The Calvin cycle is the second stage of photosynthesis, where plants use energy from light reactions to fix carbon dioxide into sugar. This process, driven by the enzyme RuBisCO, converts inorganic carbon into organic compounds that fuel plant growth and sustain ecosystems.
Carbon fixation occurs in the chloroplast stroma, where CO2 combines with a 5-carbon sugar. The resulting 3-carbon molecules undergo further reactions, producing glyceraldehyde 3-phosphate (G3P). This versatile molecule forms the basis for various plant biomolecules and metabolic processes.
Carbon Fixation by RuBisCO
RuBisCO's Role in Carbon Fixation
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- RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) functions as the primary enzyme in carbon fixation
- Catalyzes the first major step of carbon fixation in the Calvin cycle
- Considered the most abundant enzyme on Earth due to its crucial role in photosynthesis
- Located in the stroma of chloroplasts in plant cells
- Consists of eight large subunits and eight small subunits, forming a large protein complex
- Exhibits a relatively slow catalytic rate, compensated by its high abundance in plants
Carbon Fixation Process
- Carbon fixation incorporates inorganic carbon from the atmosphere into organic compounds
- Occurs in the stroma of chloroplasts during the light-independent reactions of photosynthesis
- Involves the addition of carbon dioxide to a 5-carbon sugar, ribulose 1,5-bisphosphate
- Produces an unstable 6-carbon compound that immediately splits into two 3-carbon molecules
- Represents a critical step in converting atmospheric CO2 into usable energy for plants
- Serves as the primary source of organic carbon for most ecosystems (autotrophs and heterotrophs)
Ribulose 1,5-bisphosphate and Carbon Dioxide Interaction
- Ribulose 1,5-bisphosphate acts as the primary CO2 acceptor molecule in the Calvin cycle
- Contains five carbon atoms and two phosphate groups
- Synthesized from the regeneration of 3-carbon sugars in the Calvin cycle
- Binds to the active site of RuBisCO along with CO2
- Carbon dioxide enters the leaf through small pores called stomata
- Diffuses into the stroma of the chloroplast where it reacts with ribulose 1,5-bisphosphate
Products of the Calvin Cycle
Formation of 3-Phosphoglycerate
- 3-Phosphoglycerate emerges as the first stable product of carbon fixation
- Contains three carbon atoms and one phosphate group
- Produced when the unstable 6-carbon compound formed by RuBisCO splits
- For every three turns of the Calvin cycle, six molecules of 3-phosphoglycerate are formed
- Serves as a precursor for various biomolecules, including amino acids and glucose
- Undergoes further reactions in the Calvin cycle to produce other important compounds
Synthesis of Glyceraldehyde 3-phosphate
- Glyceraldehyde 3-phosphate (G3P) represents the primary end product of the Calvin cycle
- Contains three carbon atoms and one phosphate group
- Formed through the reduction of 3-phosphoglycerate using ATP and NADPH
- Acts as a versatile molecule used in various metabolic pathways
- Serves as a precursor for glucose synthesis and other carbohydrates
- Can be used to regenerate ribulose 1,5-bisphosphate, continuing the Calvin cycle
- Excess G3P exported from chloroplasts for use in other cellular processes (starch synthesis)
Energy and Reducing Power
ATP as an Energy Source
- ATP (Adenosine Triphosphate) provides the energy required for the Calvin cycle reactions
- Produced during the light-dependent reactions of photosynthesis
- Hydrolyzed to ADP and inorganic phosphate, releasing energy for carbon fixation
- Used in the phosphorylation of 3-phosphoglycerate to 1,3-bisphosphoglycerate
- Consumed in the regeneration of ribulose 1,5-bisphosphate from 3-carbon sugars
- For every three turns of the Calvin cycle, 9 ATP molecules are utilized
NADPH as a Reducing Agent
- NADPH (Nicotinamide Adenine Dinucleotide Phosphate) serves as the primary electron donor in the Calvin cycle
- Generated during the light-dependent reactions of photosynthesis
- Provides the reducing power necessary to convert 3-phosphoglycerate to glyceraldehyde 3-phosphate
- Oxidized to NADP+ after donating electrons, then recycled in the light-dependent reactions
- Essential for the reduction of 1,3-bisphosphoglycerate to glyceraldehyde 3-phosphate
- For every three turns of the Calvin cycle, 6 NADPH molecules are consumed