The energy source for photosynthesis is light, primarily from the sun. Plants, algae, and some bacteria capture particles of light called photons and convert that energy into the chemical energy stored in sugars. A 2023 study published in Nature confirmed what scientists had long suspected: a single photon is enough to kick-start the photosynthetic reaction. Every photosynthetic organism on Earth, from oak trees to ocean algae, relies on this same fundamental energy input.
How Plants Capture Light Energy
Sunlight is a mixture of wavelengths, but plants don’t use all of them equally. The useful range, called photosynthetically active radiation (PAR), spans wavelengths from 400 to 700 nanometers, which covers violet through red on the visible spectrum. Within that range, leaves absorb red and blue light most strongly. Green light sits in the middle of the spectrum and is absorbed the least, which is why leaves look green to our eyes.
Chlorophyll, the main light-absorbing pigment, has peak absorption in the 400 to 500 nanometer range (blue-violet) and again near 680 to 700 nanometers (red). Other pigments help fill in the gaps. Carotenoids, the orange-yellow pigments found in carrots and autumn leaves, absorb purple to blue light effectively. Anthocyanins, which give some leaves a reddish tint, absorb green wavelengths between 500 and 600 nanometers. Together, this collection of pigments lets a leaf harvest energy across most of the visible spectrum.
One counterintuitive detail: although blue photons carry more energy per particle than red photons, plants convert both colors into chemical energy at the same rate per absorbed photon. The extra energy in blue light gets released as heat rather than being put to productive use.
What Happens When a Photon Hits a Leaf
When a photon strikes a chlorophyll molecule inside a leaf cell, it excites an electron, bumping it to a higher energy level. This happens in specialized protein structures called photosystems, which sit inside the membranes of chloroplasts. Plants use two photosystems working in sequence, and each one absorbs light at a slightly different peak wavelength.
In the first step, a photon excites the chlorophyll pair at the core of Photosystem II, loosening one of its electrons. That electron gets passed to a nearby molecule, creating a charge separation: one side now has extra energy, the other has a missing electron. The chlorophyll that lost its electron is now in a high-energy, electron-hungry state, and it recovers by pulling an electron from water. This is where the oxygen you breathe comes from. Water molecules are split apart to resupply the electrons that light energy knocked loose.
The energized electrons then travel through a chain of proteins, releasing energy along the way. That energy pumps protons across a membrane, building up a concentration gradient that drives the production of ATP, the cell’s energy currency. A second photon hit at Photosystem I re-energizes the electrons, which are then used to produce NADPH, another energy-carrying molecule. For every pair of electrons that passes through both photosystems, the cell generates roughly 1 to 1.5 molecules of ATP.
From Light Energy to Sugar
ATP and NADPH are not the final product. They’re the energy carriers that power the next stage, where carbon dioxide from the air is assembled into sugar molecules. This second stage, called the Calvin cycle, doesn’t require light directly. It runs on the chemical energy that the light-dependent reactions produced. Carbon dioxide enters through tiny pores on the leaf surface, and enzymes in the chloroplast stitch carbon atoms together into three-carbon sugars, which the plant later converts into glucose, starch, and other carbohydrates.
So the full energy pathway looks like this: sunlight provides photons, photons energize electrons in chlorophyll, those electrons drive the production of ATP and NADPH, and ATP and NADPH power the construction of sugar from carbon dioxide and water. The light energy is now locked in the chemical bonds of sugar, available for the plant to use or for any animal that eats it.
How Much Solar Energy Plants Actually Use
Plants are not especially efficient solar collectors. The theoretical maximum conversion of solar energy into plant biomass is about 4.6% for most common crops (C3 plants like wheat and rice) and about 6% for tropical grasses and crops like corn and sugarcane (C4 plants). In practice, the best full-season efficiencies measured in real fields are around 2.4% for C3 crops and 3.7% for C4 crops. Brief peak periods can push slightly higher, reaching 3.5% and 4.3% respectively.
Several factors explain the gap. Plants can’t use infrared or ultraviolet light, which together make up a large share of solar radiation. Some absorbed light energy is lost as heat during the electron transfer steps. And at high light intensities, the photosynthetic machinery hits a ceiling. Most plants reach their maximum processing rate somewhere between 800 and 1,600 micromoles of photons per square meter per second, depending on the species. Full midday sunlight delivers around 2,000 to 2,500 micromoles, meaning leaves are often receiving more light than they can use. The excess energy has to be safely dissipated as heat to avoid damage.
Can Artificial Light Replace Sunlight?
Yes. Plants don’t distinguish between a photon from the sun and a photon from an LED bulb. What matters is the wavelength and intensity. LED grow lights used in indoor farming can deliver photon levels four to five times greater than natural sunlight for specific colors, though plants saturate well before those extremes.
Research on different LED colors reveals some surprises. Blue light drives the most efficient conversion of photon energy into chemical energy within the chloroplast, about 35% more efficient than red, green, or white light at the molecular level. Yet blue light produces the lowest overall rate of carbon fixation. Red and green light, despite being less efficient at the initial energy conversion step, result in higher rates of sugar production. The likely explanation is that blue light diverts more energy into other plant processes, like producing protective compounds, rather than growth.
For indoor growers, this means a mix of red and blue LEDs (often supplemented with some white or green) tends to give the best balance of healthy growth and energy efficiency. The underlying energy source is still photons. The only difference is whether those photons originated from nuclear fusion in the sun or from electricity running through a diode.