Photosynthesis is the biological process by which green plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose, while also releasing oxygen as a byproduct. It occurs in specialized organelles called chloroplasts within plant cells. The process of photosynthesis consists of two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle).
1. Light-Dependent Reactions:
Light-dependent reactions occur in the thylakoid membranes of the chloroplasts.
These reactions require light energy captured by pigments, primarily chlorophyll, to drive the synthesis of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), which are energy carriers.
The light-dependent reactions consist of the following steps:
Photon Absorption: Chlorophyll molecules absorb photons (light energy) from sunlight.
Water Splitting: Light energy is used to split water molecules into oxygen, protons (H⁺ ions), and electrons (e⁻). Oxygen is released as a byproduct.
ATP Synthesis: The energy released during electron transport is used to pump protons across the thylakoid membrane, generating a proton gradient. ATP synthase enzymes then utilize this proton gradient to synthesize ATP from ADP (adenosine diphosphate) and inorganic phosphate (Pi).
NADPH Formation: Electrons from the water-splitting reaction are transferred to NADP⁺, reducing it to NADPH, a high-energy electron carrier.
2. Light-Independent Reactions (Calvin Cycle):
The Calvin cycle, or light-independent reactions, occur in the stroma of the chloroplast.
These reactions use the ATP and NADPH produced during the light-dependent reactions to convert carbon dioxide (CO₂) into glucose.
The Calvin cycle consists of the following steps:
Carbon Fixation: CO₂ molecules are captured and incorporated into a five-carbon sugar molecule called ribulose-1,5-bisphosphate (RuBP), catalyzed by the enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase).
Reduction: The six-carbon compound formed is unstable and immediately splits into two molecules of 3-phosphoglycerate (3-PGA). ATP and NADPH from the light-dependent reactions are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar.
Regeneration of RuBP: Some G3P molecules are used to regenerate RuBP, ensuring that the Calvin cycle can continue. The remaining G3P molecules are used to synthesize glucose and other carbohydrates.
Glucose Production: Glucose is synthesized from G3P molecules through additional enzymatic reactions, providing the plant with a source of energy and building blocks for growth and metabolism.
In summary, photosynthesis in plants involves two main stages: the light-dependent reactions, which capture light energy and produce ATP and NADPH, and the light-independent reactions (Calvin cycle), which use ATP and NADPH to convert CO₂ into glucose. Together, these reactions enable plants to convert solar energy into chemical energy, providing essential nutrients for growth and metabolism while releasing oxygen into the atmosphere.
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