1. ENERGY FLOW IN ECOSYSTEMS
2.4
All organisms require energy to stay alive and function.
Radiant energy from the Sun is the source of almost all this energy.
The Sun emits invisible radiant energy, including ultraviolet, and visible radiant energy, or light energy.
About 70 % of radiant energy is absorbed by the hydrosphere and lithosphere and converted into thermal energy, warming Earth.
radiant energy
light energy
thermal energy
The distribution of incoming solar radiation

2. Photosynthesis
2.4
Thermal energy keeps Earth warm, but cannot provide organisms with energy to grow and function.
Chemical energy is used by all organisms to perform functions such as growth and reproduction.
Many organisms can convert light energy into chemical energy using the process of photosynthesis.
Organisms that photosynthesize are called producers.
On land, the main producers are green plants.
In water, the main producers are microscopic organisms.
Photosynthesis
producers

3. Producers Store Chemical Energy through Photosynthesis
2.4
Most producers contain chlorophyll, which they use to capture light energy.
During photosynthesis, producers use light energy to convert carbon dioxide and water into sugars, releasing oxygen gas as a by-product.
The sugar produced through photosynthesis contains stored chemical energy.
Plants store chemical energy in the form of sugars and starches in roots, stems, leaves, and seeds.
light energy
carbon dioxide + water sugar + oxygen

4. Cellular Respiration
2.4
To make stored energy available for use, plants perform the complementary reaction called cellular respiration.
During cellular respiration, sugar and oxygen are rearranged to form carbon dioxide and water.
Cellular respiration releases energy that can be used by plant cells.
Photosynthesis occurs only in the presence of light energy, but cellular respiration occurs continuously.
cellular respiration
sugar + oxygen carbon dioxide + water + energy

5. Consumers
2.4
Many organisms cannot photosynthesize. Instead, they obtain energy by eating other organisms. These organisms are called consumers.
Only producers undergo photosynthesis, but both producers and consumers undergo cellular respiration.
Plants trap light energy and convert it to food energy, and release oxygen into the environment.
Humans are consumers. We depend on photosynthesizing organisms for food and oxygen.
Not all organisms require oxygen. Many soil micro-organisms use other chemical pathways that do not use oxygen to release chemical energy from sugar.
consumers