The light-independent reactions of photosynthesis, commonly referred to as the Calvin cycle, are pivotal for life on Earth. Occurring in the stroma of chloroplasts, these complex reactions convert carbon dioxide into glucose and other organic compounds. They do not directly require light, thus called light-independent.
Carbon Fixation in the Calvin Cycle
Carbon Dioxide Fixation
- Initial step: Carbon dioxide from the atmosphere is captured.
- Ribulose bisphosphate (RuBP): A 5-carbon sugar that acts as the CO2 acceptor.
- Enzyme rubisco: Facilitates the bonding of CO2 with RuBP.
- Result: Unstable 6-carbon compound breaks into two 3-carbon molecules (3-PGA).
Reduction Phase
- ATP and NADPH from light-dependent reactions provide energy.
- Each 3-PGA is phosphorylated by ATP, then reduced by NADPH, forming glyceraldehyde-3-phosphate (G3P).
- Energy storage: Part of the energy captured in light-dependent reactions is stored in G3P's high-energy bonds.
Regeneration of RuBP
- Some G3P molecules are converted into RuBP.
- Requires complex rearrangement and ATP consumption.
- Crucial step: RuBP's regeneration enables the continuation of the cycle.
Role of Rubisco
- Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase): Most abundant enzyme on Earth.
- Main function: Catalyses carbon fixation in the Calvin cycle.
- Photorespiration: Can also bind with O2 instead of CO2, which can decrease efficiency.
- Evolutive adaptations: Different plants have rubisco variations to suit their specific environments.
Formation of G3P
- G3P is a central product of the Calvin cycle.
- Serves as the building block for glucose and other carbohydrates.
- Three turns of the Calvin cycle are required to net one molecule of G3P.
Regeneration of RuBP
- RuBP is regenerated from G3P through a series of enzyme-catalysed reactions.
- Interconnectedness: Showcases how light-dependent and light-independent reactions are tightly connected.
- ATP consumption: Ensures the Calvin cycle's continuation.
Glucose Formation
- Glucose synthesis: Not directly formed in the Calvin cycle.
- Two G3P molecules: Combine to form one glucose molecule.
- Multiple turns: Six turns of the Calvin cycle are required to produce enough G3P for one glucose molecule.
- Other compounds: G3P serves as a precursor for other sugars and organic compounds.
Significance in Carbon Capture and Food Production
Carbon Transformation
- Converts inorganic carbon (CO2) into organic molecules.
- Essential for life: Supports growth and reproduction in plants and fuels food chains.
Food Production
- Agricultural importance: Understanding this process is vital for enhancing crop yield.
- Biotechnological applications: Targeting Calvin cycle enzymes can help improve photosynthetic efficiency.
Ecosystem Balance
- Carbon sequestration: Plays a role in mitigating climate change by capturing atmospheric CO2.
- Biomass formation: Fundamental in the formation of plant biomass, a crucial ecological factor.
FAQ
If rubisco were inhibited in a plant, the Calvin cycle would come to a halt because rubisco catalyses the first step of carbon fixation. Without this enzyme, carbon dioxide would not be incorporated into organic molecules, and the entire process of photosynthesis would be disrupted. This would result in reduced glucose production, leading to stunted growth, poor development, and eventually the death of the plant if the inhibition were prolonged.
Six rounds of the Calvin cycle are required to produce one molecule of glucose. Each round of the cycle captures one carbon atom from CO2, and since glucose is a six-carbon sugar, six rounds are necessary. These rounds also generate enough G3P to form one glucose molecule while maintaining the cycle by regenerating RuBP.
C4 and CAM plants do carry out the Calvin cycle, but they have adaptations to overcome the inefficiency of rubisco in hot or dry environments. C4 plants separate the initial CO2 fixation from the Calvin cycle spatially, while CAM plants do it temporally. Both still utilize the Calvin cycle for carbon fixation but have additional steps to concentrate CO2 around rubisco, reducing photorespiration.
Photorespiration is considered inefficient because it consumes oxygen and releases carbon dioxide without producing ATP or NADPH. It occurs when rubisco binds oxygen instead of carbon dioxide, leading to a pathway that reverses part of the Calvin cycle. In conditions where oxygen is more available or carbon dioxide levels are low, photorespiration can decrease the overall efficiency of photosynthesis as it essentially undoes part of the work of the Calvin cycle without yielding energy.
The Calvin cycle is often referred to as the dark reaction as it does not directly require light to proceed. It's a misconception that it only occurs in the dark. The Calvin cycle uses the ATP and NADPH generated in the light-dependent reactions of photosynthesis, so it is indirectly dependent on light. However, the reactions themselves do not need photons, and therefore, the Calvin cycle can occur as long as ATP and NADPH are available.
Practice Questions
Carbon fixation in the Calvin cycle involves the capturing of carbon dioxide from the atmosphere and its conversion into an organic molecule. It begins with the bonding of CO2 with a 5-carbon sugar called ribulose bisphosphate (RuBP), facilitated by the enzyme rubisco. This forms an unstable 6-carbon compound that breaks into two 3-phosphoglycerate (3-PGA) molecules. Rubisco's role is vital as it catalyses the reaction between RuBP and CO2, enabling the organic incorporation of carbon into the cycle. The enzyme is not completely efficient and can also bind O2, leading to a process known as photorespiration.
Glucose formation in the Calvin cycle is significant as it allows plants to store energy in a stable form that can be transported and used for growth and reproduction. Two molecules of G3P, produced in the cycle, combine to form one glucose molecule. Carbon capture in the Calvin cycle converts inorganic carbon into organic molecules, supporting life and contributing to carbon sequestration. Regeneration of RuBP is essential for the continuity of the cycle; it ensures that the cycle can proceed by converting some G3P molecules back to RuBP, using ATP, allowing more CO2 to be fixed in subsequent turns of the cycle.
