2.4
ENERGY FLOW IN ECOSYSTEMS
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All organisms require energy to stay alive and function.
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Radiant energy from the Sun is the source of almost all
this energy.
radiant energy
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The Sun emits invisible radiant energy, including ultraviolet,
and visible radiant energy, or light energy.
thermal energy
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About 70 % of radiant energy is absorbed by the hydrosphere
and lithosphere and converted into thermal energy, warming Earth.
light energy
The distribution of incoming solar radiation
Photosynthesis
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Thermal energy keeps Earth warm, but cannot provide
organisms with energy to grow and function.
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Chemical energy is used by all organisms to perform
functions such as growth and reproduction.
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Many organisms can convert light energy into chemical
energy using the process of photosynthesis.
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Organisms that photosynthesize are called producers.
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On land, the main producers are green plants.
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In water, the main producers are microscopic organisms.
2.4
Photosynthesis
producers
Producers Store Chemical Energy
through Photosynthesis
2.4
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Most producers contain chlorophyll, which they use to capture
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light
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During
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carbon dioxide and water into sugars, releasing oxygen gas
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as a by-product.
light energy
carbon dioxide + water
sugar + oxygen
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The sugar produced through photosynthesis contains stored chemical
energy.
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Plants store chemical energy in the form of sugars and starches in roots,
stems, leaves, and seeds.
Cellular Respiration
2.4
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To make stored energy available for use, plants perform the
complementary reaction called cellular respiration.
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During cellular respiration, sugar and oxygen are rearranged
to form carbon dioxide and water.
sugar + oxygen
carbon dioxide + water + energy
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Cellular respiration releases energy that can be used by plant cells.
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Photosynthesis occurs only in the presence of
light energy, but cellular respiration occurs
continuously.
cellular respiration
Consumers
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Many organisms cannot photosynthesize. Instead, they
obtain energy by eating other organisms. These organisms
are called consumers.
2.4
consumers
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Only producers undergo photosynthesis, but both producers
and consumers undergo cellular respiration.
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Plants trap light energy and convert it to food energy, and release oxygen into
the environment.
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Humans are consumers. We depend on photosynthesizing organisms for food
and oxygen.
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Not all organisms require oxygen. Many soil micro-organisms use other
chemical pathways that do not use oxygen to release chemical energy from
sugar.