The chemical energy used by most organisms for metabolism and growth ultimately comes from __________.

• A. The moon
• B. The ocean
• C. Chemical energy stored in fossils
• D. The sun

Answer: (D)

Energy for metabolism in most organisms ultimately comes from the sun because sunlight fuels photosynthesis. Plants, algae, and some bacteria capture light energy and convert it into chemical energy stored in sugars and other organic molecules. When organisms eat these producers (or eat organisms that have eaten them), they obtain that chemical energy and use cellular respiration to convert the energy stored in glucose into ATP, the molecule that powers most cellular processes involved in metabolism and growth. Without sunlight, the producers wouldn’t build the organic molecules that supply nearly all energy for life.

The moon has no role in providing chemical energy for metabolism, and the ocean itself doesn’t supply direct metabolic energy—it's a medium for nutrients, not a primary energy source. Fossil fuels contain energy that originally came from ancient sunlight captured by ancient organisms, but they are not the typical energy source for the majority of living organisms today.