• In many ecosystems, the sun is the source of energy. Primary producers use photosynthesis to synthesize glucose from sunlight, water and carbon dioxide. This energy is then transferred to other organisms in the ecosystem through consumption.

    Energy Input into Ecosystems

    Ecosystems require a continual input of energy. Energy is continually converted by living organisms into heat and work.

    Source of Energy

    In many recognizable ecosystems, sunlight is the source of energy. This sunlight is used by photosynthetic organisms such as plants and algae.


    Photosynthesis occurs in two stages: light reactions and dark reactions. During light reactions, sunlight is used to generate high-energy electrons, which allow the production of high-energy molecules for use in the dark reactions. In dark reactions, the high-energy molecules produced during the light reactions are used to synthesize glucose from carbon dioxide.

    Transfer of Energy through the Food Web

    The glucose molecules produced in photosynthesis are used to create additional organic molecules and energy for life processes. Grazers consuming a primary producer use the energy stored in the molecules for their own life processes. Predators of the grazers utilize the energy stored in the cells of the grazers for themselves.

    Loss of Energy

    Some of the energy used in life processes is lost. This continual loss of energy requires that energy be continually put into the system.


    Energy Flow Through the Ecosystem

    Food Chains and Food Webs

    Marine Food Webs

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