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.
4. They are typically found in dark or extreme environments such as deep-sea hydrothermal vents, volcanic springs, caves, and sediments.
5. Chemoautotrophs form the base of food webs in ecosystems where sunlight is absent.
6. They often exist in symbiotic relationships, such as with giant tube worms (Riftia pachyptila), where bacteria inside the worms use hydrogen sulfide to produce food that sustains both organisms.

Heterotrophs

1. Heterotrophs are organisms that cannot produce their own food and must obtain carbon compounds from other organisms.
2. They depend directly or indirectly on autotrophs for energy and nutrients.
3. Heterotrophs occupy various trophic levels within food chains — as consumers and decomposers.

Types of Heterotrophs

1. Herbivores feed directly on autotrophs (plants and algae) to obtain organic carbon.
2. Carnivores feed on other animals to obtain energy and nutrients.
3. Omnivores feed on both plants and animals.
4. Decomposers (such as fungi and bacteria) break down dead organic matter, returning carbon and nutrients to the environment.

Ecological Importance

1. Both photoautotrophs and chemoautotrophs serve as primary producers, forming the foundation of energy flow in ecosystems.
2. They are responsible for the conversion of inorganic carbon (CO₂) into organic forms that fuel all other life.
3. Photoautotrophs dominate light-abundant environments, while chemoautotrophs sustain light-deprived ecosystems.
4. These processes demonstrate the adaptability of life to both sunlit and dark environmental conditions.