Photosynthesis converts light energy into chemical energy, which is stored in organic compounds. Plants, algae, and certain bacteria perform this conversion, taking in water and carbon dioxide to create their own food source. This process also releases oxygen as a byproduct. Understanding this energy transformation requires examining the three distinct and highly coordinated stages.
Essential Components and Location
The inputs for photosynthesis are sunlight, water, and carbon dioxide. The entire process takes place within specialized compartments inside plant cells called chloroplasts. These organelles contain a fluid known as the stroma, which houses many necessary enzymes.
Suspended within the stroma are flattened, disc-like sacs called thylakoids, where the first two stages occur. A stack of thylakoids is referred to as a granum. The thylakoid membranes and the stroma provide the environment needed to separate the light reactions from the sugar-building reactions.
Stage 1: Capturing Light Energy
The process begins when pigments, primarily chlorophyll, absorb photons of light within the thylakoid membranes. These pigments are organized into photosystems, which act like light-harvesting antennae. When chlorophyll absorbs a photon, the energy excites one of its electrons to a higher energy level.
This energized electron is passed to a primary electron acceptor, initiating the flow of energy. To replace the lost electron, water molecules are split in a process called photolysis. The splitting of water releases protons (hydrogen ions) into the thylakoid interior and releases oxygen as a byproduct.
Stage 2: Energy Conversion
The energized electrons are channeled through the Electron Transport Chain (ETC), a series of protein complexes embedded in the thylakoid membrane. As electrons move down the ETC, they release energy used to pump protons from the stroma into the thylakoid lumen, creating a high concentration gradient. This concentration difference represents a form of potential energy.
The flow of these protons back into the stroma is harnessed by ATP synthase to synthesize adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate. This process is known as chemiosmosis. At the end of the ETC, electrons are re-energized by a second photosystem and transferred to the electron carrier NADP\(^+\), reducing it to NADPH. Both ATP and NADPH are temporary chemical energy carriers required for the final stage.
Stage 3: Building Sugars
The final stage is a light-independent process that takes place in the stroma. Known as the Calvin cycle, its purpose is to convert atmospheric carbon dioxide into stable sugar molecules. The cycle begins with carbon fixation, where an enzyme combines carbon dioxide with a five-carbon molecule called ribulose-1,5-bisphosphate (RuBP).
This immediate product splits into two molecules of a three-carbon compound. In the next phase, reduction, ATP provides the energy and NADPH supplies electrons to convert these molecules into a sugar precursor. For every six turns of the cycle, enough carbon is fixed to produce one molecule of glucose. The final phase involves the regeneration of the original RuBP molecule, a process that requires additional ATP.