What Is Photosynthesis? How Plants Turn Sunlight Into Food

The Overall Equation: Deceptively Simple

Photosynthesis can be summarized in one deceptively simple equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. In plain English: six molecules of carbon dioxide plus six molecules of water, powered by sunlight, produce one molecule of glucose (sugar) and six molecules of oxygen.

This equation hides enormous complexity. Photosynthesis involves over 100 individual chemical reactions, dozens of specialized proteins, and two distinct stages that occur in different parts of the chloroplast — the organelle (sub-cellular structure) where photosynthesis takes place. Chloroplasts are found in the cells of leaves and other green plant tissues, typically 20-100 per cell.

Chloroplasts have a fascinating evolutionary origin. They were originally free-living cyanobacteria (photosynthetic bacteria) that were engulfed by early eukaryotic cells roughly 1.5-2 billion years ago in a process called endosymbiosis. Instead of being digested, the cyanobacteria survived inside the host cell, providing it with sugars in exchange for protection. Over billions of years, they became fully integrated organelles — chloroplasts still have their own DNA, separate from the plant cell's nuclear DNA, as evidence of their bacterial ancestry.

The oxygen produced by photosynthesis is released as a waste product — plants don't need it for photosynthesis itself. Earth's atmosphere was originally almost devoid of oxygen. The Great Oxygenation Event, roughly 2.4 billion years ago, occurred when cyanobacteria had produced enough oxygen through photosynthesis to fundamentally transform the atmosphere, enabling the evolution of all oxygen-breathing life — including us.

The Light Reactions: Capturing Solar Energy

Photosynthesis occurs in two stages. The light reactions (which require sunlight) happen in the thylakoid membranes — stacked, flattened discs inside the chloroplast. The Calvin cycle (which doesn't directly require light) happens in the stroma — the fluid surrounding the thylakoids.

The light reactions begin when chlorophyll — the green pigment in plants — absorbs photons of light. Chlorophyll absorbs red and blue light most efficiently and reflects green light, which is why plants appear green. Each chlorophyll molecule is part of a photosystem — a complex of proteins and pigments that acts as a solar antenna, funneling light energy to a reaction center.

When a photon hits the reaction center, it excites an electron to a higher energy state. This energized electron is passed along an electron transport chain — a series of proteins embedded in the thylakoid membrane — releasing energy at each step. This energy is used to pump hydrogen ions (H⁺) across the membrane, creating a concentration gradient. The ions flow back through ATP synthase (a molecular turbine), generating ATP — the universal energy currency of cells.

Simultaneously, water molecules are split (photolysis): 2H₂O → 4H⁺ + 4e⁻ + O₂. This is where the oxygen comes from. The electrons replace those lost by chlorophyll, the hydrogen ions contribute to the gradient, and the oxygen is released as gas.

The light reactions also produce NADPH — another energy carrier — by transferring electrons to NADP⁺. The outputs of the light reactions (ATP and NADPH) power the next stage: the Calvin cycle.

The Calvin Cycle: Building Sugar From Air

The Calvin cycle (named after Melvin Calvin, who won the 1961 Nobel Prize for mapping it) uses the ATP and NADPH produced by the light reactions to convert carbon dioxide from the atmosphere into glucose. This process is called carbon fixation — literally fixing carbon from gas into solid organic molecules.

The cycle begins when an enzyme called RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) attaches a CO₂ molecule to a 5-carbon sugar called RuBP. RuBisCO is the most abundant protein on Earth — estimated at 700 million tonnes globally — because it's relatively slow (processing only about 3 CO₂ molecules per second) so plants compensate by producing enormous quantities of it.

The resulting 6-carbon compound immediately splits into two 3-carbon molecules (G3P). Using energy from ATP and NADPH, these are converted through a series of reactions. For every three turns of the cycle (fixing 3 CO₂), one G3P molecule is exported to be used in glucose synthesis. The remaining G3P molecules are recycled to regenerate RuBP, keeping the cycle running.

The Calvin cycle runs continuously during daylight. To produce one molecule of glucose (6 carbons), the cycle must fix 6 CO₂ molecules, requiring 6 turns. Each glucose molecule represents stored solar energy that the plant can later release through cellular respiration — or that an animal can access by eating the plant.

Why Photosynthesis Matters for Everything

Photosynthesis is the foundation of virtually all food chains on Earth. Plants (and algae and cyanobacteria) are primary producers — they convert solar energy into chemical energy that all other organisms depend on. Herbivores eat plants, carnivores eat herbivores, and decomposers break down dead organisms — but the original energy always traces back to photosynthesis.

Marine photosynthesis is often overlooked. Phytoplankton — microscopic photosynthetic organisms in the ocean — produce approximately 50% of Earth's oxygen and fix roughly 50 billion tonnes of carbon per year. The Amazon rainforest is often called 'the lungs of the Earth,' but the ocean's phytoplankton contribute equally to global oxygen production.

Fossil fuels are ancient photosynthesis. Coal, oil, and natural gas are the remains of organisms (mostly plants and marine phytoplankton) that photosynthesized millions of years ago. When we burn fossil fuels, we're releasing solar energy captured by photosynthesis 50-300 million years ago — and releasing CO₂ that was removed from the atmosphere over those same millions of years, in just a few centuries.

Artificial photosynthesis is an active area of research. Scientists are attempting to replicate photosynthesis's light-harvesting and carbon-fixing capabilities in synthetic systems to produce clean fuels directly from sunlight and CO₂. Current artificial systems are far less efficient than natural photosynthesis, but progress is accelerating. If successful, artificial photosynthesis could provide clean energy while simultaneously removing CO₂ from the atmosphere.

Sources: Blankenship, 'Molecular Mechanisms of Photosynthesis' (2021), Nobel Prize archives (Calvin, 1961), NASA Earth Observatory, Field et al. (Science, 1998), Royal Society of Chemistry.