4.1.2 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 method by which an organism gains organic molecules to fuel respiration is known as its mode of nutrition
• There are two main modes of nutrition; autotrophy and heterotrophy

Autotrophs

• An autotroph synthesises, or produces, its own organic molecules from simple inorganic substances in its environment
• Photosynthetic organisms use light energy to convert carbon dioxide from the air into organic molecules such as carbohydrates
  • Some autotrophs use energy from the oxidation of inorganic compounds instead of light energy
  • Autotrophs that use light energy are known as photoautotrophs, while those that use energy from oxidation of chemicals are known as chemoautotrophs
• Because autotrophs make their own organic molecules without relying on other organisms, they are known as producers
• Most green plants are autotrophs, along with algae such as seaweeds, and photosynthetic bacteria such as cyanobacteria

Heterotrophs

• Heterotrophic organisms gain their organic molecules by ingesting the tissues of other organisms
• There are several types of heterotroph, including consumers, detritivores, and saprotrophs

Mixing modes of nutrition

• Some organisms are able to make use of more than one mode of nutrition, such as auto- and heterotrophy
  • These organisms are referred to as mixotrophs
• Euglena is a single-celled eukaryotic organism 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 is a single-celled eukaryote that makes use of autotrophic and heterotrophic nutrition

NOS: Looking for patterns, trends and discrepancies; plants and algae are mostly autotrophic but some are not

• The majority of plants and algae are photosynthetic, meaning that they are autotrophs that rely on energy from the sun to convert carbon dioxide in the air into organic molecules in their tissues
• Their photosynthetic cells contain pigments which absorb light energy
  • The main pigment in green plants is chlorophyll, which primarily absorbs light at the red and blue ends of the visible spectrum, reflecting green light
  • Green plants also have carotenoid pigments, known as accessory pigments, which extend the range of light wavelengths that can be absorbed; these pigments appear to be red, yellow, or purple and remain in mature leaves after chlorophyll degrades
  • Brown algae, such as the seaweed kelp, contain a brown pigment called fucoxanthin
  • Red algae and green algae have pigments called phycobilins
• There are some unusual exceptions to the autotrophic mode of nutrition used by most plants and algae
  • Some plants parasitise the roots of other plants, tapping into the roots of these plants to gain their organic molecules
    • E.g. groundcone plants look like upright pine cones sitting on the ground, but are in fact parasitic plants, having no photosynthetic pigments of their own, and gaining their organic molecules from the roots of surrounding trees

  • Some plants parasitise fungi, a feeding mode known as mycoheterotrophy, gaining their organic molecules from the network of fungal cells in the soil
    • The rare plant Epipogium aphyllum, also known as the ghost orchid, has no leaves and no chlorophyll, gaining its organic molecules from the fungi that form associations with tree roots
• When exceptions to accepted trends are observed in the natural world, it can sometimes mean that established modes of thinking are incorrect, so it is important to consider discrepancies carefully
  • In the case of non-photosynthetic plants and algae:
    • They are rare
    • They appear to have evolved on multiple occasions from autotrophic ancestors
  • There is not enough evidence to disprove the mode of thinking that says that plants and algae are autotrophs, but we can say that there are a few exceptions to this rule

• There are several ways in which heterotrophs gain organic molecules from other organisms

Consumers

• Consumers gain their organic molecules by ingesting the tissues of other living organisms or recently dead organisms
• The consumers that eat plants are known as herbivores, and the consumers that eat other animals are known as carnivores

Detritivores

• Detritivores gain organic molecules by ingesting the tissues of dead organisms or ingesting animal waste
• Detritivores carry out internal digestion, meaning that they digest their food inside their bodies
• Examples of detritivores include earthworms, woodlice and dung beetles

Saprotrophs

• Saprotrophs also ingest the tissues of dead organisms and waste material, but they secrete enzymes onto their food, and digest it externally
• The products of this external digestion are then absorbed
• Examples of saprotrophs include fungi and bacteria
• Saprotrophs secrete a wide range of digestive enzymes that allow them to hydrolyse (break down) a large variety of biological molecules, releasing a large range of products as a result
  • 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
• Instead, some of the products remain in the surrounding soil and become available to other organisms such as plants
  • This is why saprotrophs are such an essential component of ecosystems and food webs
  • Without them, the nutrients locked up in dead and waste matter would never be made available again and producers such as plants would not have access to sufficient nutrients