Photosynthesis is the fundamental biological process used by plants, algae, and certain bacteria to transform light energy into chemical energy. This conversion relies on water absorbed from the environment and carbon dioxide taken from the atmosphere. The entire complex reaction is a sequence of coordinated chemical events divided into two distinct, interconnected stages. The products of the first stage provide the necessary energy and reducing power to drive the synthesis of sugars in the second stage.
Cellular Location of the Reactions
The photosynthetic process in plants occurs within specialized organelles known as chloroplasts. These organelles separate the two main reaction stages into different physical spaces. The first stage takes place on the thylakoid membranes, which are flattened, interconnected sacs containing light-capturing pigments and protein complexes. These membranes often stack up to form structures called grana. The second stage occurs in the stroma, the dense, aqueous fluid surrounding the thylakoids, which contains the enzymes required to convert carbon dioxide into sugar.
The Light-Dependent Reactions
This first stage converts solar energy into chemical energy carriers, initiated by the absorption of light. Pigments, primarily chlorophyll, capture photons within photosystems embedded in the thylakoid membranes. This energy excites electrons, initiating their flow through the electron transport chain.
To replace lost electrons, water molecules are split (photolysis), yielding hydrogen ions and oxygen gas. As electrons move, their energy pumps hydrogen ions into the thylakoid space, creating a concentration gradient. The flow of these ions back out through the enzyme ATP synthase synthesizes adenosine triphosphate (ATP).
Electrons are re-energized at photosystem I and transferred to reduce the carrier molecule NADP\(^+\) into NADPH. ATP and NADPH are the energy-rich products generated by the light-dependent reactions that power the subsequent stage.
The Light-Independent Reactions
The second stage, often called the Calvin Cycle, uses the chemical energy (ATP and NADPH) generated in the thylakoids to construct sugar molecules from carbon dioxide. These reactions occur within the stroma and rely entirely on the continuous supply of products from the first stage.
The process begins with carbon fixation, where the enzyme RuBisCO catalyzes the binding of atmospheric carbon dioxide to the five-carbon molecule ribulose bisphosphate (RuBP). This combination immediately breaks down into two molecules of the three-carbon compound 3-phosphoglycerate (3-PGA).
The 3-PGA molecules are then converted into the sugar glyceraldehyde-3-phosphate (G3P) in a reduction phase that requires ATP and NADPH. For every six turns of the cycle, one G3P molecule leaves to be used for the synthesis of glucose and other carbohydrates. The remaining G3P molecules are recycled, utilizing more ATP to regenerate the starting molecule, RuBP.