El CICLO de CALVIN-BENSON - [Paso a paso con repaso y resumen final]

Calvin-Benson Cycle Explained

Overview of Photosynthesis and the Calvin Cycle

• The video introduces the Calvin-Benson cycle, explaining its role in photosynthesis after light reactions that produce ATP and NADPH.

• The cycle is often referred to as "dark reactions" or "light-independent reactions," but this can be misleading since it requires products from light reactions to function.

• Cellular reserves of ATP and NADPH are short-lived, lasting only seconds to minutes, necessitating continuous replenishment from light reactions.

Carbon Fixation Process

• Carbon is available for photosynthetic cells as carbon dioxide (CO2), which algae absorb directly from water while terrestrial plants use stomata on leaves and stems.

• The process involves reducing CO2 through a series of reactions to form glucose, utilizing energy stored in ATP and NADPH produced during the light phase.

• Initial incorporation of CO2 into organic compounds is known as carbon fixation, occurring cyclically in the stroma of chloroplasts.

Mechanism of the Calvin Cycle

• The cycle regenerates ribulose 1,5-bisphosphate (RuBP), a five-carbon sugar with two phosphates. Each step is catalyzed by specific enzymes.

• Rubisco enzyme adds CO2 to RuBP, forming an unstable six-carbon compound that splits into two molecules of 3-phosphoglycerate (PGA).

Enzymatic Reactions and Energy Transfer

• Each PGA molecule contains three carbon atoms; hence the cycle is also called the C3 pathway. Rubisco constitutes over 25% of chloroplast proteins, making it one of Earth's most abundant proteins.

• Two ATP molecules convert PGA into 1,3-bisphosphoglycerate (BPG), followed by reduction using two NADPH molecules to form glyceraldehyde 3-phosphate (G3P).

Final Steps and Outcomes

• From three cycles of Calvin's process, enough carbon is fixed for one G3P molecule to exit for further sugar synthesis while maintaining sufficient carbon for RuBP regeneration.

• Most G3P continues through additional steps in the cycle where sugars with four, six, or seven carbons are generated before regenerating RuBP using ATP.

Summary of Carbon Fixation Cycles

• A complete turn incorporates one CO2 molecule; three cycles yield one G3P molecule through combining three RuBP with three CO2 leading to six PGA formation.

Ciclo de Calvin y su Eficiencia

Introducción al Ciclo de Calvin

• El ciclo de Calvin utiliza ATP y NADPH producidos en la primera etapa de la fotosíntesis, donde las moléculas regeneradas están disponibles para los fotosistemas 1 y 2.

Productos del Ciclo de Calvin

• La ecuación general del ciclo refleja que se requieren tres moléculas de dióxido de carbono, nueve moléculas de ATP y seis moléculas de NADPH para producir glucosa, un azúcar esencial en las células vivas.

Revoluciones Necesarias para la Producción de Azúcar

• Se necesitan seis revoluciones del ciclo con la introducción de seis moléculas de dióxido de carbono para generar una molécula equivalente a un azúcar de seis carbonos como la glucosa.

Mecanismos Regulatorios del Ciclo

• Existen mecanismos que evitan que el ciclo degrade ATP durante la noche; enzimas como rubisco son reguladas indirectamente por luz, pH óptimo y concentración iónica.

Eficiencia Fotosintética

• La eficacia del ciclo se mide comparando energía utilizada para fijar CO2 con energía luminosa necesaria para activar el ciclo. Se estima que el máximo teórico es del 35%, pero muchas plantas alcanzan solo entre 1% y 4%.

Fotorespiración