Photosynthesis is the process by which plants, algae, and certain bacteria convert sunlight, water, and carbon dioxide into sugar and oxygen. It is an endothermic reaction, meaning it requires energy input (light) to proceed. Virtually all life on Earth depends on it, either directly or indirectly, because it produces both the food that fuels ecosystems and the oxygen that most organisms breathe.
The Basic Chemical Equation
The overall reaction is straightforward: six molecules of carbon dioxide plus six molecules of water, powered by light energy, yield one molecule of glucose (a simple sugar) and six molecules of oxygen. Written out, that’s 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Carbon dioxide and water are the reactants. Glucose and oxygen are the products. Light energy is what drives the entire reaction forward.
Where It Happens Inside the Cell
Photosynthesis takes place inside chloroplasts, specialized structures found in plant and algae cells. A chloroplast has three key compartments. The thylakoid membranes are stacked into coin-like columns called grana, and this is where light energy is captured. The stroma is the fluid surrounding those stacks, where carbon dioxide gets converted into sugar. The thylakoid lumen is the interior space within the thylakoid membranes, where protons accumulate to help generate chemical energy.
The Two Stages of Photosynthesis
Light-Dependent Reactions
These occur in the thylakoid membranes and require direct sunlight. Chlorophyll and other pigments absorb light energy and use it to split water molecules. This splitting releases oxygen as a waste product (the oxygen you breathe). The energy extracted from light gets stored in two chemical carriers, ATP and NADPH, which act like rechargeable batteries for the next stage.
The Calvin Cycle (Light-Independent Reactions)
This stage takes place in the stroma and does not require light directly, though it depends on the ATP and NADPH produced by the light-dependent reactions. During the Calvin cycle, carbon dioxide from the air is “fixed,” meaning it gets incorporated into an organic molecule. The enzyme responsible for this step, called RuBisCO, is the most abundant protein on the planet. More than 90% of the inorganic carbon converted into living matter passes through this single enzyme. The end product is a three-carbon sugar that the plant uses to build glucose, starch, and other carbohydrates.
Why Plants Are Green
Chlorophyll absorbs red and blue wavelengths of light most effectively while reflecting green wavelengths (500 to 570 nm). That reflected green light is what your eyes see. Plants also contain accessory pigments called carotenoids, which absorb wavelengths shorter than about 520 nm (blue and some green). Carotenoids serve double duty: they pass some absorbed energy to chlorophyll for photosynthesis and they protect the cell from damage caused by excess light energy, dissipating the surplus as heat.
Not Just Plants
Plants are the most familiar photosynthesizers, but they are far from the only ones. Algae, ranging from single-celled species to giant kelp, perform photosynthesis using the same chlorophyll-based system. Cyanobacteria, sometimes called blue-green algae, are ancient single-celled organisms that photosynthesize in essentially the same way plants do. In fact, the chloroplasts inside plant cells originally evolved from cyanobacteria that were engulfed by early cells billions of years ago.
Some bacteria perform a different version called anoxygenic photosynthesis. Instead of splitting water and releasing oxygen, these organisms use other substances as their energy source: hydrogen sulfide, hydrogen gas, or organic compounds. Purple sulfur bacteria, green sulfur bacteria, and heliobacteria all fall into this category. They use a pigment called bacteriochlorophyll instead of chlorophyll and thrive in environments like hot springs, deep ocean vents, and stagnant ponds where oxygen is scarce.
Three Ways Plants Handle Carbon
Not all plants photosynthesize identically. There are three main strategies, each adapted to different climates.
- C3 plants use the standard Calvin cycle. RuBisCO fixes carbon dioxide directly, producing a three-carbon molecule. This works well in cool, moist environments but becomes inefficient in heat and drought. Under those conditions, RuBisCO mistakenly grabs oxygen instead of carbon dioxide, triggering a wasteful process called photorespiration that can reduce photosynthetic efficiency by up to 40%.
- C4 plants evolved an extra step to solve that problem. They first fix carbon dioxide into a four-carbon molecule in one type of cell, then shuttle it to a second cell type where it gets released directly to RuBisCO at high concentrations. This suppresses photorespiration and makes C4 plants dominant in hot, sunny, tropical grasslands. Corn and sugarcane are C4 plants.
- CAM plants take a different approach entirely. They open their pores at night to collect carbon dioxide (minimizing water loss in the cool dark) and store it as an acid. During the day, they close their pores and release the stored carbon dioxide internally for the Calvin cycle. Cacti, pineapples, and many succulents use CAM photosynthesis.
Efficiency of Energy Conversion
Plants convert a surprisingly small fraction of the sunlight they receive into usable chemical energy. The theoretical maximum efficiency is about 9.4% for C3 plants and 12.3% for C4 plants under optimal conditions. In practice, actual crops fall well below even half of those theoretical limits. Corn, one of the most productive food crops, averages about 4.9% efficiency. Most food crops perform worse than that. The gap between theoretical and real-world efficiency is one reason researchers are interested in improving photosynthetic performance to increase crop yields.
Global Oxygen Production
Roughly half of all the oxygen produced on Earth comes from the ocean, not from forests. Marine phytoplankton, tiny photosynthetic organisms drifting near the surface, are responsible for this enormous output. One genus of cyanobacteria alone, Prochlorococcus, produces up to 20% of all the oxygen in the biosphere, more than all tropical rainforests combined. The other half comes from terrestrial plants, with tropical and boreal forests contributing the largest shares on land.