Photosynthesis is the process by which plants, algae, and some bacteria transform light energy into chemical energy. This conversion involves steps that provide energy for life on Earth. Organisms capable of photosynthesis convert atmospheric carbon dioxide and water into energy-rich organic compounds, primarily sugars, and release oxygen as a byproduct. This mechanism underpins most food webs and maintains atmospheric oxygen levels.
Capturing Light Energy
The initial step in photosynthesis involves the absorption of light energy by pigment molecules within the thylakoid membranes of chloroplasts. Chlorophyll, the most abundant green pigment, absorbs blue-violet and red light, reflecting green light, which makes plants appear green. Other pigments, like carotenoids, also absorb light at different wavelengths and transfer this energy to chlorophyll.
When a photon strikes a pigment molecule, the absorbed energy excites an electron to a higher energy state. This excited electron is unstable. To prevent energy loss, pigment molecules are organized into photosystems.
Two main photosystems are involved: Photosystem II (PSII) and Photosystem I (PSI), named in order of discovery, though PSII functions first. Each photosystem has an antenna complex, a collection of pigment molecules that gather light energy and funnel it to a central reaction center. At the reaction center, special chlorophyll a molecules receive the energy, exciting their electrons, which are then passed to a primary electron acceptor, initiating the energy transfer chain.
To replace electrons lost from chlorophyll in Photosystem II, water molecules are split in a process called photolysis. This reaction yields electrons, which replenish the photosystem, along with protons (hydrogen ions) and oxygen (O₂), which is released into the atmosphere as a byproduct.
The Electron Transport Chain and Energy Carriers
After light capture and electron excitation, these high-energy electrons move through an electron transport chain. This chain consists of protein complexes embedded within the thylakoid membrane. As electrons move along this chain, they gradually release energy.
The energy released by the moving electrons pumps protons (hydrogen ions) from the stroma, the fluid-filled space outside the thylakoids, into the thylakoid lumen, the space inside the thylakoids. This creates a high concentration of protons within the thylakoid lumen, establishing a proton gradient across the thylakoid membrane. This gradient represents stored potential energy.
The accumulated protons in the thylakoid lumen then flow back into the stroma, moving down their concentration gradient through ATP synthase. This movement drives the synthesis of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate. This process, known as chemiosmosis, is a primary mechanism for generating chemical energy.
After the electron transport chain, electrons are re-energized by Photosystem I, which also absorbs light. These re-energized electrons are then transferred to NADP+ (nicotinamide adenine dinucleotide phosphate). NADP+ accepts electrons and hydrogen ions to form NADPH, another energy-carrying molecule. Both ATP and NADPH are used in the subsequent stage of photosynthesis, serving as immediate chemical energy carriers produced during the light-dependent reactions.
Building Sugars with Stored Energy
The chemical energy now stored in ATP and NADPH is subsequently used to synthesize glucose and other carbohydrate molecules in a series of reactions known as the Calvin cycle. These reactions occur in the stroma of the chloroplast, the fluid-filled region surrounding the thylakoid membranes, and are often referred to as light-independent reactions because they do not directly require sunlight. While light is not directly needed, the Calvin cycle relies on the short-lived ATP and NADPH produced during the light-dependent reactions.
The Calvin cycle begins with carbon fixation, a process where carbon dioxide (CO₂) from the atmosphere is incorporated into organic molecules. This initial step is catalyzed by the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase). RuBisCO combines a molecule of CO₂ with a five-carbon sugar called ribulose-1,5-bisphosphate (RuBP), forming an unstable six-carbon compound that quickly splits into two molecules of a three-carbon compound called 3-phosphoglycerate (3-PGA).
In the next phase, reduction, the 3-PGA molecules are converted into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar. This conversion requires energy supplied by ATP and electrons provided by NADPH, both generated during the light reactions. ATP provides the necessary chemical energy, while NADPH provides the reducing power by donating electrons and hydrogen ions. For every three molecules of CO₂ incorporated, there is a net gain of one G3P molecule that can be used to build sugars.
A significant portion of the G3P molecules produced does not immediately exit the cycle to form sugars. Instead, five out of every six G3P molecules are used to regenerate the starting molecule, RuBP. This regeneration also requires energy, which is supplied by ATP. The continuous regeneration of RuBP ensures that the Calvin cycle can proceed uninterrupted as long as ATP, NADPH, and CO₂ are available. The glucose and other carbohydrates formed from G3P represent the stable, long-term storage of the light energy initially captured, providing the energy source for the plant and, indirectly, for most other life forms on Earth.