Energy flow: how sunlight powers a food chain

0:00Where does the energy in a food chain come from, and where does it go?

0:05Almost every food chain starts with the Sun. Producers, like grass, capture its light energy by photosynthesis.

0:14Consumers get energy by eating: a mouse eats the grass, a snake eats the mouse, and a hawk eats the snake.

0:22Each feeding step is called a trophic level: producers, then primary, secondary and tertiary consumers.

0:31Say the grass stores 1000 kilojoules, or kJ, of energy.

0:36Only about 10 percent of it passes on to the mouse: 100 kilojoules.

0:42The other 90 percent is not passed on. Most of it is used for living, and it ends up lost as heat.

0:50The same happens at every step: only 10 kilojoules reach the snake, and just 1 reaches the hawk.

0:59This is an energy pyramid. With so little energy at the top, there are only a few top predators, like the hawk.

1:07So energy flows one way: in as sunlight, out as heat.

1:14Matter is different: it cycles from producers to consumers to decomposers, which break down dead remains, and back to producers.

1:24A common mistake is to think energy is recycled like matter. It is not: producers cannot use heat, so the Sun must keep supplying energy.

1:35One exception: at deep-sea vents, bacteria make food from chemical energy, not sunlight. This is called chemosynthesis.

1:44In short: energy flows one way from the Sun, about 10 percent at each step, while matter cycles.