Photosynthesis is the fundamental biological process through which plants, algae, and some bacteria convert light energy into chemical energy, stored in sugar molecules. This conversion takes place inside specialized cellular compartments called chloroplasts. The process is divided into two interconnected stages: the light-dependent reactions and the light-independent reactions. The light-independent reactions use the energy captured from sunlight to create organic compounds from atmospheric carbon dioxide.
The Site of the Reaction
The light-independent reactions occur in the stroma, the dense, fluid-filled interior space of the chloroplast. This gel-like matrix surrounds the internal membranes known as thylakoids. The stroma provides the appropriate environment, including the necessary \(\text{pH}\) and concentration of molecules, for the carbon-fixing chemistry to take place.
This location contrasts with the light-dependent reactions, which are confined to the thylakoid membranes. The stroma contains a high concentration of the enzymes required to fix carbon dioxide and synthesize sugar precursors. The spatial separation ensures that both reaction phases operate efficiently. The fluid nature of the stroma allows molecules to diffuse freely, facilitating the movement of reactants and products.
The Calvin Cycle
The light-independent reaction is formally known as the Calvin Cycle, named after Melvin Calvin. Its function is to convert inorganic carbon dioxide (\(\text{CO}_2\)) from the atmosphere into a stable three-carbon sugar precursor, glyceraldehyde-3-phosphate (\(\text{G3P}\)). The cycle accomplishes this through a series of biochemical steps organized into three phases.
Carbon Fixation
The first phase is carbon fixation. An enzyme called \(\text{RuBisCO}\) catalyzes the attachment of a \(\text{CO}_2\) molecule to the five-carbon acceptor molecule, ribulose-1,5-bisphosphate (\(\text{RuBP}\)). This combination creates an unstable six-carbon compound that immediately splits into two molecules of 3-phosphoglycerate (3-PGA). \(\text{RuBisCO}\) is considered the most abundant protein on Earth, highlighting the importance of this initial step.
Reduction
The second phase is reduction, converting the 3-PGA molecules into \(\text{G3P}\). This conversion requires chemical energy and reducing power. The reaction uses energy from adenosine triphosphate (\(\text{ATP}\)) and electrons supplied by nicotinamide adenine dinucleotide phosphate (\(\text{NADPH}\)). The \(\text{G3P}\) molecules are the final sugar precursors, and for every six molecules created, only one exits the cycle to be used by the plant for synthesizing glucose and other organic molecules.
Regeneration
The final phase is regeneration. The remaining five \(\text{G3P}\) molecules are rearranged to re-form the original three molecules of the five-carbon acceptor, \(\text{RuBP}\). This step is necessary because it ensures the cycle can continue to fix more \(\text{CO}_2\) from the atmosphere. Regeneration consumes additional \(\text{ATP}\) to provide the energy for the molecular restructuring.
Fueling the Process
The Calvin Cycle is not independent of light, as it is entirely dependent on the products generated by the light-dependent reactions. These reactions, which occur in the thylakoids, produce the high-energy molecules \(\text{ATP}\) and \(\text{NADPH}\). These molecules act as the energy carriers and reducing agents that fuel the light-independent reactions in the stroma.
The proximity of the thylakoids to the stroma is a major factor in the efficiency of photosynthesis. Once \(\text{ATP}\) and \(\text{NADPH}\) are synthesized on the thylakoid membranes, they diffuse quickly into the surrounding stroma. This short distance is necessary because these molecules, particularly \(\text{NADPH}\), are unstable and have a short lifespan.
The \(\text{ATP}\) provides the energy for the fixation and regeneration steps, while the \(\text{NADPH}\) provides the electrons needed for the reduction of 3-PGA into \(\text{G3P}\). Once their energy is spent, the lower-energy molecules, adenosine diphosphate (\(\text{ADP}\)) and \(\text{NADP}^+\), cycle back to the thylakoid membranes. They are recharged by the light-dependent reactions, ensuring a continuous supply of fuel for the carbon-fixing cycle in the stroma.