A food chain represents a single, linear pathway of energy transfer through successive trophic levels, in the correct direction from primary producer (autotroph, such as phytoplankton) to primary consumer (herbivorous zooplankton) to secondary consumer (planktivorous fish) to a higher-level predator, distinguishing it from a food web (which involves multiple interconnected feeding pathways) and requiring that the sequence of organisms listed follow both correct trophic level progression and correct direction of energy flow (from producer toward higher consumers, not the reverse).