All organisms need organic molecules, both as a source of energy and as building materials. They differ in how they obtain them.Autotrophs make their own complex organic molecules, such as glucose, from simple inorganic molecules (carbon dioxide and water), using an external energy source. They are the producers in food chains.
• Photoautotrophs use light energy, by photosynthesis. Examples are green plants, algae and cyanobacteria.
• Chemoautotrophs use energy released by chemical reactions, such as the oxidation of ammonium ions or iron compounds. Examples are the nitrifying bacteria of the nitrogen cycle and bacteria around deep-sea vents.Heterotrophs cannot make organic molecules from inorganic ones. They must obtain ready-made organic molecules from other organisms, living or dead. They are the consumers and decomposers.
• Animals ingest food and digest it internally.
• Fungi and many bacteria are saprotrophs: they secrete enzymes onto dead organic matter and absorb the soluble products (extracellular digestion).Heterotrophs depend on autotrophs, directly or indirectly, because autotrophs are the only organisms that bring new carbon and energy into a food web. Almost all the energy in a food web first enters as light absorbed during photosynthesis.Both groups respire. An autotroph makes organic molecules and then respires some of them to make ATP; a heterotroph respires organic molecules it has eaten or absorbed.
• Photoautotrophs use light energy, by photosynthesis. Examples are green plants, algae and cyanobacteria.
• Chemoautotrophs use energy released by chemical reactions, such as the oxidation of ammonium ions or iron compounds. Examples are the nitrifying bacteria of the nitrogen cycle and bacteria around deep-sea vents.Heterotrophs cannot make organic molecules from inorganic ones. They must obtain ready-made organic molecules from other organisms, living or dead. They are the consumers and decomposers.
• Animals ingest food and digest it internally.
• Fungi and many bacteria are saprotrophs: they secrete enzymes onto dead organic matter and absorb the soluble products (extracellular digestion).Heterotrophs depend on autotrophs, directly or indirectly, because autotrophs are the only organisms that bring new carbon and energy into a food web. Almost all the energy in a food web first enters as light absorbed during photosynthesis.Both groups respire. An autotroph makes organic molecules and then respires some of them to make ATP; a heterotroph respires organic molecules it has eaten or absorbed.