Photosynthesis is a fundamental biological process that sustains most life on Earth. It is the mechanism by which plants, algae, and some bacteria convert light energy into chemical energy. This chemical energy is stored in organic compounds, primarily sugars, which serve as food. Beyond creating their own sustenance, photosynthetic organisms also release oxygen as a byproduct, a gas essential for the respiration of many living things, including humans.
Naming the Second Phase
The second phase of photosynthesis is known as the Light-Independent Reactions. Instead, they rely on the energy-carrying molecules produced during the first phase of photosynthesis, which does require light. Another, more specific name for this phase is the Calvin Cycle, named after Melvin Calvin.
The older term “dark reactions” is sometimes used, but it can be misleading because these reactions do not require darkness; they can proceed in both light and dark conditions as long as the necessary energy carriers are available. The goal of this phase is to use carbon dioxide to synthesize sugars.
The Calvin Cycle Explained
The Calvin Cycle, occurring in the stroma—the fluid-filled space within chloroplasts—is a series of biochemical reactions that convert carbon dioxide into sugar. This cycle operates through three main stages.
The first stage is carbon fixation, where an enzyme called RuBisCO combines carbon dioxide with a five-carbon molecule, ribulose-1,5-bisphosphate (RuBP). This combination forms an unstable six-carbon compound that quickly splits into two molecules of a three-carbon compound called 3-phosphoglycerate (3-PGA).
The second stage is reduction. Here, the 3-PGA molecules are converted into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar. This conversion uses ATP and NADPH, both of which are products of the light-dependent reactions. For every three molecules of carbon dioxide fixed, one molecule of G3P leaves the cycle to be used for the synthesis of glucose and other organic molecules.
The final stage is the regeneration of RuBP. The remaining G3P molecules are rearranged and, with additional ATP, are converted back into RuBP. The overall input for the Calvin cycle includes carbon dioxide, ATP, and NADPH, while the main outputs are glucose precursors (G3P), along with recycled ADP and NADP+ which return to the light-dependent reactions.
Connecting the Phases
The Light-Dependent Reactions and the Light-Independent Reactions, or Calvin Cycle, are linked. The first phase, the Light-Dependent Reactions, takes place in the thylakoid membranes within the chloroplasts. During this phase, light energy is absorbed by pigments like chlorophyll and converted into chemical energy. This energy is then stored in two molecules: ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).
These energy-rich molecules, ATP and NADPH, are then transported from the thylakoid membranes to the stroma, where the Calvin Cycle occurs. ATP provides the necessary chemical energy to power the reactions of the Calvin Cycle, while NADPH supplies the high-energy electrons required for the reduction steps in sugar synthesis. Therefore, even though the Calvin Cycle does not directly use light, it cannot proceed without the continuous supply of ATP and NADPH generated by the light-dependent reactions. This flow of energy and reducing power from the first phase to the second highlights their symbiotic relationship, allowing for the complete process of photosynthesis to synthesize organic molecules from carbon dioxide.