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Food Chains and Photosynthesis

Living things must have energy to survive. In an ecosystem, the path that energy takes as it moves from one organism to another is called a food chain. The Sun is the major source of energy for most food chains. Organisms that can capture the Sun’s energy are called producers, or autotrophs, because they are able to produce food molecules. Living things that cannot capture energy must eat food and are referred to as consumers, or heterotrophs. Heterotrophs that eat plants are herbivores, and those that eat animals are carnivores. Organisms that eat plants and animals are described as omnivores.

When living things die, another group of organisms in the food chain—the decomposers, or detritivores—uses the energy tied up in the lifeless bodies. Detritivores break down dead or decaying matter, returning the nutrients to the environment. Nutrients in ecosystems are constantly recycled through interlocking food chains called food webs. Energy, on the other hand, cannot be recycled. It is eventually lost to the system in the form of heat.

Autotrophs can capture the Sun’s energy because they contain the green pigment chlorophyll. During photosynthesis, detailed in Figure below, autotrophs use the Sun’s energy to rearrange the carbon atoms from carbon dioxide gas to form glucose molecules. Glucose is the primary food or energy source for living things. The hydrogen and oxygen atoms needed to form glucose come from molecules of water. Producers give off the extra oxygen atoms that are generated during photosynthesis as oxygen gas.

During photosynthesis, the energy of sunlight is used to rearrange the components of carbon dioxide and water molecules to form glucose, water, and oxygen.
During photosynthesis,
the energy of sunlight is used to rearrange the components of
carbon dioxide and water molecules to form glucose, water,
and oxygen.


Autotrophs usually make more glucose than they need, so they store some for later use. Heterotrophs consume this stored glucose to support their own life processes. In the long run, it is an ecosystem’s productivity that determines the types and numbers of organisms that can live there.

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