Photosynthesis requires three main substances: carbon dioxide, water, and light energy. These are the raw inputs that plants use to produce sugar and oxygen. The overall reaction is straightforward: six molecules of carbon dioxide combine with six molecules of water, powered by light, to create one molecule of glucose and six molecules of oxygen. But the full picture includes several supporting players, from the green pigment that captures light to minerals pulled from the soil.
Carbon Dioxide: The Carbon Source
Carbon dioxide supplies the carbon atoms that become sugar. It enters leaves through stomata, microscopic pores on the leaf surface formed by pairs of specialized guard cells. These pores open and close in response to light intensity, humidity, temperature, and water availability, letting the plant balance its need for carbon dioxide against the risk of losing too much water through evaporation.
Once inside, carbon dioxide diffuses through air spaces within the leaf and reaches the chloroplasts, where an enzyme called RuBisCO attaches it to an existing five-carbon molecule. That molecule splits into two three-carbon compounds, which are then rebuilt into glucose through a series of steps known as the Calvin cycle. RuBisCO is responsible for fixing more than 90% of the inorganic carbon that ends up as living biomass on Earth.
The concentration of carbon dioxide in outdoor air is currently around 400 parts per million. Photosynthesis rates increase as that concentration rises, typically reaching a saturation point near 1,000 ppm, beyond which adding more carbon dioxide doesn’t help. In enclosed greenhouses during winter, heavy plant use can drop levels to around 200 ppm, which noticeably slows growth.
Water: The Electron Donor
Water plays a less obvious but essential role. Plants absorb it through their roots and transport it to the leaves, where it participates directly in the light-dependent reactions of photosynthesis. Inside the chloroplast, water molecules are split apart in a process called photolysis. This splitting releases oxygen (the oxygen you breathe is a byproduct of water being torn apart), donates electrons to power the photosynthetic machinery, and provides hydrogen ions that help drive energy production.
The water-splitting step happens at a cluster containing four manganese atoms and one calcium atom, embedded in a protein complex called Photosystem II. Four electrons are removed sequentially from this cluster, building up enough oxidizing power to break the strong bonds holding water together. It takes four light-driven cycles to extract the four electrons needed from two water molecules, releasing one molecule of oxygen in the process.
Light: The Energy That Drives It All
Light is not a chemical substance, but it is an absolute requirement. It provides the energy that powers every other step. Chlorophyll, the green pigment in leaves, absorbs light primarily in two ranges: blue light between 400 and 500 nanometers and red light between 650 and 680 nanometers. Green wavelengths are mostly reflected, which is why plants look green.
Chlorophyll works across a broader window of roughly 400 to 700 nanometers. When it absorbs a photon, the energy quickly funnels down to a lower energy state (equivalent to red light) regardless of the original wavelength absorbed. That energy then drives the transfer of electrons through two protein complexes, Photosystem II and Photosystem I, which together produce two critical energy-carrying molecules: ATP and NADPH. These act as the battery packs for the Calvin cycle, providing the energy and electrons needed to convert carbon dioxide into sugar.
As light intensity increases, the rate of photosynthesis rises until it hits a saturation point where the plant can’t process photons any faster. That saturation point shifts depending on carbon dioxide availability. Higher carbon dioxide concentrations let plants use stronger light more effectively.
Chlorophyll and Its Magnesium Core
Chlorophyll itself is built from specific substances the plant must obtain. At the center of every chlorophyll molecule sits a magnesium ion, chelated within a ring-shaped structure called a porphyrin. This magnesium flattens and stabilizes the molecule’s shape, directly influencing how it absorbs light. Without adequate magnesium from the soil, plants cannot build chlorophyll, and leaves turn yellow.
Attached to the ring is phytol, a long carbon chain that anchors the pigment into the membrane of the chloroplast. Removing either the magnesium or the phytol tail makes the molecule floppy and less functional. So while magnesium is a trace element, it is structurally non-negotiable for photosynthesis.
Minerals That Support the Process
Beyond magnesium, several other soil nutrients play direct roles in photosynthesis. Nitrogen is a major component of the chlorophyll molecule itself, so nitrogen-deficient plants quickly lose their ability to capture light. Iron is essential for the heme-containing enzymes and for ferredoxin, a protein involved in electron transfer during the light reactions. Manganese, as mentioned, sits at the heart of the water-splitting complex in Photosystem II, where it participates directly in photolysis.
These minerals are needed in relatively small amounts compared to water and carbon dioxide, but their absence creates bottlenecks. A plant with plenty of sunlight, water, and carbon dioxide will still photosynthesize poorly if its soil lacks iron or manganese, because the molecular machinery that handles electrons simply cannot be built without them.
How the Substances Work Together
Photosynthesis happens in two linked stages. In the first stage (the light reactions), chlorophyll captures light energy and uses it to split water, releasing oxygen and generating ATP and NADPH. In the second stage (the Calvin cycle), those energy carriers power the conversion of carbon dioxide into glucose. The two stages depend on each other completely: without light, there is no ATP or NADPH; without carbon dioxide, those energy carriers have nothing to build.
The substances needed for photosynthesis, then, fall into three categories. The primary reactants are carbon dioxide and water. The energy input is light. And the supporting cast includes magnesium, nitrogen, iron, and manganese, all of which are woven into the enzymes and pigments that make the reaction possible. Remove any one of these, and the process slows or stops entirely.