What is the metabolic pathway in the stroma of the chloroplast where carbon enters as CO2 and leaves as sugar?

The Calvin cycle is the metabolic pathway that takes place in the stroma of chloroplasts, where carbon dioxide (CO2) is fixed and converted into glucose and other sugars. This process is fundamental for photosynthesis and is not dependent on light, which is why it occurs in the stroma instead of the thylakoid membranes where the light reactions take place.

During the Calvin cycle, CO2 is captured by a five-carbon sugar called ribulose bisphosphate (RuBP) through a reaction catalyzed by the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO). Through a series of reactions, the fixed carbon is eventually transformed into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar that can be further processed to produce glucose and other carbohydrates.

While other metabolic processes, such as glycolysis, occur in the cytoplasm and involve the breakdown of glucose, and the Krebs cycle occurs in the mitochondria and is part of cellular respiration, the Calvin cycle specifically focuses on carbon fixation and sugar production in the chloroplasts of plant cells. This makes it the correct pathway in the context of photosynthesis and carbon assimilation.