Calvin cycle: carbon fixation and reduction phase, synthesis of sugar
The dark reactions occur in the stroma of the chloroplast. These reactions do not need light directly and can happen in the existence or absence of light provided the assimilatory power in the form of ATP and NADPH, produced during light reactions is available.
The energy of these substances is utilized in the formation of carbohydrates from CO2, and hence stored in them.
The details of the path of carbon in these reactions were found by Melvin Calvin and his co-workers at the University of California. Calvin was granted Nobel Prize in 1961.
The cyclic series of reactions, catalyzed by particular enzymes, by which the carbon is fixed and reduced resulting in the synthesis of sugar during the dark reactions of photosynthesis is called Calvin Cycle.
The Calvin cycle can be divided into three stages: Carbon fixation, Reduction, and Regeneration of CO2 acceptor (RuBP).
Phase 1: Carbon fixation
Carbon fixation describes the initial incorporation of CO2 into organic material. Keep in mind that we are following 3 molecules of CO2 through the reaction (because 3 molecules of CO2 are needed to produce one molecule of carbohydrate, a triose).
The Calvin cycle begins when a molecule of CO2 reacts with a highly reactive phosphorylated five-carbon sugar called ribulose bisphosphate (RuBP). This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase, likewise known as Rubisco (it is the most abundant protein in chloroplasts, and probably the most abundant protein on Earth).
The product of this reaction is an extremely unsteady, 6 – carbon intermediate that instantly breaks into two molecules of 3 – carbon compound called 3 -phosphoglycerate (phosphoglyceric acid – PGA). The carbon that was originally part of CO2molecule is now a part of an organic molecule; the carbon has actually been “fixed”. Since the product of initial carbon fixation is a 3 – carbon compound, the Calvin cycle is also called the C3 pathway.
Phase 2:Reduction
Each molecule of phosphoglyceric acid(PGA) receives an extra phosphate from ATP of light reaction, forming 1,3 -bisphosphoglycerate as the product. 1,3 bisphosphoglycerate is reduced to glyceraldehyde 3-phosphate(G3P) by getting a pair of electrons contributed from NADPH of light reactions.
G3P is the same three-carbon sugar which is formed in glycolysis (first stage of cellular respiration) by the splitting of glucose. In this way fixed carbon is reduced to energy-rich G3P with the energy and reducing the power of ATP and NADPH (both the products of light-dependent reactions), having the energy stored in it. Actually, G3P, and not glucose, is the carbohydrate produced straight from the Calvin cycle.
For every 3 molecules of CO2 going into the cycle and integrating with 3 molecules of five-carbon RuBP, six molecules of G3P (including 18 carbon in all) are produced. However, only one molecule of G3P can be counted as a net gain of carbohydrates.
Out of every 6 particles of G3P formed, only one particle leaves the cycle to be utilized by the plant for making glucose, sucrose starch or other carbohydrates, and other natural compounds; the other five molecules are recycled to restore the three molecules of five-carbon RuBP, the CO2 acceptor.
Phase 3: Regeneration of CO2 acceptor, RuBP
Through a complex series of reactions, the carbon skeletons of five molecules of three-carbon G3P are reorganized into 3 molecules of five-carbon ribulose phosphate (RuP). Each RuP is phosphorylated to ribulose bisphosphate (RuBP), the actual five-carbon CO2 acceptor with which the cycle began. Once again three more molecules of ATP of light reactions are used for this phosphorylation of 3 RuP molecules. These RuBP are now prepared to receive CO2 again, and the cycle continues.
Sugar synthesis
FAQs: Light Independent or Dark Reactions (Calvin Cycle)
1. What are Light-Independent Reactions in Photosynthesis? Light-independent reactions, also known as dark reactions or the Calvin Cycle, occur in the stroma of chloroplasts. Unlike the light-dependent reactions, these processes do not directly require light and can take place in the presence or absence of light, given the availability of ATP and NADPH generated during the light reactions.
2. Who Discovered and Explored the Calvin Cycle? The Calvin Cycle was elucidated by Melvin Calvin and his research team at the University of California. For his groundbreaking work in uncovering the details of the carbon path in these reactions, Melvin Calvin was awarded the Nobel Prize in 1961.
3. What is the Calvin Cycle? The Calvin Cycle refers to the cyclic series of reactions catalyzed by specific enzymes that fix and reduce carbon, ultimately leading to the synthesis of sugars during the dark reactions of photosynthesis.
4. How is the Calvin Cycle Divided? The Calvin Cycle consists of three stages: Carbon Fixation, Reduction, and Regeneration of CO2 acceptor (RuBP). Each phase plays a crucial role in the overall process of converting carbon dioxide into carbohydrates.
5. What Happens in the Carbon Fixation Phase of the Calvin Cycle? In the Carbon Fixation phase, a molecule of CO2 reacts with ribulose bisphosphate (RuBP) to produce a highly unstable 6-carbon intermediate. This intermediate rapidly breaks into two molecules of 3-phosphoglycerate (PGA), marking the initial incorporation of CO2 into organic material.
6. How is Carbon Reduced in the Calvin Cycle? During the Reduction phase, each molecule of 3-phosphoglycerate (PGA) receives an additional phosphate from ATP, forming 1,3-bisphosphoglycerate. This compound is then reduced to glyceraldehyde 3-phosphate (G3P) by obtaining a pair of electrons from NADPH generated in the light reactions.
7. What is the Role of Regeneration of CO2 Acceptor (RuBP)? The Regeneration phase involves a complex series of reactions where the carbon skeletons of G3P molecules are rearranged into ribulose phosphate (RuP) and then phosphorylated back to ribulose bisphosphate (RuBP). This prepares RuBP to receive CO2 again, ensuring the continuity of the Calvin Cycle.
8. How Efficient is the Calvin Cycle in Carbohydrate Production? For every 3 molecules of CO2 entering the Calvin Cycle, six molecules of G3P are produced. However, only one molecule of G3P is counted as a net gain of carbohydrates, with the other five molecules being recycled to regenerate the CO2 acceptor, RuBP.
9. What Are the End Products of the Calvin Cycle? The primary end product of the Calvin Cycle is glyceraldehyde 3-phosphate (G3P), a three-carbon sugar. This G3P can be used by the plant to synthesize various carbohydrates, such as glucose, sucrose, starch, or other organic compounds.
Multiple Choice Questions (MCQs) with Answers
- Where do the dark reactions or Calvin Cycle occur in photosynthesis?
- a. Thylakoid membranes
- b. Stroma of chloroplasts
- c. Mitochondria
- d. Cell wall
Answer: b. Stroma of chloroplasts
- What is the primary source of energy for the Calvin Cycle?
- a. Sunlight
- b. ATP and NADPH
- c. Oxygen
- d. Carbon dioxide
Answer: b. ATP and NADPH
- Who discovered the details of the path of carbon in the Calvin Cycle?
- a. Robert Hooke
- b. Melvin Calvin
- c. Gregor Mendel
- d. Charles Darwin
Answer: b. Melvin Calvin
- What is the Calvin Cycle?
- a. Light-dependent reactions
- b. Cyclic pathway for carbon fixation and reduction
- c. Breakdown of glucose in cellular respiration
- d. None of the above
Answer: b. Cyclic pathway for carbon fixation and reduction
- In which phase of the Calvin Cycle does the initial incorporation of CO2 into organic material occur?
- a. Reduction
- b. Regeneration
- c. Carbon Fixation
- d. Dark phase
Answer: c. Carbon Fixation
- Where do the dark reactions take place within the chloroplast?
- a. Grana
- b. Thylakoid lumen
- c. Stroma
- d. Intermembrane space
Answer: c. Stroma
- What is the role of Rubisco in the Calvin Cycle?
- a. ATP production
- b. Carbon fixation
- c. Oxygen release
- d. Electron transport
Answer: b. Carbon fixation
- Which enzyme is responsible for the formation of 3-phosphoglycerate in the Calvin Cycle?
- a. ATP synthase
- b. Rubisco
- c. NADP reductase
- d. Phosphoglycerate kinase
Answer: b. Rubisco
- What is the product of the carbon fixation phase in the Calvin Cycle?
- a. 3-phosphoglycerate
- b. Glyceraldehyde 3-phosphate
- c. Ribulose bisphosphate
- d. RuBP
Answer: a. 3-phosphoglycerate
- Which stage of the Calvin Cycle involves the reduction of 1,3-bisphosphoglycerate to glyceraldehyde 3-phosphate?
- a. Carbon fixation
- b. Reduction
- c. Regeneration
- d. Photolysis
Answer: b. Reduction
- What is the fate of most G3P molecules produced in the Calvin Cycle?
- a. Used for ATP production
- b. Released as oxygen
- c. Recycled to regenerate RuBP
- d. Translocated to the mitochondria
Answer: c. Recycled to regenerate RuBP
- Which molecule serves as the actual CO2 acceptor in the Calvin Cycle?
- a. G3P
- b. RuP
- c. RuBP
- d. PGA
Answer: c. RuBP
- How many molecules of ATP are used in the regeneration of RuBP during the Calvin Cycle?
- a. One
- b. Two
- c. Three
- d. Four
Answer: c. Three
- What is the overall purpose of the Calvin Cycle in photosynthesis?
- a. ATP production
- b. Oxygen release
- c. Carbon fixation and sugar synthesis
- d. Chlorophyll synthesis
Answer: c. Carbon fixation and sugar synthesis
- What term is used to describe the Calvin Cycle’s ability to occur in the absence of light?
- a. Light-dependent reactions
- b. Dark reactions
- c. Cyclic phosphorylation
- d. Photorespiration
Answer: b. Dark reactions
Summary: Calvin Cycle – Light Independent or Dark Reactions
The Calvin Cycle, also known as the light-independent or dark reactions, unfolds in the stroma of chloroplasts, demonstrating a remarkable independence from direct light exposure. These reactions are pivotal for the synthesis of carbohydrates from carbon dioxide, with the assimilatory power derived from ATP and NADPH, products of the light reactions, serving as their energy source.
Melvin Calvin and his colleagues at the University of California made significant contributions to unraveling the intricate path of carbon within these dark reactions, earning Calvin the Nobel Prize in 1961.
The cyclic series of reactions, orchestrated by specific enzymes, constitutes the Calvin Cycle. It comprises three distinct phases:
- Carbon Fixation (Phase 1): In this initial step, three molecules of CO2 are incorporated into organic material. The cycle commences when CO2 reacts with the phosphorylated five-carbon sugar, ribulose bisphosphate (RuBP), catalyzed by the enzyme Rubisco. The resulting product is a highly unstable 6-carbon intermediate, which promptly splits into two 3-carbon compounds, known as 3-phosphoglycerate (PGA).
- Reduction (Phase 2): Each molecule of PGA receives an additional phosphate from ATP, forming 1,3-bisphosphoglycerate. Subsequently, 1,3-bisphosphoglycerate is reduced to glyceraldehyde 3-phosphate (G3P) by obtaining electrons from NADPH. G3P, a three-carbon sugar, stores energy derived from ATP and NADPH, representing the direct product of the Calvin Cycle. However, it’s important to note that out of every 6 molecules of G3P produced, only one exits the cycle for further utilization, while the remaining five are recycled to regenerate the CO2 acceptor, RuBP.
- Regeneration of CO2 Acceptor, RuBP (Phase 3): The final phase involves the complex rearrangement of the carbon skeletons of five G3P molecules into three molecules of five-carbon ribulose phosphate (RuP). Each RuP is then phosphorylated to regenerate ribulose bisphosphate (RuBP), the actual CO2 acceptor. This regeneration process necessitates the utilization of three ATP molecules derived from the light reactions. The now regenerated RuBP is poised to initiate the cycle again by accepting CO2.
The Calvin Cycle, with its meticulous orchestration of chemical reactions, underscores its crucial role in the conversion of carbon dioxide into energy-rich carbohydrates, contributing to the sustenance of plant life and the overall balance of the Earth’s ecosystems.