Reacciones Oscuras de la Fotosíntesis: Ciclo de Calvin

Dark Reactions of Photosynthesis

Overview

The dark reactions, also known as the light-independent reactions or the Calvin cycle, are the second stage of photosynthesis. These reactions occur in the stroma of the chloroplast and are essential for converting carbon dioxide into glucose and other organic compounds, providing energy for the plant. Unlike the light-dependent reactions, the dark reactions do not require light and can occur under low-light conditions. They are named after the American chemist Melvin Calvin, who, along with co-workers, conducted crucial experiments to understand the process.

The Calvin Cycle

The Calvin cycle, also known as the Calvin cycle, is the series of chemical reactions that occur in the dark reactions of photosynthesis. It is a cyclic process that begins with the conversion of carbon dioxide (CO2) into an organic form, primarily in the form of glucose. The cycle consists of three main stages: CO2 fixation, reduction, and regeneration.

1. CO2 fixation: The first stage involves the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) capturing carbon dioxide (CO2) and converting it into two molecules of 3-phosphoglycerate (3PGA).

2. Reduction: In the second stage, the 3PGA molecules are reduced by ATP and NADPH, which were produced during the light-dependent reactions. This reduction process converts the 3PGA molecules into glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).

3. Regeneration: In the third stage, the G3P and DHAP molecules are rearranged and converted back into 3PGA, which can then be used to fix more CO2. The remaining ATP and NADPH molecules are used for other cellular processes.

CO2 Fixation

The process of CO2 fixation is essential for photosynthesis. The enzyme RuBisCO, which is the most abundant enzyme on Earth, plays a crucial role in this process. RuBisCO is responsible for catalyzing the reaction between CO2 and ribulose-1,5-bisphosphate (RuBP), which is a six-carbon compound. This reaction produces two molecules of 3PGA, each containing three carbons.

Reduction

After CO2 fixation, the 3PGA molecules are reduced by ATP and NADPH, which were produced during the light-dependent reactions. This reduction process is catalyzed by the enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH). The two 3PGA molecules are converted into one molecule of glyceraldehyde-3-phosphate (G3P) and one molecule of dihydroxyacetone phosphate (DHAP).

Regeneration

The final stage of the Calvin cycle involves the regeneration of the RuBP molecules used in CO2 fixation. This process is necessary to enable the cycle to continue and is catalyzed by the enzyme triose phosphate isomerase (TPI). The G3P and DHAP molecules are rearranged and converted back into 3PGA, which can then be used to fix more CO2.

Energy Requirements

The dark reactions require ATP and NADPH, which were produced during the light-dependent reactions. The ATP and NADPH molecules provide the energy needed to reduce the 3PGA molecules into G3P and DHAP. Without the energy from the light-dependent reactions, the dark reactions could not occur.

Summary

The dark reactions of photosynthesis, also known as the Calvin cycle, are a series of biochemical reactions that convert carbon dioxide into glucose and other organic compounds. These reactions occur in the stroma of the chloroplast and are essential for providing energy for the plant. The cycle consists of three main stages: CO2 fixation, reduction, and regeneration, and is named after the American chemist Melvin Calvin. The dark reactions require ATP and NADPH, which were produced during the light-dependent reactions, and are crucial for photosynthesis to occur.

Descubre más sobre las reacciones oscuras de la fotosíntesis, también conocidas como ciclo de Calvin. Aprende sobre la fijación de CO2, la reducción de 3PGA y la regeneración de RuBP, y cómo estas etapas esenciales convierten el dióxido de carbono en glucosa y otros compuestos orgánicos en las plantas.