Photosynthesis and mixotrophic nutritionSpec B4.2.3, B4.2.5
In short
Photosynthesis is the mode of nutrition in plants, algae and several groups of photosynthetic prokaryotes, which make their own carbon compounds from carbon dioxide using light energy. Mixotrophic protists such as Euglena are both autotrophic and heterotrophic. Some mixotrophs are obligate, needing both modes to grow, while facultative mixotrophs can survive using either mode alone.
Photosynthesis is the mode of nutrition in plants, algae and several groups of photosynthetic prokaryotes such as cyanobacteria. These organisms are autotrophs: they use light energy to make their own carbon compounds from carbon dioxide and other simple inorganic substances.
Mixotrophic nutrition
A mixotroph is an organism that is both autotrophic and heterotrophic. Some protists do this. Euglena is a well-known freshwater example: it has chloroplasts and photosynthesises in the light, but it can also absorb carbon compounds such as sugars from its surroundings. Many other mixotrophic species form part of oceanic plankton.
- Obligate mixotrophs need both modes of nutrition to grow and survive.
- Facultative mixotrophs can survive using either mode alone, switching to heterotrophy when light is short or to autotrophy when food is scarce. Euglena can live heterotrophically in the dark.
Linking question: what are the relative advantages of specificity and versatility? A facultative mixotroph is versatile, surviving changes in light or food that would kill a strict autotroph or heterotroph.
Written and checked against the IB Biology HL specification · Updated October 2026