Final answer:
Energy in the food web originates from the sun and is acquired by living things through photosynthesis, chemosynthesis, and consumption. Only about 10% of the energy is passed from one trophic level to the next, with the rest used for metabolism or given off as heat. This limits the number of trophic levels in a food chain or web.
Step-by-step explanation:
Energy in the food web originates from the sun and is acquired by living things through three ways: photosynthesis, chemosynthesis, and the consumption and digestion of other living or previously living organisms by heterotrophs. Energy is passed up a food chain or web from lower to higher trophic levels, but only about 10% of the energy at one level is available to the next level. The other 90% of energy is used for metabolic processes or given off to the environment as heat. This loss of energy explains why there are rarely more than four trophic levels in a food chain or web.