Photosynthesis is the fundamental process by which photopigment-bearing organisms convert light energy into chemical energy, using carbon dioxide and water to produce sugars and oxygen. Many people associate this process almost exclusively with trees and flowers, leading to the common misconception that only plants possess this ability. The direct answer to whether only plants photosynthesize is no. A wide array of other organisms, from single-celled microbes to complex sea creatures, have independently evolved or acquired this energy-capturing mechanism. This biological innovation is spread across multiple domains of life.
The Standard Model: Photosynthesis in Plants
The mechanism in terrestrial plants serves as the template for oxygen-producing photosynthesis. Within the plant cells, the process is compartmentalized into specialized organelles known as chloroplasts. These structures house the green pigment chlorophyll, which absorbs light energy, particularly in the blue and red wavelengths.
The absorbed light energy is channeled through Photosystem II and Photosystem I, which operate sequentially. In Photosystem II, water molecules are split, releasing gaseous oxygen as a byproduct. The energy from the electrons is then used to generate adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). The Calvin cycle, which occurs in the stroma of the chloroplast, uses the ATP and NADPH to fix carbon dioxide into glucose, the plant’s primary energy storage molecule.
Photosynthesis in Protists and Algae
The largest group of photosynthesizers is found in aquatic environments: the protists and algae. This diverse collection includes massive kelp, microscopic diatoms, and motile euglenoids. They are all eukaryotes and utilize chloroplasts and chlorophyll in a manner structurally similar to land plants. They lack the complex tissues, roots, stems, and leaves characteristic of terrestrial flora.
Algae and other photosynthetic protists are ecological powerhouses, often contributing significantly more to global oxygen production than all terrestrial plants combined. Microscopic phytoplankton, which are primary producers in the ocean, use the same chlorophyll-based oxygenic process to convert solar energy and dissolved carbon dioxide into organic matter. This marine photosynthesis is responsible for generating an estimated 50 to 80 percent of the oxygen in Earth’s atmosphere.
Photosynthesis in Bacteria: Prokaryotic Variation
Photosynthesis also occurs in the prokaryotic domain of bacteria, where the process exhibits fundamental differences from the plant model. Some bacteria, like cyanobacteria, perform oxygenic photosynthesis exactly like plants, utilizing chlorophyll and releasing oxygen from water. Cyanobacteria are believed to be the ancestors of plant chloroplasts and were responsible for the “Great Oxygenation Event” that changed Earth’s atmosphere billions of years ago.
Other bacterial groups, such as the purple and green sulfur bacteria, perform anoxygenic photosynthesis, which does not produce oxygen. Instead of using water as an electron donor, these microbes use compounds like hydrogen sulfide or organic molecules, releasing elemental sulfur or sulfate. They employ pigments called bacteriochlorophylls, which are chemically distinct from plant chlorophyll and absorb light in different regions, often the infrared. Since bacteria lack chloroplasts, the photosynthetic pigments and reaction centers are embedded directly within the folds of the cell membrane, called chromatophores or thylakoids.
Borrowing the Power: Kleptoplasty and Symbiosis
The ability to photosynthesize can be acquired through biological partnerships, either temporary or permanent. Symbiosis is a long-term relationship where a non-photosynthetic host harbors a photosynthetic partner. A well-known example is the relationship between corals and zooxanthellae, which are single-celled algae that live within the coral tissue. The algae provide the coral with up to 90 percent of its energy needs in the form of sugars, while the coral provides the algae with protection and carbon dioxide.
A more surprising method is kleptoplasty, meaning “stolen plastids,” where an organism physically steals photosynthetic machinery from its food. The sea slug Elysia chlorotica feeds on a specific alga, ingests the functional chloroplasts, and then integrates them into the cells lining its digestive tract. The slug can retain these stolen organelles for weeks or months, using them to capture solar energy and produce its own food. This temporary, borrowed ability allows the slug to function as a solar-powered animal.