. A food chain shows the transfer of energy from one organism to the next, starting with a producer.. The source of all energy in a food chain is light energy from the Sun.. The arrows in a food chain show the transfer of energy from one trophic level to the next.. In the food chain above lettuce is the producer which obtains its energy from th sun. Snails are the primary consumer which obtains its energy from the producer (lettuce). Duck is the secondary consumer which further obtains its energy from the primary consumer (snails). At last humans are the tertiary consumer which obtains its energy from the secondary consumer (Duck).