Brent Cornell

    

Autotrophs, such as all plants and algae, convert inorganic carbon dioxide into organic compounds via photosynthesis

• These organic compounds include the carbohydrates, lipids and proteins required by the organism for survival

Since autotrophs use carbon dioxide for photosynthesis, the levels of carbon dioxide within the organism should always be low

• In other words, carbon dioxide should always be at a higher concentration in the atmosphere (or water)
• This concentration gradient ensures that carbon dioxide will passively diffuse into the autotrophic organism as required
• In aquatic producers, CO₂ can usually diffuse directly into the autotroph; whereas in terrestrial plants, diffusion occurs at stomata

Heterotrophs cannot synthesise their own organic molecules and instead obtain carbon compounds via feeding

    

All organisms may produce the chemical energy (ATP) required to power metabolic processes via the process of cell respiration

• Cell respiration involves the breakdown of organic molecules (e.g. sugars) and produces carbon dioxide as a by-product
• The build up of CO₂ in respiring tissues creates a concentration gradient, allowing it to be removed by passive diffusion

In autotrophs, the uptake of CO₂ by photosynthesis may at times be balanced by the production of CO₂ by respiration

• This is known as the compensation point, at which the net carbon dioxide assimilation is zero (intake = output)
  

Similarly, the amount of carbon dioxide in the environment will be determined by the level of these two processes:

• If there is more net photosynthesis than cell respiration occuring in the biosphere, atmospheric carbon dioxide levels should drop
• If there is more net respiration than overall photosynthesis occuring, atmospheric carbon dioxide levels should increase

Photosynthesis versus Cell Respiration