Summary
Highlights
The Calvin cycle, also known as the dark reactions, takes place in the stroma and converts carbon dioxide into sugar. It consists of three main steps: carbon fixation, carbon reduction, and the regeneration of RuBP.
Photosynthesis is the process plants use to convert carbon dioxide, water, and sunlight into food and oxygen. This process occurs in two main stages: the light-dependent reactions and the Calvin cycle, both taking place in the chloroplast. The chloroplast contains thylakoids, organized into grana, with the stroma being the fluid surrounding the thylakoids and the lumen being the fluid inside them.
The light-dependent reactions occur in the thylakoid membrane. Key components include Photosystem I and II with chlorophyll to capture light energy, the electron transport chain (ETC) for electron transfer, ATP synthase for ATP production, and NADP+ as an electron carrier, which is reduced to NADPH when it gains electrons.
The process begins with Photosystem II absorbing sunlight, energizing electrons that move to the ETC. Water photolysis in Photosystem II replaces these electrons, splitting water into protons, electrons, and oxygen. The electrons transfer through the ETC, releasing energy to pump protons from the stroma into the lumen, creating a proton gradient. This gradient drives ATP synthesis via chemiosmosis. Electrons then reach Photosystem I, are re-energized by light, and transfer to NADP+, forming NADPH. The overall products are oxygen, NADPH, and ATP.
In carbon fixation, the enzyme RuBisCO attaches CO2 to RuBP, forming an unstable 6-carbon molecule that immediately splits into two 3PGA molecules. In carbon reduction, ATP and NADPH from the light-dependent reactions are used. ATP phosphorylates 3PGA, making it more reactive, and NADPH donates electrons to reduce it to G3P. For every three CO2 molecules, six G3P molecules are produced.
In the regeneration of RuBP, five of the six G3P molecules are converted back into RuBP using ATP, ensuring the cycle continues. The remaining one G3P molecule exits the cycle to be used in the synthesis of sugars like glucose, sucrose, starch, or cellulose. Two G3P molecules are required to make one glucose molecule, meaning the cycle must run twice for each glucose produced.