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First exams 2025

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Methods of Nutrition (HL IB Biology)

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Naomi H

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Naomi H

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Photosynthesis

Methods of nutrition

  • Organisms need energy in the form of ATP to survive
  • The energy stored in ATP comes from other organic molecules, such as carbohydrates, and is transferred during the process of respiration
  • The way by which an organism gains organic molecules to fuel respiration is known as its method, or mode, of nutrition
  • There are two main modes of nutrition; autotrophy and heterotrophy
    • An autotroph synthesises, or produces, its own organic molecules from simple inorganic substances in its environment
      • E.g. autotrophs that use light energy are known as photoautotrophs, while those that use energy from oxidation of chemicals are known as chemoautotrophs
    • A heterotroph gains organic molecules from the tissues of other organisms

Photosynthesis

  • Photosynthetic organisms are autotrophs that use light energy to convert carbon dioxide from the air into organic molecules such as carbohydrates
    • Photosynthetic pigments such as chlorophyll absorb light energy, enabling this process

Photosynthesis word equation

  • Because photosynthetic organisms make their own organic molecules without relying on other organisms, they are known as producers
    • Photosynthesis is a crucial process because it transfers light energy into a chemical form that can be used by living organisms
    • Producers can then be eaten by other living organisms, continuing the process of energy transfer
  • In addition to providing the crucial bridge between non-living matter and living organisms, photosynthesis is also responsible for the release of oxygen into Earth's atmosphere, enabling aerobic respiration
  • Photosynthetic organisms include
    • Plants, both terrestrial and aquatic
    • Algae, including single-celled algae and multicellular seaweeds
    • Photosynthetic bacteria such as cyanobacteria

Examiner Tip

Be careful with your language when discussing energy; you should NEVER say that energy is produced or created, only that it is transferred from one form to another. Photoautotrophs do not produce energy, they produce their own food by transferring light energy to chemical energy.

Holozoic Nutrition

  • Heterotrophs are organisms that gain organic molecules from the tissues of other organisms
    • E.g. animals are all heterotrophs
  • Organisms that use holozoic nutrition are heterotrophs that gain organic molecules by ingesting, digesting, absorbing, and assimilating molecules from the tissues of other organisms
    • Ingestion = eating
    • Digesting = breaking down larger molecules into smaller molecules
    • Absorbing = the transport of molecules from the digestive tract into the cells
    • Assimilation = using molecules to build cells and tissues
  • The crucial point to remember here is that holozoic nutrition involves internal digestion
    • There are a few examples of animals, e.g. house flies, that secrete enzymes onto their food before absorbing the products; animals like this are heterotrophs, but they do not use holozoic nutrition as digestion takes place externally

Mixotrophs

  • Some organisms are able to make use of more than one method of nutrition, such as auto- and heterotrophy; these organisms are referred to as mixotrophs
  • Obligate mixotrophs must constantly have access to both methods of nutrition
  • Facultative mixotrophs can survive using one method of nutrition, which is supplemented by the other
  • E.g. euglena is a single-celled eukaryotic organism found in fresh water that makes use of both autotrophy and heterotrophy
    • Euglena cells can take in bacterial cells by endocytosis, and then digest them using digestive enzymes stored in lysosomes
    • Euglena cells also contain a light-sensitive spot that enables them to position themselves so that maximum light reaches their chloroplasts

Euglena diagram

Euglena

Euglena is a mixotroph that makes use of autotrophic and heterotrophic nutrition

  • Other examples of mixotrophs include
    • Carnivorous plants
      • These plants build organic molecules using both photosynthesis and using molecules from the tissues of digested insects
    • Corals
      • Coral polyps gain organic molecules from their symbiotic photosynthetic algae, and by filter feeding from the surrounding water
    • Marine plankton, e.g. dinoflagellates
      • These are microscopic organisms that float in the water of the ocean
      • Many are mixotrophs, e.g. feeding on bacteria while also carrying out photosynthesis

Saprotrophs

  • Saprotrophs are heterotrophs that ingest the tissues of dead organisms and waste material by secreting enzymes onto their food and digesting it externally before absorbing the products of this digestion
    • Note that this is different to holozoic nutrition as here the digestion takes place externally
  • Examples of saprotrophs include fungi and bacteria
    • These organisms can be described as decomposers
  • Saprotrophs secrete a wide range of digestive enzymes that allow them to hydrolyse a large variety of biological molecules, and so to release a large range of products
  • Examples of these products include mineral ions, such as ammonium ions and phosphate ions
  • Importantly, not all of the products of external digestion get absorbed by saprotrophs, leaving some minerals in the surrounding soil for absorption by other organisms, such as plants
  • Without saprotrophs the nutrients locked up in dead and waste matter would never be released and plants would not have access to sufficient minerals
    • This is why saprotrophs are such an essential component of ecosystems and food webs

Examiner Tip

Note that decomposers, such as fungi, do NOT use the same method of nutrition as detritivores such as earthworms. Both decomposers and detritivores feed on dead organisms and waste material, but decomposers are saprotrophs and use external digestion, while detritivores digest their food internally using holozoic nutrition

Nutrition in Archaea

  • The archaea are a diverse group of single-celled organisms that make up one of the three domains
  • Different groups of archaea vary metabolically, e.g.
    • Phototrophic archaea
    • Chemotrophic archaea
      • This includes chemoautotrophs and chemoheterotrophs
    • Heterotrophic archaea

Phototrophic archaea

  • Phototrophic archaea use energy from light to generate ATP
  • E.g. Halobacteria use a pigment called bacteriorhodopsin to absorb light energy and to pump H+ ions across a membrane; the resulting ion gradient leads to the production of ATP by the enzyme ATP synthase in a similar way to oxidative phosphorylation and photophosphorylation
  • Note that this is not the same as oxygen-releasing photosynthesis, and Halobacteria are not considered to be autotrophic, but could instead be described as photoheterotrophs
    • While they use light energy to produce ATP, Halobacteria gain carbon compounds to build their cell structure from other organisms

Chemotrophic archaea

  • Some archaea can produce their own carbon compounds using chemosynthesis
    • These archaea are chemoautotrophs
      • They use energy released from chemicals in the environment
      • They produce their own carbon compounds
  • Chemosynthesis releases energy from chemicals which is transferred to carbon compounds
    • These carbon compounds can then be used for ATP production
  • Chemicals that can act as energy sources for chemosynthetic archaea include
    • Hydrogen gas
    • Ammonia
    • Methane
    • Hydrogen sulfide
  • Some chemotrophic archaea use energy from chemicals to directly drive ATP production in a similar way to the phototrophic archaea described above
    • These archaea are chemoheterotrophs; they use chemicals to produce ATP but they gain their carbon compounds from other organisms

Heterotrophic archaea

  • Heterotrophic archaea gain their carbon compounds from other organisms, and then use these carbon compounds to generate ATP
  • E.g. archaea that break down and absorb carbon compounds in dead plant material

Examiner Tip

Note that you are not expected to give examples of archaea at the species level.

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Naomi H

Author: Naomi H

Expertise: Biology

Naomi graduated from the University of Oxford with a degree in Biological Sciences. She has 8 years of classroom experience teaching Key Stage 3 up to A-Level biology, and is currently a tutor and A-Level examiner. Naomi especially enjoys creating resources that enable students to build a solid understanding of subject content, while also connecting their knowledge with biology’s exciting, real-world applications.