In the carbon fixation stage, carbon dioxide is attached to RuBP by the enzyme rubisco. The resulting 6-carbon product quickly splits into two molecules of a. Jan 20, Rubisco catalysis the connection of the small molecule ribulose bisphosphate phosphate (RuBP) to carbon dioxide - therefore fixing the. The Calvin cycle is the term used for the reactions of photosynthesis that use the RuBisCO catalyzes a reaction between CO2 and RuBP, which forms a.
The Calvin cycle
Unlike the light reactions, which take place in the thylakoid membrane, the reactions of the Calvin cycle take place in the stroma the inner space of chloroplasts. Here is a general diagram of the cycle: Diagram of the Calvin cycle, illustrating how the fixation of three carbon dioxide molecules allows one net G3P molecule to be produced that is, allows one G3P molecule to leave the cycle.
This reaction is catalyzed by the enzyme rubisco. One G3P molecule leaves the cycle and will go towards making glucose, while five G3Ps must be recycled to regenerate the RuBP acceptor. Regeneration involves a complex series of reactions and requires ATP. Fructose-1,6-bisphosphate phosphatase hydrates the substrate to remove a phosphate and produce fructosephosphate. We now have a six-carbon sugar-phosphate. Transketolase now combines the fructosephosphate with another PGAL in a reaction involving 9 carbon atoms.
These fall apart as a molecule of erythrosephosphate a four-carbon sugar-phosphate and a molecule of xylulosephosphate a five-carbon sugar-phosphate. We will keep the xylulosephosphate in mind for later--it is ready for the last steps in regeneration.
But we will focus more on the erythrosephosphate instead for now. Aldolase combines erythrosephosphate with a dihydroxyacetonephosphate to make sedoheptulose-1,7-bisphosphate.
If you are counting carbons correctly, this product has seven carbon atoms and two phosphate groups. Sedoheptulose-1,7-bisphosphate phosphatase hydrates the seven-carbon sugar-phosphate and removes one of its phosphates. This leaves us with sedoheptulosephosphate Transketolase combines sedoheptulosephosphate with another PGAL to assemble 10 carbons total.
These fall apart as a xylulosephosphate and a ribosephosphate We will also recall that we have another xylulosephosphate made earlier. Ribulosephosphate epimerase will convert xylulosephosphate produced from both previous steps into ribulosephosphate. This enzyme just reorganizes hydroxyl positions in space to convert the 5-carbon sugar-phosphate isomer.
Meanwhile, ribosephosphate isomerase will linearize the ribosephosphate molecule from an earlier step to make an additional ribulosephosphate.
Plant Life: Calvin cycle
We now have a total of three ribulosephosphate molecules. These are phosphorylated with ATP from the light reactions by ribulosephosphate kinase to make the three ribulose-1,5-bisphosphates needed to regnerate enough Calvin cycle activity to explain the production of one PGAL for the plant cell to use!
The Calvin cycle is a source of complex carbohydrates Of course, the number of cycles through the Calvin cycle will be much higher than just three.
This process is ongoing and iterating repeatedly under good lighting conditions. Because of this, there are small pools of all the intermediates named and diagrammed in the figures on this page!
Because these small pools of intermediates are around at any given moment, any of these molecules may be siphoned off for biochemical use elsewhere. In our regeneration diagram above, you will notice ribosephosphate the sugar-phosphate forming the backbone of RNA. It can be deoxygenated to make deoxyribosephosphate for DNA. You also see sugar-phosphates of 3, 4, 5, 6, and 7 carbons that can form the organic skeletons of a range of secondary molecules.
Some of these will be critical in the polymers for cell walls and pectins. Others are for lignin and so on. So anyone who tells you that only glucose is made by photosynthesis is grossly oversimplifying what plants can produce through the Calvin cycle. We often think of photosynthesis as the source of sucrose and polysaccharides. But technically these products are NOT made by the Calvin cycle.
In fact the transport sugar, sucrose, is not even synthesized in the chloroplast. Yet most of the photosynthate is arguably used to make sucrose and starch. Starch can be synthesized within the chloroplast The triose phosphates from the Calvin cycle can be combined into fructose-1,6-bisphosphate and fructosephopshate in portions of the regeneration phase.
The fructosephosphate can be converted by the enzyme, hexose phosphate isomerase into glucosephosphate. This, in turn, can be converted to glucosephosphate by the enzyme, phosphoglucomutase. This is accomplished by no less than nine separate enzyme-catalyzed steps, involving intermediate compounds containing two, three, four, five, or seven carbon atoms derived from the PGAL molecules.
At the end of this complex process, ATP is used to add another phosphate group to each five-carbon molecule, thus regenerating the required amount of RuBP, the original organic starting material for the cycle.
A continuous supply of phosphate must be made available in order to continue running the Calvin cycle. Ultimately, all this phosphate must be supplied to the organism from the environment in the short term, however, it is gleaned from other biochemical reactions, such as sugar biosynthesis. These six-carbon sugar phosphates then react to form sucrose, a twelve-carbon molecule ordinary table sugar.
A sucrose molecule consists of one molecule each of the six-carbon sugars fructose and glucose. Phosphate is released from the sugar phosphates during the formation of sucrose. The phosphate can then be returned to the chloroplast, where it is needed for the formation of ATP. Most of the sucrose is transported out of the cell and flows to various parts of the plant, such as the fruits or the roots.
The Calvin cycle (article) | Photosynthesis | Khan Academy
The transport of sucrose out of the cell requires energy derived from ATP. The accumulation of sucrose in the water outside the cell causes the hydrostatic pressure to rise. This pressure drives the flow of the water and sucrose sap through the phloem away from the leaves and toward the fruit or roots.
The accumulation of sucrose in the fruit accounts for a large part of the nutritional value of plants. When conditions do not favor the formation of sucrose, the triose phosphates may remain inside the chloroplast.
These can react to form six-carbon sugar bisphosphates that, in turn, can react in several steps to form an insoluble carbohydrate storage compound called starch.
The conversion of six-carbon sugar bisphosphate into starch releases phosphate.
The phosphate released can then participate in the synthesis of more ATP, permitting continued operation of the Calvin cycle even when sucrose is not being formed. The accumulation of starch is another major source of nutritional value in plants. In the morning, when plants begin receiving light, the amounts of phosphoglycerate and six carbon sugar phosphates increase dramatically.