The chloroplast is the organelle within plant and algal cells where photosynthesis takes place, converting light energy into chemical energy. This double-membraned structure contains an internal framework. The stroma is the dense, aqueous fluid that fills the space inside the inner membrane of the chloroplast. It serves as the reaction medium for the second major phase of photosynthesis.
Location and Contents of the Stroma
The stroma is an enzyme-rich matrix that occupies the entire internal volume of the chloroplast, excluding the space enclosed by the thylakoids. It is the fluid surrounding the stacks of coin-like membrane structures known as grana, which are bundles of individual thylakoids. This internal fluid provides a buffered, regulated environment necessary for the biochemical reactions that occur there.
The stroma contains the organelle’s own circular DNA (cpDNA) and ribosomes, which synthesize certain chloroplast proteins. It also holds starch granules, which act as temporary storage for sugars produced during photosynthesis.
The Stroma’s Primary Function
The light-independent reaction, known as the Calvin cycle, occurs within the stroma. While this process does not require light directly, it relies completely on the energy molecules produced by the light-dependent reactions. The cycle converts carbon dioxide from the atmosphere into a stable organic sugar molecule.
This fundamental step, called carbon fixation, converts inorganic carbon into organic compounds. The process is initiated by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known as RuBisCO. RuBisCO, one of the most abundant proteins on Earth, catalyzes the reaction between carbon dioxide and a five-carbon sugar molecule. This reaction starts the synthesis of glucose, leading to the formation of a stable sugar product.
Integration with Light-Dependent Reactions
Photosynthesis requires a continuous exchange between the stroma and the thylakoids. The first stage, the light-dependent reactions, occurs in the thylakoid membranes, capturing light energy. This phase converts light energy into chemical energy stored in two carrier molecules: adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH).
The stroma is the destination for these energy carriers. ATP and NADPH diffuse from the thylakoid membranes into the stroma, providing the energy and reducing power to drive the Calvin cycle. Once spent during carbon fixation, the depleted molecules (ADP and NADP+) are recycled. They return to the thylakoid membranes to be re-energized by light, ensuring continuous operation.