IB Biology SL

Revision Notes

4.2.1 Carbon Cycle: Carbon Dioxide

Carbon Fixation

  • Carbon is present in the atmosphere in the form of carbon dioxide gas
  • Carbon is taken out of the atmosphere by plants to be used in photosynthesis
  • Plants are autotrophs
  • Autotrophs use light energy to convert carbon dioxide from the air into carbon compounds such as
    • Carbohydrates
    • Lipids
    • Amino acids
  • This reduces the amount of carbon present in the atmosphere and stores it in the tissues of plants

Carbon Dioxide in Solution

  • Where the atmosphere comes into contact with bodies of water, carbon dioxide in the atmosphere can dissolve in water
    • The oceans are thought to store significantly more carbon than the atmosphere
  • Some of the carbon in aquatic systems is in the form of dissolved carbon dioxide
  • Some dissolved carbon dioxide reacts with water to form carbonic acid, (H2CO3), which then dissociates to produce hydrogen carbonate ions (HCO3), and hydrogen (H+) ions
    • H+ ions in the water cause it to become more acidic; as more carbon dissolves in the oceans this can cause problems for some marine organisms
  • Aquatic producers can absorb dissolved carbon dioxide and hydrogen carbonate ions for photosynthesis, using light energy from the sun to convert this carbon into other carbon compounds such as carbohydrates

Carbon Dioxide Absorption

  • The processes of respiration and photosynthesis both rely on the exchange of oxygen and carbon dioxide

Gas exchange during respiration

  • All living cells respire
  • The process requires the uptake of oxygen and the release of carbon dioxide

 

Word equation for aerobic respiration, IGCSE & GCSE Chemistry revision notes

The respiration equation

  • The oxygen diffuses down the concentration gradient from a high concentration (outside the leaf) to a low concentration (inside the leaf)
    • The cells use oxygen in respiration so the concentration is always low inside the respiring cells
  • The carbon dioxide diffuses down the concentration gradient from a high concentration (inside the leaf) to a low concentration (outside the leaf)

Gas exchange in respiration, downloadable IGCSE & GCSE Biology revision notes

In respiration, carbon dioxide diffuses out of the leaf and oxygen diffuses into the leaf

Gas exchange during photosynthesis

  • Plant cells photosynthesize when there is enough sunlight
  • The process requires the uptake of carbon dioxide and the release of oxygen

Photosynthesis word equation, IGCSE & GCSE Biology revision notes

The photosynthesis equation

  • The carbon dioxide diffuses down the concentration gradient from a region of high concentration (outside the leaf) to a region of low concentration (inside the leaf)
    • The cells use carbon dioxide in photosynthesis so the concentration is always low inside the photosynthesizing cells
  • The oxygen diffuses down the concentration gradient from a high concentration (inside the leaf) to a low concentration (outside the leaf)

Gas exchange in photosynthesis, downloadable IGCSE & GCSE Biology revision notes

In photosynthesis, carbon dioxide diffuses into the leaf and oxygen diffuses out of the leaf

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