Phosphoglycerate kinase phosphorylates 3-PG to 1,3-bisphosphoglycerate. Under conditions of low CO 2 and high O 2 in the local atmosphere, oxygen, instead of carbon dioxide, binds to rubisco and forms 3-PG and 2-phosphoglycolate from its reaction with RuBP.
From there, it undergoes a series of reactions in the peroxisomes and mitochondria to transform it to 3-PG, which can then go in the chloroplast and participate in the Calvin cycle. Unfortunately for the cell, in the course of these reactions, NADH and ATP are used, thus lowering the energy availability inside the cell. This is a particular problem in hot climates, because the oxygenase activity of rubisco increases more than the carboxylase activity as the temperature increases.
This leads to an interesting side effect: in C3 plants, as the temperature rises and CO 2 is outcompeted by O 2 for rubisco binding, the stomata of the leaves need to remain open for longer in order to allow for acquisition of enough CO 2 from the atmosphere. This in turn allows more water vapor from inside the cell to escape, leading to dehydration.
C3 plants are thus at a competitive disadvantage in hot dry climates in comparison to plants that do not use rubisco for carbon fixation.
Both of these molecules return to the nearby light-dependent reactions to be reused and reenergized. At this point, only one of the G3P molecules leaves the Calvin cycle and is sent to the cytoplasm to contribute to the formation of other compounds needed by the plant. But each turn makes two G3Ps, thus three turns make six G3Ps. One is exported while the remaining five G3P molecules remain in the cycle and are used to regenerate RuBP, which enables the system to prepare for more CO 2 to be fixed.
Three more molecules of ATP are used in these regeneration reactions. Learning Objectives Describe the Calvin Cycle. The Rights Holder for media is the person or group credited.
Tim Gunther, Illustrator. For information on user permissions, please read our Terms of Service. If you have questions about how to cite anything on our website in your project or classroom presentation, please contact your teacher. They will best know the preferred format. When you reach out to them, you will need the page title, URL, and the date you accessed the resource.
If a media asset is downloadable, a download button appears in the corner of the media viewer. If no button appears, you cannot download or save the media. Text on this page is printable and can be used according to our Terms of Service. Any interactives on this page can only be played while you are visiting our website. You cannot download interactives. Trophic levels provide a structure for understanding food chains and how energy flows through an ecosystem. At the base of the pyramid are the producers, who use photosynthesis or chemosynthesis to make their own food.
Herbivores or primary consumers, make up the second level. This type of reaction is called a reduction reaction, because it involves the gain of electrons. A reduction is the gain of an electron by an atom or molecule. One of the G3P molecules leaves the Calvin cycle to contribute to the formation of the carbohydrate molecule, which is commonly glucose C 6 H 12 O 6. Because the carbohydrate molecule has six carbon atoms, it takes six turns of the Calvin cycle to make one carbohydrate molecule one for each carbon dioxide molecule fixed.
The remaining G3P molecules regenerate RuBP, which enables the system to prepare for the carbon-fixation step. Figure 2. The Calvin cycle has three stages. In stage 1, the enzyme RuBisCO incorporates carbon dioxide into an organic molecule. In stage 2, the organic molecule is reduced. In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue.
In summary, it takes six turns of the Calvin cycle to fix six carbon atoms from CO 2. Check out this animation of the Calvin cycle. Figure 3. Living in the harsh conditions of the desert has led plants like this cactus to evolve variations in reactions outside the Calvin cycle. These variations increase efficiency and help conserve water and energy.
The shared evolutionary history of all photosynthetic organisms is conspicuous, as the basic process has changed little over eras of time. Even between the giant tropical leaves in the rainforest and tiny cyanobacteria, the process and components of photosynthesis that use water as an electron donor remain largely the same. Photosystems function to absorb light and use electron transport chains to convert energy. The Calvin cycle reactions assemble carbohydrate molecules with this energy.
However, as with all biochemical pathways, a variety of conditions leads to varied adaptations that affect the basic pattern.
0コメント