In any ecosystem, plants synthesise organic compounds from atmospheric, or…

The idea

Producers form the entry point of carbon and chemical energy into an ecosystem. In terrestrial ecosystems, plants synthesise organic compounds using carbon dioxide from the atmosphere. In aquatic ecosystems, producers utilise dissolved carbon dioxide and hydrogencarbonate ions. Through photosynthesis, this inorganic carbon is converted into organic compounds, primarily sugars. While some sugars are respired, the remainder are used to synthesise other biological molecules, forming the plant's biomass. This biomass represents the chemical energy available to consumers and decomposers. Therefore, the rate at which producers synthesise these organic compounds directly determines the energy available to the rest of the food web.

What earns the marks

• Identifying carbon dioxide as the inorganic carbon source for terrestrial plants.
• Identifying dissolved carbon dioxide and hydrogencarbonate ions as the carbon source for aquatic producers.
• Explaining that plants synthesise organic compounds, such as sugars, from these inorganic carbon sources.
• Linking the synthesis of organic compounds to the production of producer biomass.
• Stating that this organic carbon and chemical energy then becomes available to consumers and decomposers in the ecosystem.

Common mistakes and how to put them right

• Treating the carbon source as atmospheric carbon dioxide only.

  Put it right: Aquatic producers use dissolved carbon dioxide and hydrogencarbonate ions.

• Describing the detailed biochemistry of the Calvin cycle when asked about energy transfer.

  Put it right: Focus on the synthesis of organic compounds and biomass for ecosystem questions.

• Assuming all synthesised organic compounds become biomass.

  Put it right: A significant proportion of the sugars produced are used in respiration by the plant itself.

Exam tips

• For aquatic producers, always specify dissolved carbon dioxide or hydrogencarbonate ions rather than just carbon dioxide.
• When the question is about ecosystems, explicitly link the synthesis of organic compounds to the creation of producer biomass and energy availability for consumers.