Organisms that make their own food through photosynthesis are called autotrophs, or more specifically, photoautotrophs. This group includes all green plants, algae, and certain bacteria. They convert light energy, water, and carbon dioxide into sugar and oxygen, forming the energy foundation for nearly every ecosystem on Earth.
How Photosynthesis Produces Food
The basic recipe is simple: six molecules of carbon dioxide plus six molecules of water, powered by light, yield one molecule of sugar and six molecules of oxygen. The sugar fuels the organism’s growth, repair, and reproduction. The oxygen is released as a byproduct, which is why photosynthetic life is responsible for the breathable atmosphere we depend on.
The key ingredient that makes this possible is chlorophyll, a green pigment that absorbs light most efficiently in two bands: blue light (400 to 500 nanometers) and red light (650 to 680 nanometers). Green wavelengths bounce off, which is why leaves and algae look green to our eyes. Chlorophyll sits inside tiny cellular structures called chloroplasts, which act as miniature solar panels.
Plants: From Mosses to Towering Trees
Every plant you can picture, from a backyard dandelion to a redwood, makes its own food through photosynthesis. But the group extends far beyond familiar flowering plants. Mosses, liverworts, and hornworts are ancient, non-vascular plants that photosynthesize without roots, stems, or the internal plumbing that moves water through larger plants. About 20,000 species of these small, ground-hugging organisms thrive everywhere from tropical forests to arctic tundra.
In larger plants, photosynthesis happens primarily in leaves. Tiny pores called stomata open to let carbon dioxide in and release oxygen. Beneath the surface, spongy mesophyll cells are arranged with air pockets between them so that gas can flow freely. Each stomatal pore must sit directly above these air pockets, not over solid cells, to form a working channel. The density and size of stomata, along with the internal architecture of the mesophyll, determine how efficiently a leaf can exchange gases and capture carbon dioxide.
Algae: The Ocean’s Food Factories
Algae are not plants, but they photosynthesize using the same basic chemistry. They range from single-celled organisms invisible to the naked eye to giant kelp stretching dozens of meters. In the ocean, microscopic floating algae collectively called phytoplankton are enormously productive. Major groups include diatoms, which are found in both saltwater and freshwater environments worldwide, dinoflagellates that dominate open ocean waters, and green algae that thrive in freshwater and estuaries.
NOAA estimates that roughly half of all oxygen production on Earth comes from the ocean, driven largely by these microscopic photosynthesizers. One bacterial species alone, Prochlorococcus (technically a cyanobacterium, not an alga), produces up to 20% of the oxygen in the entire biosphere. That single microbe outperforms all the world’s tropical rainforests combined.
Cyanobacteria: The Original Photosynthesizers
Cyanobacteria are bacteria, not plants or algae, yet they perform full oxygen-producing photosynthesis. They were likely doing so at least 2.6 to 2.7 billion years ago, based on chemical signatures found in ancient rock. That timing matters because it slightly predates the first major rise of oxygen in Earth’s atmosphere, about 2.3 billion years ago. In other words, cyanobacteria are almost certainly the organisms that made our planet’s air breathable in the first place.
They are also the evolutionary ancestors of chloroplasts. At some point in deep history, a larger cell engulfed a cyanobacterium, and instead of digesting it, the two formed a partnership. That ancient cyanobacterium eventually became the chloroplast found inside every plant and algal cell today. Four other groups of bacteria also perform photosynthesis (purple bacteria, heliobacteria, green sulfur bacteria, and green non-sulfur bacteria), but none of them produce oxygen. Only cyanobacteria split water molecules to release oxygen the way plants do.
Why Photosynthetic Organisms Matter
Photosynthetic organisms are called primary producers because they form the base of virtually every food chain. They capture light energy and lock it into sugar molecules, creating the only entry point for new energy in most ecosystems. Herbivores eat them directly. Predators eat the herbivores. Decomposers break down what’s left. Without primary producers, no other organisms that can’t capture light energy on their own would have access to food.
This role is hard to overstate. The total amount of organic material that photosynthetic organisms build each year, called primary production, sets the upper limit on how much life an ecosystem can support. A desert with sparse plant cover supports fewer animals than a rainforest. A nutrient-poor stretch of open ocean with little phytoplankton supports far less marine life than a coastal upwelling zone teeming with microscopic algae.
Plants That Lost the Ability
Not every plant photosynthesizes. A small number of species have abandoned the process entirely and become parasites. Beechdrops, for instance, has no leaves and no chlorophyll. It survives by tapping into the roots of beech trees using specialized organs called haustoria, which penetrate the host’s tissues and siphon off water and nutrients.
Indian pipe (sometimes called ghost plant or corpse plant) takes the arrangement a step further. It is completely white, lacking any pigment, and parasitizes fungi that are themselves partnered with trees. So the Indian pipe is ultimately feeding off a tree, but through a fungal middleman. These exceptions are rare, and they highlight just how unusual it is for a plant lineage to give up its defining trick. Photosynthesis is so efficient that the vast majority of plants have kept it running for hundreds of millions of years.