2.2J Modes of nutrition (HL) Notes

1. All living organisms depend on carbon compounds to survive, but they differ in how they obtain them.
2. Organisms are therefore classified into two main groups, autotrophs and heterotrophs, based on their mode of nutrition and carbon source.

Autotrophs

1. Autotrophs are organisms that synthesize their own carbon compounds from inorganic sources such as carbon dioxide, water, and mineral ions.
2. They are known as producers because they form the base of all food chains and support the flow of energy through ecosystems.
3. Autotrophs can convert inorganic substances into organic compounds like glucose, amino acids, and lipids, which store energy and provide structure.
4. They use external energy sources, such as light (in photosynthesis) or chemical energy (in chemosynthesis), to power this conversion.

Photoautotrophs

1. Photoautotrophs are organisms that use light as an external energy source to convert inorganic molecules (like carbon dioxide and water) into organic compounds such as glucose.
2. This process is known as photosynthesis and takes place in specialized structures such as chloroplasts.
3. They contain pigments such as chlorophyll that capture sunlight and transform it into chemical energy.
4. They are found in sunlit environments — terrestrial ecosystems (plants) and aquatic ecosystems (algae, phytoplankton).
5. Photoautotrophs are the primary source of oxygen and organic matter for almost all ecosystems.

Chemoautotrophs

1. Chemoautotrophs use energy from inorganic chemical reactions rather than sunlight to produce organic molecules, a process called chemosynthesis.
2. They oxidize inorganic compounds such as hydrogen sulfide, ammonia, or methane to release energy.
3. This energy is then used to fix carbon dioxide into organic compounds.