Not all organisms feed the same. Some make their own food and energy, such as photosynthesising plants. These are autotrophs meaning they make their own nutrients, either through light energy or chemical energy.
Some bacteria living in extreme conditions such as deep sea vents rely on chemical reactions to derive energy. Although still autotrophic, these are chemoautotrophic while the photosynthesising organisms are photoautotrophic. Both are under the autotrophic umbrella.
Organisms that cannot make their own organic molecules using simpler molecules found in their environment, rely on feeding on those that can. These are heterotrophs and include humans. Since heterotrophs lack any ability to convert light energy or the electron donating potential from certain chemicals into reactions to produce the carbohydrates, lipids and proteins needed for life, they extract them from primary producers such as plants, or other heterotrophs at higher trophic levels.
Some heterotrophs secrete enzymes into the environment to break down waste materials from other organisms, then absorb the nutrients. This happens in soil bacteria and fungi and is termed saprotrophic nutrition.
There are two other subtypes of nutrition under the hetrotrophic umbrella: holozoic and parasitic. Holozoic nutrition is the depicted nutrition involving the ingestion and internal digestion of food from various sources.
Parasitic nutrition involves a one-sided scenario where one organism takes advantage of another for its nutrient needs. An example of this is the malaria parasite residing in the anopheles mosquito in order to get nutrients and reach the developmental stage required before infecting human red blood cells. Another example is the tapeworm which can deprive its host of nutrients and can cause weight loss, anaemia and damage to the host intestine.
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