The Calvin cycle, also known as the Calvin Benson cycle, is an integral part of the process of photosynthesis in plants, algae, and photosynthetic bacteria. Named after its discoverer, Melvin Calvin of the University of California at Berkeley, its principal product is a three-carbon compound called glyceraldehyde 3-phosphate, or PGAL. Sugars are synthesized using PGAL as a starting material.
Light, absorbed by chlorophyll, is used to synthesize the high-energy compounds adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH). Chlorophyll and the enzymes that are used for synthesis of ATP and NADPH are associated with internal membranes in all photosynthetic cells.
The ATP and NADPH, once formed, are released from the membrane-bound enzymes and diffuse into the surrounding solution inside the cell. The Calvin cycle takes place in this solution, using the ATP and NADPH molecules as a source of energy to drive the conversion of carbon dioxide into PGAL.
All the steps in the Calvin cycle and sugar biosynthesis are catalyzed by specific enzyme molecules. The carbon dioxide molecules react with a five-carbon sugar-phosphate molecule called ribulose bisphosphate (RuBP) to form a six-carbon intermediate.
The reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase/oxygenase (Rubisco). The six-carbon intermediate reacts with water and decomposes into two identical three-carbon molecules called phosphoglycerate. These, in turn, react with ATP and NADPH to produce PGAL molecules. Some of these leave the Calvin cycle and are used for the formation of sugars.
The ADP and NADP molecules, produced when PGAL is formed, diffuse back to the chlorophyll containing membranes, where they can be used to regenerate a supply of ATP and NADPH for the next round of the Calvin cycle. The remaining PGAL molecules are used for the regeneration of sufficient amounts of RuBP to permit the reactions of the Calvin cycle to be repeated.
The regeneration of RuBP from PGAL involves the rearrangement of the carbon atoms in three-carbon containing molecules to form five-carbon molecules. For example, if there are ten three-carbon molecules remaining after two three-carbon molecules have been removed from the cycle, then six five-carbon molecules are synthesized by the Calvin cycle.
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.
The principal product of the Calvin cycle is not sugar but PGAL. In higher plants, the Calvin cycle takes place inside chloroplasts, and the PGAL molecules are transported across the membranes of the chloroplasts and released into the solution between the chloroplast membranes and the cell’s outer membrane.
In this solution, called the cytosol, the PGAL molecules react to form six-carbon sugar phosphates. 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 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. The amounts of other intermediates in the cycle do not change as much.
This suggests that some (but not all) steps in the Calvin cycle shut down in the dark and are activated in the light. The Calvin cycle also operates in nonphotosynthetic bacteria that use environmental chemicals as an energy source for the synthesis of ATP and other high-energymolecules.
Although these organisms are responsible only for a minor proportion of the total carbon dioxide converted to organic form every year, their existence is interesting because it suggests that the Calvin cycle evolved before the origin of photosynthesis. The starch that builds up in the chloroplasts during the day is converted to sucrose at night and is then exported from the leaf.
A significant complication must be taken into account when discussing the Calvin cycle: Oxygen can also react with RuBP, because the active site of Rubisco has affinity for both oxygen and carbon dioxide.
Under normal conditions in many higher plants, three out of ten RuBP molecules react with oxygen instead of reacting with carbon dioxide. Under conditions where carbon dioxide levels are lower than normal and oxygen levels are higher than normal, oxygen may even react more frequently than carbon dioxide.
This has deleterious consequences, because each RuBP molecule that reacts with oxygen is cleaved into two parts. One part is PGA, identical to that produced by the reaction of RuBP with carbon dioxide. The other part, however, is a two-carbon compound called phosphoglycolate.
The latter molecule subsequently is cleaved to produce carbon dioxide; only one of the two carbons in phosphoglycolate is salvaged by the cell in a complex series of reactions called photorespiration. Because the photorespiratory reactions use energy, the chlorophyll-containing membranes must produce more ATP and NADPH than would otherwise be needed for the Calvin cycle.
Evolution of the Cycle
The Calvin cycle is believed to have originated more than 3.5 billion years ago in marine bacteria that were using very simple carbon compounds as an energy source. Some of the descendants of these bacteria later acquired the ability to synthesize ATP and NADPH (or their equivalents), using light as an energy source. As long as one billion years ago, some of these photosynthetic bacteria are believed to have established mutually beneficial, or symbiotic, relationships with other cells.
These symbiotic relationships became stabilized and led to the evolution of algae and higher plants. The photosynthetic organelles of plants and algae, the chloroplasts, are thought to be the direct descendants of the symbiotic photosynthetic bacteria.
The early atmosphere of the earth probably had significantly less oxygen than it does now, so the existence of the oxygenase activity of Rubisco would not have been a problem. When the oxygen concentration in the atmosphere rose to its present level about 1.7 billion years ago, however, the losses of energy caused by the oxygenase reaction became significant.
Some organisms evolved mechanisms to prevent these losses: In algae, for example, there are molecular pumps which, in effect, concentrate carbon dioxide in the cell so that the oxygenase reaction is inhibited. In sugarcane, corn, and certain other plants that are specialized to live in hot, dry climates, a similar effect is achieved by C4 photosynthesis.
Not content with the results of evolution, biotechnologists are interested in altering Rubisco. They reason that if this enzyme can be genetically engineered to lower its oxygenase activity, the net photosynthetic rates of some plants could be improved.
Possibly a 30 percent increase in plant productivity could be expected if such strategies prove successful, provided other materials (nitrogen, phosphorus, and other nutrients) are present in sufficient supply to permit the extra growth.
Thus, although the Calvin cycle is the major route of entry of inorganic carbon into the biosphere, it is also something of a bottleneck. It remains to be seen whether the Calvin cycle can be made to function more efficiently.