The Calvin cycle is the term used for the reactions of photosynthesis that use the energy stored by the light-dependent reactions to form glucose and other carbohydrate molecules. Oxygenic photosynthesis is composed of two stages: the light-dependent reactions and the light-independent reactions. The C4 mechanism, also called the Hatch-Slack pathway, utilizes two sets of cells, an outer layer mesophyll that takes in air and fixes the CO 2 to PEP and produces malate, and an inner layer of cells bundle sheath that takes the malate, and decarboxylates it for its rubisco enzyme.
The two cells are connected via plasmodesmata see Cell-cell Interactions chapter. Although energetically more expensive than carbon fixation by C3 plants in cooler climates, the C4 pathway overtakes C3 in efficiency as temperatures rise and photorespiration increases.
Variations of this pathway have been found in which aspartate is transported to the bundle-sheath cells instead of malate. After decarboxylation of the malate by malic enzyme NAD-dependent in some species, NADP-dependent in others to release the CO 2 for rubisco, the resulting pyruvate is shuttled back to the mesophyll cell where it is phosphorylated by pyruvate-phosphate dikinase to generate PEP for re-entry into the C4 cycle. Desert plants go one step further than C4 plants.
Living in environments that are extremely hot and dry during the day, but relatively cool at night, many desert succulents like cacti are diurnal, and only open their stomata at night when temperatures are significantly lower and water evaporates far more slowly for CO 2 gathering, which is then fixed via the CAM pathway to malate.
Then in the daylight hours, CO 2 is released from the malate and used in the Calvin cycle to generate carbohydrates. Keep a tally of how many carbon atoms enter the cycle, and how many were added via fixation, how many.
The Calvin cycle regenerates RuBP to begin the next cycle. For every three carbon dioxide molecules that are fixed, three molecules of RuBP were needed. Thus, at the end of the cycle there must be three molecules of RuBP or the cycle would get out of balance. The three molecules of RuBP that began the cycle had a total of three carbons multiplied by five molecules, or 15 atoms of carbon. Three molecules of carbon were then fixed for a surplus of three carbons in the cycle.
Those three carbons are expelled from the cycle as one molecule of G3P. The remaining 15 carbons are still in the form of G3P. Therefore, they must be converted back to RuBP to start the process over. More ATP, as well as many steps involving enzymes, are necessary to do this regeneration. Consider the different enzymes, reactants, reducing agents, and sources of energy involved in the Calvin Cycle.
Draw the Calvin Cycle including all enzymes, reactants, etc. Your browser may not support display of this image. Why is the Calvin Cycle also called the dark reactions of photosynthesis? Do you think that this is misleading?
The Calvin Cycle is also called the dark reaction because it can occur in the absence of light, that is, without the use of photons or pigments. However, this name could be misleading because the Calvin Cycle can occur in the presence of light.
It does not need to occur in the dark. What is the global carbon cycle? How have humans allegedly put the global carbon cycle out of balance? Carbon dioxide is constantly being fixed into sugars and other macromolecules which, in turn, are oxidized back into CO2. This relationship, on a global scale, is termed the carbon cycle. However, humans are burning fossil fuels at a faster rate than plants can fix them back into sugars and other carbon molecules, so many scientists claim that the global carbon cycle is out of balance.
What are two views on global warming? Do these views necessarily oppose one another? Some scientists believe that humans are burning fossil fuels at a faster rate than plants can fix them back into sugars and other carbon molecules, so humans have caused the global carbon cycle to become out of balance. Other scientists think that the current rise in CO2 levels is part of a natural cycle i. Thus, there are cycles within cycles, each interacting with and affecting the others. It is possible that both of these processes are occurring at the same time.
The viewpoints do not necessarily conflict. Cram has partnered with the National Tutoring Association Claim your access. Highlight Links Highlight Links. Change Contrast Change Contrast. Top Stories. CURE at MSU: Bringing the laboratory experience to undergraduate classrooms Researchers are integrating their work into undergraduate cell and molecular biology laboratory courses at Michigan State University through the use of Arabidopsis mutant screenings.
Recently discovered protein enhances understanding of photosynthesis MSU-DOE Plant Research Laboratory PRL scientists have published a new study that furthers our understanding of how plants make membranes in chloroplasts, the photosynthesis powerhouse.
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