IB Environmental systems and societies 2.2J Modes of Nutrition Notes

Modes of nutrition: how an organism gets its carbon splits all life into two camps

1. Every organism needs carbon compounds for energy and structure, but organisms differ in where those compounds come from.
2. This carbon source divides all life into autotrophs, which make their own, and heterotrophs, which take theirs from other organisms.

Autotrophs

1. Autotrophs synthesise their own carbon compounds from inorganic sources such as carbon dioxide, water, and mineral ions.
2. They are the producers that form the base of every food chain.
3. They need an external energy source to drive this conversion, and this source defines the two autotroph subtypes.
4. Photoautotrophs use light energy through photosynthesis.
   1. Examples are plants, algae, and cyanobacteria.
5. Chemoautotrophs use chemical energy from inorganic reactions through chemosynthesis.
   1. Examples are nitrifying bacteria and deep-sea hydrothermal-vent bacteria.

Photoautotrophs

1. Photoautotrophs use light as their energy source to convert carbon dioxide and water into glucose.
2. This process is photosynthesis, and it uses pigments such as chlorophyll to capture sunlight.
3. They live in sunlit environments, both on land (plants) and in water (algae and phytoplankton).
4. They supply almost all of the oxygen and organic matter that other organisms depend on.

Chemoautotrophs

1. Chemoautotrophs use energy from inorganic chemical reactions instead of light, a process called chemosynthesis.
2. They oxidise inorganic compounds such as hydrogen sulfide, ammonia, or methane to release energy.
3. That energy then fixes carbon dioxide into organic compounds.
4. They live in dark or extreme environments such as deep-sea hydrothermal vents, volcanic springs, and caves.
5. They form the base of food webs wherever sunlight is absent.
6. They often live in symbiosis, as in giant tube worms whose internal bacteria use hydrogen sulfide to feed both partners.

Heterotrophs

1. Heterotrophs cannot make their own food and must obtain carbon compounds from other organisms.
2. They depend directly or indirectly on autotrophs for their energy and nutrients.
3. They are sorted by their feeding mode, which is how they take in and digest their food.

Feeding modes of heterotrophs

1. Holozoic consumers ingest food and then digest it internally.
   1. Herbivores feed on autotrophs such as plants and algae.
   2. Carnivores feed on other animals.
   3. Omnivores feed on both plants and animals.
2. Detritivores ingest dead organic matter, or detritus, and digest it internally.
   1. Examples are earthworms and woodlice.
3. Saprotrophs, also called decomposers, secrete enzymes onto dead matter and absorb the digested products.
   1. Examples are fungi and many bacteria.
4. Parasites feed on a living host, usually harming it.
   1. Examples are tapeworms and ticks.

Why decomposers matter

1. Decomposers, meaning saprotrophs and detritivores together, break down dead organic matter.
2. They recycle carbon and mineral nutrients back into the ecosystem.
3. This returns nutrients to the soil and water where producers can reuse them.

Ecological importance: nutrition modes keep energy and matter moving

1. Autotrophs are the primary producers that convert inorganic carbon dioxide into organic forms that fuel all other life.
2. Photoautotrophs power light-rich ecosystems, while chemoautotrophs power light-free ecosystems.
3. Consumers pass this energy and carbon along the food chain from one trophic level to the next.
4. Decomposers close the loop by returning nutrients to the producers.

Together the nutrition modes make ecosystems self-sustaining, because producers supply the carbon and decomposers recycle it.