Autotrophs and heterotrophs at Higher Level: The same ideas as SL, with different examples: phytoplankton, krill and blue whales in the Southern Ocean, and dodder, a plant that feeds on other plants. At HL, expect to trace carbon from carbon dioxide into a consumer.
Practise this as you read
- Classify any organism by where its carbon compounds come from.
- Trace carbon from CO2 through an autotroph to heterotrophs.
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Self-feeders: An autotroph builds its own food. It takes simple inorganic substances and turns them into the carbon compounds that make up its body.
The points to remember
- An autotroph makes its own carbon compounds: sugars, starch, cellulose, proteins and fats.
- It builds them from inorganic carbon: carbon dioxide from the air or dissolved in water.
- It also takes in other elements as simple ions from soil or water, such as nitrate and phosphate.
- It needs an external energy source: for plants and algae this is light, used in photosynthesis.
- Autotrophs are the producers: plants, algae, phytoplankton and some bacteria.
Remember it as: Auto = self: it makes its own.
Real example: tiny phytoplankton carry out about half of all the photosynthesis on Earth, making carbon compounds from carbon dioxide dissolved in seawater.
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A heterotroph cannot build carbon compounds from carbon dioxide. It has to take them, ready-made, from other organisms, by eating them or by absorbing them.
The points to remember
- A heterotroph cannot make carbon compounds from inorganic carbon.
- It obtains carbon compounds from other organisms, living or dead.
- Consumers eat them: herbivores eat producers, carnivores eat other animals.
- Decomposers, such as fungi and many bacteria, absorb them from dead matter.
- It digests what it takes in and builds its own compounds from the pieces.
- All animals and fungi, and most bacteria, are heterotrophs.
Remember it as: Hetero = other: it takes from others.
Real example: in the Southern Ocean, Antarctic krill eat phytoplankton, and a blue whale eats several tonnes of krill a day in the feeding season.
All living organisms can be sorted into these two groups. One question decides it: where do its carbon compounds come from?
The points to remember
- Every living organism is either an autotroph or a heterotroph.
- The test: are its carbon compounds made from CO2, or taken from another organism?
- Fungi are not plants: they absorb carbon compounds, so they are heterotrophs.
- A plant that catches insects, such as the Venus flytrap, is still an autotroph: insects give it nitrogen, not carbon.
- A plant with almost no green pigment that feeds on another plant, such as dodder, is a heterotroph.
Two common mix-ups: 'Fungi are plants' is wrong: fungi are heterotrophs.
Autotroph does not mean 'green' or 'plant' alone: say it makes carbon compounds from inorganic carbon.
Real example: dodder wraps around other plants and pushes feeding tubes into their stems to take their sugars: a plant that is a heterotroph.
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How this comes up: Paper 2, Section A: trace carbon from an inorganic source to a consumer [3].
In the Southern Ocean, phytoplankton are eaten by krill, and krill are eaten by blue whales.
Explain how carbon from carbon dioxide dissolved in seawater becomes part of a blue whale.
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