When plants utilize light energy to reduce carbon dioxide to carbohydrates, they are called photosynthetic autotrophs. There are some bacteria which can utilize chemical energy released during biological oxidation of certain inorganic substances to reduce carbon dioxide to carbohydrate. These bacteria are called chemosynthetic autotrophs.

This is found in many colourless bacteria and because they use chemical energy to reduce carbon dioxide, this process of carbohydrate synthesis is known as chemosynthesis.

Chemosynthesis is the method of carbon assimilation when the reduction of CO2 is carried out in darkness, utilizing the energy obtained from oxidation of inorganic substances, such as H2S and NH3.

The common chemosynthetic forms are:

  • Nitrifying bacteria. Nitrosomonas and Nitrobactoroxidise NH3 to NO2
  • Sulphur bacteria
  • Iron bacteria
  • Hydrogen and methane producing bacteria

Chemiosmotic Synthesis

This is a process in which energy stored as a hydrogen ion gradient across a membrane is used to synthesise ATP from ADP and Pi. The enzyme which uses the energy is ATP synthase and the energy or power source is the difference in the concentration of H+ ions on opposite sides of the membrane. The membrane is the inner membrane of the mitochondrion or the chloroplast.

Chloroplasts use chemiosmosis to generate ATP during photosynthesis. The prokaryotes lack the organelles mitochondria and chloroplast to generate H+ gradients across plasma membranes and cannot use it for ATP synthesis.