Plants are autotrophs. They produce their own food by converting sunlight, water, and carbon dioxide into sugar through photosynthesis. This makes them fundamentally different from animals, fungi, and other organisms that must consume food to survive. That said, the plant kingdom includes some fascinating exceptions where certain species have partially or fully abandoned photosynthesis and live as heterotrophs, feeding off other organisms.
How Plants Make Their Own Food
Photosynthesis happens in two stages. In the first, pigments called chlorophylls absorb sunlight and use that energy to split water molecules, releasing oxygen as a byproduct and storing the captured energy in chemical form. In the second stage, the plant uses that stored energy to pull carbon dioxide from the air and convert it into glucose, a simple sugar that fuels growth and metabolism. The entire process essentially turns light, air, and water into food.
This ability to build organic matter from inorganic ingredients is what defines an autotroph. Plants, algae, and certain bacteria all share this trait. Because they generate energy without consuming other living things, autotrophs sit at the base of nearly every food chain on Earth. Herbivores eat plants, carnivores eat herbivores, and so on. Without autotrophs producing that first round of energy, the rest of the chain collapses.
Autotrophs vs. Heterotrophs
The distinction comes down to where an organism gets its carbon and energy. Autotrophs capture energy from sunlight (or, in some bacteria, from chemical reactions) and build their own organic molecules from carbon dioxide. Heterotrophs cannot do this. They have to eat other organisms, whether plants, animals, or decomposing matter, to obtain both energy and carbon.
Heterotrophs lack chlorophyll and cannot run photosynthesis. They occupy the second and third levels of a food chain as consumers: herbivores eating plants, carnivores eating herbivores, omnivores eating both, and detritivores breaking down dead material. Every animal, every fungus, and most bacteria fall into this category.
Parasitic Plants That Live as Heterotrophs
Not every plant follows the textbook rule. Holoparasitic plants have completely lost the ability to photosynthesize and instead steal all their water and nutrients from a host plant. They are, by any functional definition, heterotrophs.
Dodder is one of the most widespread examples. It wraps around the stems of crops like sugar beet, onion, and citrus, tapping directly into the host’s vascular system. Broomrapes take a different approach, attaching to the roots of legumes, tomatoes, sunflowers, and potatoes underground where they’re invisible until they flower. These parasites cause serious agricultural losses across the Mediterranean, Asia, and beyond. Another genus, Cytinus, parasitizes shrubs in the rockrose family across the Mediterranean basin, producing small flowers that emerge directly from the host’s roots.
What unites all holoparasites is that they’ve lost functional chlorophyll. They can’t make sugar from sunlight. They depend entirely on another plant to survive, making them obligate heterotrophs despite belonging to the plant kingdom.
Carnivorous Plants Blur the Line
Venus flytraps, sundews, and pitcher plants still photosynthesize, but they also digest insects. For a long time, scientists assumed the prey only provided minerals like nitrogen and phosphorus, nutrients that are scarce in the boggy, acidic soils where these plants typically grow. The plants were still considered autotrophs because they made their own sugar from sunlight.
Recent isotope-tracing studies tell a more complex story. Researchers have shown that Venus flytraps directly absorb organic carbon from digested prey and use it to fuel their metabolism. Carnivorous plants also tend to photosynthesize at lower rates than non-carnivorous species, and genetic analysis reveals that carnivorous lineages have progressively lost genes involved in photosynthesis. This combination of reduced photosynthetic ability and direct carbon uptake from prey has led some researchers to classify certain carnivorous plants as mixotrophs, organisms that blend autotrophic and heterotrophic nutrition. Carnivory has evolved independently at least 12 separate times in flowering plants, suggesting it’s a reliable survival strategy when soil nutrients are extremely limited.
Plants That Feed Through Fungi
Over 400 species of plants have evolved to extract carbon and nutrients not from sunlight or a host plant, but from fungi. These are called myco-heterotrophs, and they exploit the underground fungal networks that normally help trees and other plants share resources.
In deeply shaded forest floors where sunlight barely reaches the ground, some plants have essentially given up on photosynthesis and instead tap into fungi connected to the roots of nearby trees. The fungus channels sugars from photosynthetic trees to the non-photosynthetic plant, making the myco-heterotroph a kind of indirect parasite. These plants span at least ten different flowering plant families, including orchids, gentians, and heaths. Indian pipe (Monotropa) is one of the most recognizable: a ghostly white plant with no chlorophyll that grows in dark forests across North America. Its color alone signals that it has nothing to do with photosynthesis.
Some species are only partially myco-heterotrophic, supplementing their photosynthesis with carbon stolen from fungi. Others have abandoned photosynthesis entirely. Scientists can distinguish them by measuring carbon and nitrogen isotopes in their tissues. Fully myco-heterotrophic plants show significantly different isotope signatures compared to their autotrophic neighbors, confirming that their carbon comes from fungi rather than from the air.
Why Plants Are Still Classified as Autotrophs
The plant kingdom is formally defined as a group of photosynthetic autotrophs with shared features: embryo production, chloroplasts, and cell walls made primarily of cellulose. The parasitic and myco-heterotrophic species evolved from photosynthetic ancestors and retain enough other plant characteristics to stay in the kingdom, even though they’ve individually lost the trait that defines the group as a whole.
Think of it this way: autotrophy is the default setting for plants, and the vast majority of the roughly 380,000 known plant species run on photosynthesis. The few hundred that have gone fully heterotrophic represent evolutionary outliers, lineages that found alternative survival strategies in extreme environments like nutrient-poor bogs, dense forest understories, or as specialized parasites on crops. They’re genuine exceptions, but they don’t change the rule. If someone asks whether plants are autotrophs or heterotrophs, the answer is autotrophs, with a handful of remarkable asterisks.