C4 Photosynthesis | BioNinja
Explain the light-dependent reactions. Mark Scheme E. electron passed from photosystem II to carriers (in thylakoid membrane);. F. production of ATP in. During light dependent reaction which is occurring in thylakoid 8 Photons of light + 2H 2 O + 2NADP + + 10 H+ stroma > O2 + 2NADH + 12 H+ lumen Ih light independent reaction, the ATP and NADPH which are formed. The light-independent reactions, or dark reactions, of photosynthesis are chemical reactions These reactions take the products (ATP and NADPH) of light-dependent reactions and perform further chemical processes on them. Steps. In the first stage of the Calvin cycle, a CO2 molecule is incorporated into one of two.
When light energy is absorbed by pigments and passed inward to the reaction center, the electron in P is boosted to a very high energy level and transferred to an acceptor molecule. The special pair's missing electron is replaced by a new electron from PSII arriving via the electron transport chain. The high-energy electron travels down a short second leg of the electron transport chain. In another form of the light reactions, called cyclic photophosphorylation, electrons follow a different, circular path and only ATP no NADPH is produced.
More on cyclic photophosphorylation The process described above, in which electrons flow in a line from water to NADPHis called non-cyclic photophosphorylation. Plants also carry out another form of the light-dependent reactions called cyclic photophosphorylation, in which electrons instead cycle repeatedly through PSI and the first portion of the electron transport chain but do not pass through PSII.
You can learn more about cyclic photophosphorylation in later sections of the article. In cyclic photophosphorylation, an excited electron leaves photosystem I and travels a short distance down the second leg of the electron transport chain. It travels back down that first leg to photosystem I, where it can repeat the process with absorption of more light energy.
The light-dependent reactions
Cyclically flowing electrons generate ATP, because passage down the first leg of the electron transport chain causes protons to be pumped into the thylakoid lumen, thus establishing a gradient.
However, cyclic electron flow does not make NADPH, nor does it involve the splitting of water or production of oxygen. Sedoheptulose-1,7-bisphosphatase one of only three enzymes of the Calvin cycle that are unique to plants cleaves sedoheptulose-1,7-bisphosphate into sedoheptulosephosphatereleasing an inorganic phosphate ion into solution.
Fixation of a third CO 2 generates two more G3P. The ketose S7P has two carbons removed by transketolasegiving ribosephosphate R5Pand the two carbons remaining on transketolase are transferred to one of the G3P, giving another Xu5P. This leaves one G3P as the product of fixation of 3 CO 2, with generation of three pentoses that can be converted to Ru5P.
Xu5P is converted into RuP by phosphopentose epimerase.Photosynthesis Light reaction, Calvin cycle, Electron Transport 3D Animation
Finally, phosphoribulokinase another plant-unique enzyme of the pathway phosphorylates RuP into RuBP, ribulose-1,5-bisphosphate, completing the Calvin cycle.
This requires the input of one ATP. Thus, of six G3P produced, five are used to make three RuBP 5C molecules totaling 15 carbonswith only one G3P available for subsequent conversion to hexose.
- Light-independent reactions
The equation of the overall Calvin cycle is shown diagrammatically below. The overall equation of the Calvin cycle black circles represent carbon atoms RuBisCO also reacts competitively with O 2 instead of CO 2 in photorespiration. The rate of photorespiration is higher at high temperatures. Photorespiration turns RuBP into 3-PGA and 2-phosphoglycolate, a 2-carbon molecule that can be converted via glycolate and glyoxalate to glycine.
Serine can be converted back to 3-phosphoglycerate. Thus, only 3 of 4 carbons from two phosphoglycolates can be converted back to 3-PGA. It can be seen that photorespiration has very negative consequences for the plant, because, rather than fixing CO 2, this process leads to loss of CO 2.
Light-independent reactions - Wikipedia
C4 carbon fixation evolved to circumvent photorespiration, but can occur only in certain plants native to very warm or tropical climates—corn, for example. Each G3P molecule is composed of 3 carbons. For the Calvin cycle to continue, RuBP ribulose 1,5-bisphosphate must be regenerated. So, 5 out of 6 carbons from the 2 G3P molecules are used for this purpose. Therefore, there is only 1 net carbon produced to play with for each turn.
What's the difference between the light dependent and independent reactions?
To create 1 surplus G3P requires 3 carbons, and therefore 3 turns of the Calvin cycle. To make one glucose molecule which can be created from 2 G3P molecules would require 6 turns of the Calvin cycle.
Surplus G3P can also be used to form other carbohydrates such as starch, sucrose, and cellulose, depending on what the plant needs. Light-dependent reactions These reactions do not occur in the dark or at night. There is a light-dependent regulation of the cycle enzymes, as the third step requires reduced NADP. There are two regulation systems at work when the cycle must be turned on or off: This happens when light is available, as the ferredoxin protein is reduced in the photosystem I complex of the thylakoid electron chain when electrons are circulating through it.
What's the difference between the light dependent and independent reactions? | MyTutor
This is a dynamic process as the same bond is formed again by other proteins that deactivate the enzymes. The implications of this process are that the enzymes remain mostly activated by day and are deactivated in the dark when there is no more reduced ferredoxin available. The enzyme RuBisCo has its own, more complex activation process.