Plants get their energy from sunlight. Through photosynthesis, they capture light and convert it into chemical energy stored in sugars, which they then use to fuel every process in their bodies. It’s a two-stage system: first, light energy is captured and converted into a usable chemical form, then that chemical energy is used to build sugar molecules from carbon dioxide and water.
How Plants Capture Sunlight
The process starts inside specialized cell structures called chloroplasts, which contain stacks of membrane discs called thylakoids. These membranes are packed with chlorophyll, the green pigment responsible for absorbing light. Plants use light in the 400 to 700 nanometer wavelength range, which covers most of the visible spectrum from violet through red. This band of usable light is known as photosynthetically active radiation, or PAR.
When a photon of light hits a chlorophyll molecule, it excites an electron to a higher energy state. That excited electron gets passed along a chain of carrier molecules embedded in the thylakoid membrane, much like a bucket brigade. As the electron moves through the chain, its energy is gradually harvested to produce two key energy-carrying molecules: ATP (the cell’s universal energy currency) and NADPH (an electron carrier). These two molecules are the direct chemical products of captured sunlight, and they power the next stage of the process.
To keep this system running, the plant needs a constant supply of fresh electrons. It gets them by splitting water molecules. This is why plants need water for more than just staying hydrated. The splitting of water releases oxygen as a byproduct, which is the oxygen you breathe.
Turning Carbon Dioxide Into Sugar
With ATP and NADPH in hand, the plant moves to the second stage of photosynthesis: building sugar. This happens in the fluid-filled space inside the chloroplast called the stroma, through a series of reactions known as the Calvin cycle.
Carbon dioxide enters the leaf through tiny pores called stomata and diffuses into the stroma. There, an enzyme called RuBisCO grabs a molecule of CO₂ and attaches it to an existing five-carbon molecule. The resulting six-carbon compound immediately splits into two three-carbon molecules. The ATP and NADPH produced during the light-capturing stage then supply the energy to convert these three-carbon molecules into a slightly different three-carbon compound called G3P.
For every six molecules of CO₂ the plant fixes, it produces enough G3P to assemble one molecule of glucose (C₆H₁₂O₆). That means the Calvin cycle must turn six times to make a single sugar molecule. The remaining G3P molecules are recycled to keep the cycle running. Glucose is the plant’s primary energy storage molecule, and it can be converted into starch for longer-term storage or broken down immediately for fuel.
How Plants Use Stored Energy at Night
Photosynthesis only works when light is available. At night, plants rely on the sugars and starches they built during the day. They break these down through cellular respiration, the same basic process animals use to extract energy from food. Mitochondria inside plant cells break apart sugar molecules, releasing the stored chemical energy as ATP.
During daylight hours, plants typically produce more sugar than they immediately need, and the surplus gets stored as starch in the chloroplasts. When darkness falls, this starch is broken down into simpler sugars, which enter glycolysis and then mitochondrial respiration to generate the ATP the plant needs for growth, repair, and maintenance overnight. This is why a plant that gets too little light will eventually starve, even if it has plenty of water and nutrients. Without enough sunlight hours to build a surplus, the plant can’t sustain itself through the dark period.
How Efficient Is the Process?
Despite being the foundation of nearly all life on Earth, photosynthesis is not particularly efficient by engineering standards. Typical land plants convert only about 3% to 6% of the total solar energy they receive into biomass. Much of the incoming sunlight is the wrong wavelength, reflected away, or lost as heat during the chemical conversion steps. Still, this modest efficiency is enough to power entire ecosystems, from forests to ocean phytoplankton.
Plants That Get Energy Without Sunlight
A small number of plants have evolved to cheat the system entirely. Myco-heterotrophic plants, like ghost pipes (the pale, waxy wildflowers sometimes found on forest floors), have lost the ability to photosynthesize. Instead, they tap into underground fungal networks to steal carbon and nutrients. Some of these plants parasitize fungi that are connected to the roots of nearby photosynthetic trees, effectively siphoning sugar that a tree produced and shared with its fungal partners. Others recruit free-living fungi that decompose dead organic matter. These plants are diverse and ancient lineages found across multiple plant families, including orchids and heaths.
Carnivorous plants like Venus flytraps take a different approach but still rely on photosynthesis for their core energy supply. The insects they catch provide mainly nitrogen and phosphorus, nutrients that are scarce in the boggy, acidic soils where these plants grow. Recent metabolic research on Venus flytraps shows that carbon from digested prey is burned immediately through respiration to power the energy-intensive process of producing digestive enzymes, rather than being incorporated into the plant’s own tissues. The organic nitrogen from prey gets redistributed throughout the plant to support the growth of new traps. So even carnivorous plants are fundamentally solar-powered; the insects are a nutritional supplement, not a primary energy source.