Photosynthesis is the biological process by which plants, algae, and certain bacteria transform light energy into chemical energy in the form of food. This conversion process is the source of energy for almost all life on Earth, powering entire ecosystems. By taking in simple inorganic molecules and building energy-rich organic compounds, photosynthesis supports the vast majority of food webs through two interconnected stages.
The Foundational Framework: Naming the Two Stages
The entire process of photosynthesis is organized into two distinct sets of reactions, named based on their requirement for light. The first stage is the Light-Dependent Reactions, which directly requires the energy from light photons. The second stage is the Light-Independent Reactions, which does not require light directly but relies on the products generated by the first stage. This second stage is also referred to as the Calvin Cycle. Together, these two stages form a continuous process where the energy captured in the first half is used to build sugar molecules.
Stage One: Capturing Light Energy
The Light-Dependent Reactions occur within the thylakoid membranes, which are flattened sacs inside the cell’s chloroplasts. This stage begins when pigment molecules, particularly chlorophyll, absorb light energy, exciting electrons to a higher energy level. These energized electrons are then passed along an electron transport chain. Water molecules are split to replace the lost electrons, releasing oxygen gas as a byproduct into the atmosphere. The energy harvested from the flowing electrons is used to create two temporary energy-storing compounds: adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). These molecules store the energy in a chemical form for use in the next stage.
Stage Two: Building Sugars
The Light-Independent Reactions, or Calvin Cycle, take place in the stroma, the fluid-filled space surrounding the thylakoid membranes within the chloroplast. Here, the plant uses the chemical energy carriers, ATP and NADPH, produced during the first stage to construct organic molecules. The cycle begins with carbon fixation, where carbon dioxide (\(\text{CO}_2\)) from the atmosphere is incorporated into an existing five-carbon organic molecule. The enzyme Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes this initial step.
Using the stored energy from ATP and the reducing power of NADPH, the newly fixed carbon is chemically reduced and rearranged. For every three molecules of \(\text{CO}_2\) fixed, the cycle produces one molecule of glyceraldehyde-3-phosphate (G3P), a three-carbon sugar precursor. G3P is then converted into complex sugars like glucose, or used to regenerate the starting five-carbon molecule to keep the cycle running.
The Global Impact of Photosynthetic Output
The outputs of photosynthesis—sugar and oxygen—sustain nearly all biological activity on the planet. Sugars, like glucose, serve as the primary source of chemical energy and structural carbon for the plant, forming the biomass that supports terrestrial and aquatic food chains. This energy is transferred to every organism that consumes the plant, establishing the foundation of all ecosystems.
The oxygen released as a byproduct during the light-dependent stage maintains the composition of the atmosphere. This oxygen is then used by most living things to perform aerobic respiration, completing the cyclical flow of energy and matter.