Students should understand that photosynthetic organisms are the producers…

The idea

Photosynthetic organisms, such as green plants, algae and some bacteria, capture light energy and convert carbon dioxide and water into glucose and oxygen. The glucose is used to build biological molecules including carbohydrates, lipids, proteins and nucleic acids, which together form biomass. Because these organisms synthesise their own organic nutrients from inorganic raw materials, they are called producers. Almost all energy in food webs enters as light energy trapped by producers, so the biomass available to consumers originates from photosynthesis. For example, a wheat plant uses glucose from photosynthesis to make starch in grains and cellulose in stems; a human eating bread obtains chemical energy stored in that biomass. Without producers, heterotrophs would have no source of organic carbon or energy.

What earns the marks

• Producers are photosynthetic organisms that synthesise organic molecules from inorganic raw materials using light energy.
• Photosynthesis converts light energy into chemical energy stored in glucose, which is then used to build biomass.
• Biomass includes the biological molecules made from glucose, such as carbohydrates, lipids, proteins and nucleic acids.
• Energy and matter pass from producers to consumers through feeding, so producer biomass is the starting point of food chains and webs.
• Examples of producers include green plants, algae and photosynthetic bacteria; not all producers are plants.
• The carbon in consumer biomass can be traced back to carbon dioxide fixed by producers during photosynthesis.

Common mistakes and how to put them right

• Stating that producers are simply 'plants'.

  Put it right: Producers are photosynthetic organisms and include algae and some bacteria as well as green plants.

• Saying that producers 'eat' or 'absorb' food from the soil.

  Put it right: Producers synthesise their own organic molecules by photosynthesis using carbon dioxide and water.

• Confusing biomass with the total mass of an organism including water.

  Put it right: Biomass is the mass of biological material, often considered as the organic matter built from glucose, and is not simply the fresh mass of the organism.

• Claiming that energy is recycled in an ecosystem.

  Put it right: Energy flows through ecosystems and is eventually lost as heat, whereas matter such as carbon is recycled.

Exam tips

• When asked why producers are essential, link your answer to the synthesis of biomass from glucose made in photosynthesis, not just to 'making oxygen'.
• Use the term 'biomass' precisely: refer to the biological molecules built from glucose rather than to the whole organism's fresh mass.
• If a question gives a food chain, identify the producer and explain that it is the only organism that synthesises organic molecules from inorganic materials.
• Practise writing a clear sequence: light energy → photosynthesis → glucose → biological molecules → biomass → available to consumers.