Every organism on Earth needs organic molecules to fuel respiration. There are only two ways to get them: build your own, or take somebody else’s. Everything on this page is a variation on those two.
📚 What you need to know
A mode of nutrition is the way an organism gains organic molecules to fuel respiration.
Autotrophs make their own organic molecules from simple inorganic substances. Heterotrophs take them from the tissues of other organisms.
Photoautotrophs use light energy; chemoautotrophs use energy from oxidising chemicals.
Holozoic nutrition is ingest, digest, absorb, assimilate — with digestion inside the body.
Saprotrophs secrete enzymes onto dead matter and digest it outside the body, then absorb the products.
Mixotrophs use more than one mode. Obligate mixotrophs need both; facultative ones can manage on one.
Why nutrition exists at all
Organisms need energy in the form of ATP. The energy stored in ATP comes from organic molecules such as carbohydrates, and it is transferred to ATP during respiration.
So every organism has the same underlying problem: it needs a steady supply of organic molecules to feed into respiration. How it solves that problem is its mode of nutrition.
Watch your language on energy. Energy is never produced or created — it is only transferred from one form to another. Photoautotrophs do not produce energy; they produce their own food by transferring light energy into chemical energy. Examiners mark this strictly.
The whole family, in one picture
Holozoic and saprotrophic are two ways of being a heterotroph. The difference between them is simply where the digestion happens.
Autotrophs
An autotroph synthesises its own organic molecules from simple inorganic substances in its environment. The two types are named after where the energy comes from.
Photoautotrophs use light energy. Plants, algae and photosynthetic bacteria such as cyanobacteria.
Chemoautotrophs use energy released by oxidising chemicals. Many are archaea and bacteria living where there is no light at all.
Photosynthesis
Photosynthetic organisms use light energy to convert carbon dioxide from the air into organic molecules such as carbohydrates. Photosynthetic pigments, chiefly chlorophyll, absorb the light energy that makes this possible.
Photosynthesis
carbon dioxide + water → glucose + oxygen requires light energy and chlorophyll
Because they make their own organic molecules without relying on any other organism, photosynthetic organisms are called producers. They matter for two reasons that go well beyond feeding themselves:
They are the bridge between non-living matter and living organisms, transferring light energy into a chemical form the rest of life can use. Everything that eats a producer is living off that transfer.
They released the oxygen into Earth’s atmosphere that makes aerobic respiration possible in the first place.
Heterotrophs
A heterotroph gains its organic molecules from the tissues of other organisms. All animals are heterotrophs, and so are all fungi.
Holozoic nutrition
Holozoic organisms take food into the body and break it down inside. Four steps, in order:
🧩 The four stages, and what each one means
Ingestion — eating; taking the food into the body.
Digestion — breaking larger molecules down into smaller ones.
Absorption — transporting those small molecules from the digestive tract into the cells.
Assimilation — using the molecules to build the organism’s own cells and tissues.
The point that carries the mark: holozoic nutrition involves INTERNAL digestion. A house fly secretes enzymes onto its food and absorbs the products. It is still a heterotroph, but it is not holozoic, because the digestion happens outside its body.
Saprotrophs
Saprotrophs are heterotrophs that feed on dead organisms and waste material. They secrete enzymes onto the food, digest it externally, and then absorb the products. Fungi and many bacteria work this way, and they are also described as decomposers.
The leftovers from external digestion stay in the soil, which is precisely how plants get their mineral ions back.
🤔 Why saprotrophs hold ecosystems together
Saprotrophs secrete a very wide range of digestive enzymes, so they can hydrolyse a huge variety of biological molecules and release a huge variety of products. Those products include mineral ions such as ammonium and phosphate.
Crucially, they do not absorb all of it. What is left stays in the surrounding soil, where plants can take it up. Without saprotrophs, the nutrients locked inside dead bodies and waste would never be released, and plants would eventually run out of minerals. That is why they are treated as an essential part of every ecosystem and food web.
Decomposers are not the same as detritivores. Both feed on dead material, but decomposers (such as fungi) are saprotrophs using external digestion, while detritivores (such as earthworms) take the material in and use holozoic, internal digestion.
Mixotrophs
Some organisms use more than one mode of nutrition — typically both autotrophy and heterotrophy. These are mixotrophs.
Obligate mixotrophs must constantly have access to both methods.
Facultative mixotrophs can survive on one method, with the other used to supplement it.
The light-sensitive spot is what makes the autotrophic half work: the cell can swim to where the light is strongest for its chloroplasts.
Other mixotrophs worth knowing:
Carnivorous plants build organic molecules by photosynthesis and from the tissues of digested insects.
Corals gain organic molecules from their symbiotic photosynthetic algae and by filter feeding from the surrounding water.
Marine plankton such as dinoflagellates often feed on bacteria while also photosynthesising.
Nutrition in archaea
Archaea form one of the three domains of life, and they are metabolically the most varied group of the three. You are not expected to name archaea at species level — only to know the categories.
Group
Where the ATP energy comes from
Where the carbon compounds come from
Phototrophic archaea
Light, absorbed by a pigment that pumps H+ ions across a membrane; the resulting gradient drives ATP synthase
From other organisms, so these are photoheterotrophs, not autotrophs
Chemoautotrophic archaea
Energy released by oxidising chemicals such as hydrogen, ammonia, methane or hydrogen sulfide
They make their own, by chemosynthesis
Chemoheterotrophic archaea
Chemicals, used to drive ATP production directly
From other organisms
Heterotrophic archaea
From the carbon compounds they take in
From other organisms, e.g. breaking down dead plant material
🧠 Decoding any of these words in two seconds
Split the word in half. The first half tells you the energy source: photo- means light, chemo- means chemicals. The second half tells you the carbon source: -autotroph means it makes its own, -heterotroph means it takes it from others. So a photoheterotroph runs on light but eats its carbon.
Worked examples
WORKED EXAMPLE
Distinguish between holozoic and saprotrophic nutrition. [3 marks]
Point 1: what they have in common
Both are modes of heterotrophic nutrition, gaining organic molecules from the tissues of other organisms.
Point 2: holozoic
In holozoic nutrition food is ingested and digestion takes place inside the body, before absorption and assimilation.
Point 3: saprotrophic
In saprotrophic nutrition enzymes are secreted onto dead material and digestion takes place outside the organism, with the soluble products then absorbed.
Same goal, opposite location for the digestionThe words “internal” and “external” are what the mark scheme is looking for. Use them explicitly.
WORKED EXAMPLE
Explain why saprotrophs are essential to ecosystems. [3 marks]
Step 1: what they break down
They secrete a wide range of enzymes onto dead organisms and waste material, hydrolysing a large variety of biological molecules.
Step 2: what that releases
This releases mineral ions such as ammonium and phosphate ions into the surrounding soil.
Step 3: why it mattersno saprotrophs → nutrients stay locked in dead matter → plants run short of minerals → food webs collapse
Because not all the products are absorbed by the saprotroph, plants can take up what is left.
They recycle nutrients that would otherwise be locked away
WORKED EXAMPLE
State the mode of nutrition of each: (a) a bread mould growing on old fruit, (b) an earthworm eating dead leaves, (c) a bacterium that oxidises hydrogen sulfide to build its own sugars. [3 marks]
(a) bread mould
A fungus feeding on dead material by secreting enzymes onto it and digesting externally.
saprotroph (a decomposer)(b) earthworm
It takes the leaves into its body and digests them internally, so despite feeding on dead matter it is not a saprotroph.
holozoic heterotroph (a detritivore)(c) the bacterium
It uses energy from oxidising a chemical, and makes its own organic molecules.
chemoautotrophSaprotroph, holozoic detritivore, chemoautotrophPart (b) is the trap. Feeding on dead material does not automatically make something a saprotroph.
💡 Exam tip
Never write that an organism “produces energy”. Say it transfers energy, or that it produces its own food.
Define a mode of nutrition by where the organic molecules come from, not by what the organism looks like.
Say internal or external digestion explicitly when comparing holozoic and saprotrophic nutrition.
Learn the four holozoic stages in order: ingestion, digestion, absorption, assimilation.
Split the long words in half. First half = energy source, second half = carbon source.
For archaea, describe the category. You are not expected to name species.
⚠ Common mix-up
“All heterotrophs are holozoic.” Saprotrophs and house flies are heterotrophs that digest externally.
Decomposers and detritivores treated as the same. Decomposers digest externally; detritivores digest internally.
“Autotrophs do not respire.” They do — that is precisely why they need organic molecules in the first place.
Confusing chemoautotroph with chemoheterotroph. Both run on chemical energy; only one makes its own carbon compounds.
Calling all archaea extremophiles or autotrophs. The domain contains all these modes.
Saying photosynthesis “creates energy”. It transfers light energy into chemical energy.
Up next: Nutrition in Hominidae (Skills) — using teeth and jaws to work out what an animal ate, including species that died out long before anyone could watch them eat.
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