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The Secret of the Dark Reaction: Why the Calvin Cycle Has Nothing to Do with Darkness?
The Calvin cycle, often called the "dark reaction", actually has nothing to do with darkness. Instead, it occurs in the presence of light and relies on the energy products of the photosynthesis process. This complex series of chemical reactions converts carbon dioxide and hydrogen-carrying compounds into plant-usable glucose, primarily in the chloroplast stroma of plant cells.
Although the name contains "dark", the reactions of the Calvin cycle actually require light-dependent products such as ATP and NADPH.
How does the Calvin cycle work? It is divided into three main stages: carbonation, reduction reaction, and ribulose-1,5-bisphosphate (RuBP) regeneration. During this process, carbon dioxide combines with the five-carbon RuBP to form a stable three-carbon compound. In this reaction, the RuBisCO enzyme plays a vital role, and the efficiency of its catalytic reaction directly affects the progress of the entire cycle.
The first step in the Calvin cycle is that the enzyme RuBisCO catalyzes the combination of RuBP with carbon dioxide, and the subsequent unstable six-carbon compound rapidly decomposes into two three-carbon compounds.
During this process, ATP and NADPH serve as energy and reducing agent providers, converting three-carbon compounds into more complex sugars. Although the end products of the reaction are mainly three-carbon sugar phosphate compounds, this does not mean that they cannot be further converted into six-carbon sugars. These three-carbon products can be used to synthesize larger carbohydrates such as sucrose and starch.
The operation of the Calvin cycle not only depends on light, but is also affected by other metabolic pathways within the plant, such as the photorespiration process, in which RuBisCO can also use oxygen as a substrate, producing unfavorable by-products, which is even more severe in high-temperature environments. obvious.
The loss of carbon dioxide in plants caused by the process of photorespiration makes plants that have specifically evolved C4 and CAM photosynthetic pathways more competitive in high-temperature environments.
How is this competitive appearance explained? Both C4 plants and CAM plants use different strategies to capture carbon dioxide to reduce the effects of photorespiration. For example, C4 plants fix carbon dioxide in different cells so that they can still perform photosynthesis efficiently in high temperature and low carbon dioxide environments.
The Calvin cycle is usually closely combined with light-dependent reactions that occur on the thylakoid membrane of chloroplasts. The ATP and NADPH produced by these reactions will be used in subsequent reactions of the Calvin cycle. Without these light-dependent reactions, the Calvin cycle would not be possible.
So, despite the "dark reaction" in the name, it is actually a process that is crucial to photosynthesis. Many plants use the energy stored in starch to stay alive at night, but this does not mean that the Calvin cycle and its processes occur in the dark. Instead, the reaction is largely hampered by a lack of light.
Various enzymes in this process are activated in the presence of light and deactivated as the light becomes darker, revealing the inseparable relationship between light and these biochemical reactions.
The discovery of the Calvin cycle, first introduced by Melvin Calvin and his colleagues as early as 1950, was an advance that advanced our understanding of photosynthesis. This discovery not only opened up the study of plant photosynthesis, but also had a profound impact on other fields of biology, showing the complexity of the operation of living systems.
With the advancement of science and technology, our understanding of the Calvin cycle continues to deepen, and this process once again proves the intertwined relationship between various life processes in nature. So, how do plants rely on these responses to adapt to changes in their environment as we face the challenge of climate change?
