Methods of Nutrition (SL IB Biology)

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

• 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

• 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

• 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 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 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

• 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