as a product of the Calvin—Benson cycle since the remaining five are required to regenerate ribulose 1,5-bisphosphate in a complex series of reactions that also require ATP. The one molecule of GAP that is produced for each turn of the cycle can be quickly converted by a range of metabolic pathways into amino acids, lipids or sugars such as glucose.
Glucose in turn may be stored as the polymer starch as large granules within chloroplasts. Overview of the biochemical pathway for the fixation of CO 2 into carbohydrate in plants. The fructose 1,6-bisphosphate is then dephosphorylated by fructose-1,6-bisphosphatase to yield fructose 6-phosphate 6C and releasing P i. Two carbons are then removed from fructose 6-phosphate by transketolase, generating erythrose 4-phosphate 4C ; the two carbons are transferred to another molecule of GAP generating xylulose 5-phosphate 5C.
Another DHAP molecule, formed from GAP by triose phosphate isomerase is then combined with the erythrose 4-phosphate by aldolase to form sedoheptulose 1,7-bisphosphate 7C.
Sedoheptulose 1,7-bisphosphate is then dephosphorylated to sedoheptulose 7-phosphate 7C by sedoheptulose-1,7-bisphosphatase releasing P i. Sedoheptulose 7-phosphate has two carbons removed by transketolase to produce ribose 5-phosphate 5C and the two carbons are transferred to another GAP molecule producing another xylulose 5-phosphate 5C.
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