The main source of energy for all living things is solar energy.

Explanation:
Solar energy, in the form of sunlight, is captured by photosynthetic organisms such as plants, algae, and some bacteria. These organisms utilize specialized pigments, such as chlorophyll, to convert solar energy into chemical energy through the process of photosynthesis.

During photosynthesis, plants and other photosynthetic organisms harness the energy from sunlight to convert carbon dioxide and water into glucose and oxygen. This glucose serves as the primary source of energy for the organism, as well as the building block for other essential organic molecules.

The energy stored in glucose and other organic compounds produced through photosynthesis is transferred through food chains and webs. When organisms consume plants or other organisms, they obtain the stored energy and convert it into ATP through cellular respiration. ATP, as the universal energy currency in cells, fuels various cellular activities and processes necessary for life, including growth, movement, reproduction, and maintenance of cellular functions.

In this way, solar energy provides the foundation for the energy flow within ecosystems and is the ultimate source of energy for sustaining life on Earth.