Not all plants feed the same way
Most plants are autotrophic — they build organic food from carbon dioxide and water using light energy. However, some soils are so poor in one nutrient (often nitrogen) that plants have evolved extra ways to obtain it.
Carnivorous plants
Carnivorous plants, such as the pitcher plant (Nepenthes) and Venus flytrap, trap and digest small animals — mainly insects — to obtain nitrogen and other minerals. They still carry out photosynthesis for their carbon and energy; the trapped prey simply supplements the mineral supply. This is why they can survive in nitrogen-poor bogs where other plants struggle.
Parasitic plants
Parasitic plants obtain nutrients directly from a living host plant. They grow special structures called haustoria that penetrate the host's vascular tissue to draw off water, minerals and food. A total parasite such as Rafflesia has no chlorophyll and depends entirely on its host, while a partial parasite still photosynthesises.
Saprophytic plants
Saprophytic nutrition means obtaining nutrients from dead and decaying organic matter. The organism secretes enzymes onto the dead material, digests it externally and absorbs the soluble products. Many fungi feed this way; some flowering plants without chlorophyll rely on saprophytic fungi to supply them.
Key idea
Carnivorous plants are photosynthetic; they eat insects only to top up nitrogen. Total parasites like Rafflesia are not photosynthetic and take everything from a host.
Helpful partnerships
Some plants gain nutrients through mutualism. Leguminous plants (beans, peas) house nitrogen-fixing bacteria (Rhizobium) in root nodules; the bacteria convert atmospheric nitrogen into usable compounds and receive sugars in return. Many roots also form mycorrhizae with fungi that help absorb minerals.
Example
A pitcher plant on a nutrient-poor mountain slope lures insects into a slippery, fluid-filled trap. Enzymes digest the insect and the plant absorbs the released nitrogen — a resource missing from the thin soil.