Final answer:
The sun is the primary source of energy for almost all living organisms, with plants converting solar energy into chemical energy via photosynthesis. In turn, animals and other organisms obtain energy by eating these plants or plant-eating organisms, connectivity that underpins food webs.
Step-by-step explanation:
The ultimate source of energy for almost all living organisms is the sun. Plants harness this energy through the process of photosynthesis, converting solar energy into chemical energy stored within organic compounds. From there, both animals and fungi obtain energy by consuming plants or other organisms that have ingested plants. Autotrophs, such as plants, capture light or chemical energy directly, while heterotrophs, including animals and fungi, acquire energy through consumption. This chain of energy transfer is fundamental to food webs and the sustenance of life on Earth.
All organisms, from bacteria to humans, use energy for their metabolic activities. Energy is necessary for growth, maintenance, and reproduction, and is a vital factor in an organism's survival. The energy captured by autotrophs or consumed by heterotrophs is used at the cellular level in metabolic pathways, importantly in the form of ATP, to build complex organic molecules necessary for life.