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All animals, certain types of fungi, and non-photosynthesizing plants are heterotrophic. In contrast, green plants , red algae , brown algae , and cyanobacteria are all autotrophs , which use photosynthesis to produce their own food from sunlight.
If the heterotroph uses chemical energy, it is a chemoheterotroph (e.g., humans and mushrooms). If it uses light for energy, then it is a photoheterotroph (e.g., green non-sulfur bacteria ). Heterotrophs represent one of the two mechanisms of nutrition ( trophic levels ), the other being autotrophs ( auto = self, troph = nutrition).
The other pathway, non-cyclic photophosphorylation, is a two-stage process involving two different chlorophyll photosystems in the thylakoid membrane. First, a photon is absorbed by chlorophyll pigments surrounding the reaction core center of photosystem II.
The non-absorbed part of the light spectrum is what gives photosynthetic organisms their color (e.g., green plants, red algae, purple bacteria) and is the least effective for photosynthesis in the respective organisms.
Photoheterotrophs generate ATP using light, in one of two ways: [6] [7] they use a bacteriochlorophyll-based reaction center, or they use a bacteriorhodopsin.The chlorophyll-based mechanism is similar to that used in photosynthesis, where light excites the molecules in a reaction center and causes a flow of electrons through an electron transport chain (ETS).
For example, cyanobacteria and many purple sulfur bacteria can be photolithoautotrophic, using light for energy, H 2 O or sulfide as electron/hydrogen donors, and CO 2 as carbon source, whereas green non-sulfur bacteria can be photoorganoheterotrophic, using organic molecules as both electron/hydrogen donors and carbon sources.
Cyanobacteria is the only prokaryotic group that performs oxygenic photosynthesis. Anoxygenic photosynthetic bacteria use PSI- and PSII-like photosystems, which are pigment protein complexes for capturing light. [5] Both of these photosystems use bacteriochlorophyll. There are multiple hypotheses for how oxygenic photosynthesis evolved.
Photosynthesis is the main means by which plants, algae and many bacteria produce organic compounds and oxygen from carbon dioxide and water (green arrow). An autotroph is an organism that can convert abiotic sources of energy into energy stored in organic compounds , which can be used by other organisms .