Obtaining Carbon Compounds in Ecosystems (SL IB Biology)

• When inorganic nutrients enter the food chain, they are converted into carbon compounds, e.g. carbohydrates and proteins, and are locked up inside the tissues of living plants and animals
• Because the supply of inorganic nutrients is finite, it is essential that when these organisms die the nutrients locked up in their tissues are released
  • The carbon compounds need to be converted back into inorganic nutrients that can be used by producers
• The process of breaking down the bodies of dead organisms and the waste products of living organisms is known as decomposition, and it enables the cycling of nutrients
  • Decomposition allows the breakdown of molecules in the bodies of dead organisms, dead parts of organisms, e.g. a fallen tree branch, and animal faeces
• The cycling of nutrients is carried out by decomposers, e.g. 
  • Detritivores often begin the process of decomposition by breaking apart tissues
  • Saprotrophs release enzymes that break down the organic molecules in the tissues, releasing inorganic nutrients
• Saprotrophs absorb some of the nutrients themselves, and what is left in the soil becomes available for other organisms, e.g. producers

Nutrient cycling diagram

Decomposers release carbon compounds from dead tissues and waste, making them available for themselves and for other organisms

• 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, e.g.
  • Photosynthetic organisms use light energy to fix carbon dioxide from the air into organic molecules such as carbohydrates; these are photoautotrophs
  • Some autotrophs use energy from the oxidation of inorganic compounds instead of light energy; organisms that use energy from oxidation of chemicals in this way are known as chemoautotrophs
• The organic molecules produced can be built up into a range of macromolecules needed to produce cells and tissues, e.g. proteins and lipids
  • Chemical reactions that build larger molecules from smaller molecules are described as anabolic
• 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

• The energy transferred to ATP during ATP synthesis comes from oxidation reactions
  • Oxidation reactions involve the loss of electrons; you may be familiar with the acronym OIL = oxidation is loss
  • The donated electrons can be used in the production of ATP
• Different types of autotroph harvest electrons in different ways:

Photoautotrophs

• Light energy is used to release electrons:
  • Light energy splits water in the process of photolysis, releasing electrons, hydrogen ions and oxygen
  • Light energy excites electrons in photosynthetic pigments, causing them to be released
• The resulting electrons and hydrogen ions can be used in the production of ATP and reduced coenzymes
• ATP and reduced coenzymes are then involved in the production of carbon compounds, such as glucose, during the light independent reactions of photosynthesis

Chemoautotrophs

• Chemoautotrophs oxidise inorganic chemicals in the environment to provide electrons for ATP production
• E.g. some bacteria produce ATP by oxidising iron
  • Iron(II), or Fe²⁺ is oxidised to iron(III), or Fe³⁺
  • The donated electrons are used in the production of ATP
• These chemoautotrophs take the role of producers in habitats where light energy is not available, e.g. in deep sea vents or caves

CC BY 4.0, via Wikimedia Commons

Chemoautotrophs are the producers in ecosystems found around deep sea volcanic vents

Heterotrophs

• Heterotrophic organisms gain their carbon compounds by ingesting the tissues of other organisms
  • This may be through eating plants, killing and consuming other animals, consuming the bodies of dead organisms, or consuming biological waste
• Heterotrophs ingest biological material from other organisms, breaking tissues down in the process of digestion before building the molecules back up into, e.g. new proteins and nucleic acids
  • Note that the process of digestion can occur either inside or outside the body of the heterotroph
• The new carbon compounds can then be assimilated into the bodies of heterotrophs, where they become available to the next trophic level
• There are several types of heterotroph, including consumers, detritivores, and saprotrophs

• Autotrophs produce their own carbon compounds, and heterotrophs gain carbon compounds from other organisms
• The chemical energy stored in these carbon compounds can be released by the process of respiration
• • The carbon compound glucose is the fuel for respiration
  • Other carbon compounds such as lipids can be converted into glucose before being respired
• Respiration releases energy by the oxidation of carbon compounds
• The energy released during respiration can be used by by autotrophs and heterotrophs to carry out the functions of life
  • Metabolism - the enzyme-catalysed reactions taking place inside cells
  • Reproduction - the sexual or asexual production of offspring
  • Homeostasis - the maintenance of internal conditions within tolerable limits
  • Growth - increasing in size
    • Note that during growth some of the chemical energy in the carbon compounds ingested by an organism is incorporated into the tissues of the organism as it grows; this stored chemical energy can be passed to the next trophic level in the food chain
  • Response - sensing and responding to the environment
  • Excretion - disposal of metabolic waste
  • Nutrition - gaining energy and nutrients
• The process of respiration also releases heat as a by-product