Photosynthesis is the process by which plants convert light energy into chemical energy. This biological process occurs in two main phases: the light-dependent reactions and the light-independent reactions, often called the Calvin cycle. The light-dependent reactions capture energy from sunlight and convert it into chemical energy as adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). These energy-carrying molecules then power the light-independent reactions, which produce sugars. This article focuses on the synthesis of NADPH during the light-dependent reactions.
The Photosynthesis Powerhouse
The light-dependent reactions occur within chloroplasts, specifically on the thylakoid membranes. These internal membrane systems within the chloroplasts are arranged into stacks of discs called grana. Embedded within these membranes are chlorophyll and other light-absorbing pigments, organized into protein complexes known as photosystems.
Two primary photosystems, Photosystem II (PSII) and Photosystem I (PSI), work in sequence to capture light energy. When light energy strikes these pigments, it excites electrons within them. Water molecules serve as the initial source of electrons for PSII, undergoing a splitting process that releases electrons, protons (H+), and oxygen gas as a byproduct.
Electron Flow and Energy Capture
Once electrons are excited in Photosystem II (PSII), they begin a journey along an electron transport chain (ETC). These electrons are then passed through a series of protein complexes between PSII and Photosystem I (PSI). As these electrons move along the ETC, they gradually lose energy.
A portion of the energy released from the electron movement is used to actively pump protons (H+) from the stroma, the fluid-filled space within the chloroplast, into the thylakoid lumen. This pumping action creates a concentration gradient of protons across the thylakoid membrane, with a higher concentration of protons accumulating inside the lumen. This proton gradient stores potential energy, which is subsequently harnessed by an enzyme called ATP synthase to produce ATP through a process known as chemiosmosis. The electrons, having lost some energy, eventually reach Photosystem I, where they are re-energized by absorbing additional light photons.
The Creation of NADPH
After being re-energized at Photosystem I, the high-energy electrons continue their path towards the final steps of NADPH formation. These electrons are transferred to a small protein called ferredoxin (Fd). Ferredoxin then passes these electrons to an enzyme known as NADP+ reductase, also referred to as ferredoxin-NADP+ reductase (FNR).
NADP+ reductase plays a direct role in catalyzing the reduction of NADP+ to NADPH. This enzymatic reaction involves the transfer of two electrons and one proton (H+) from the surrounding environment to each molecule of NADP+, converting it into its reduced form, NADPH. This process effectively captures the energy from the excited electrons in a stable chemical form, creating a crucial energy carrier molecule for the subsequent stage of photosynthesis.
NADPH’s Crucial Role
NADPH generated during the light-dependent reactions, alongside ATP, serves as an energy carrier for the plant. Its primary function is to provide reducing power, in the form of high-energy electrons, for the light-independent reactions, specifically the Calvin cycle. In the Calvin cycle, NADPH contributes electrons that help convert carbon dioxide into glucose, which is the plant’s food source. Without sufficient NADPH, the plant cannot efficiently convert carbon dioxide into sugars, impacting its growth and survival.