Metabolism and Energy Transformation in Body Cells - Renata Armenakovna Petrosova 2004

Chemosynthesis

Photosynthesis is not the only mode of autotrophic Nutrition. There is another mechanism for synthesizing organic matter from inorganic precursors that does not require light energy. Earth is inhabited by organisms that derive energy through The oxidation of various Inorganic Compounds, utilizing it to reduce carbon dioxide into organic molecules. The process of synthesizing Organic compounds from inorganic substances using the energy derived from the oxidation of inorganic compounds is called chemosynthesis. It falls under chemoautotrophic nutrition.

Chemosynthetic organisms were discovered by the Russian microbiologist S.N. Winogradsky in 1887. These are Bacteria that utilize the energy of Chemical Reactions released during the oxidation of inorganic substances to synthesize organic compounds.

Depending on which substance's oxidation yields the energy, we distinguish nitrogen-fixing bacteria, nitrifying bacteria, iron bacteria, sulfur bacteria, and so on. Since these are prokaryotic organisms, their Cells lack specialized Organelles where redox reactions can take place. Their chemosynthetic processes occur on infoldings of Cell/30.html">The Plasma Membrane known as mesosomes. The source of hydrogen in these reactions is not only Water but also inorganic compounds such as hydrogen sulfide (H₂S) and molecular hydrogen (H₂).

Let us examine the chemical processes that take place within the cells of certain chemosynthetic bacteria.

1. In iron bacteria, ferrous iron is oxidized to ferric iron, a process accompanied by the release of energy:

Fe²⁺

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Fe³⁺ + Е;

4FeCO₃ + O₂ + 6H₂O

4Fe(OH)₃ + 4СO₂ + Е.

2. In colorless sulfur bacteria, elemental sulfur or hydrogen sulfide is oxidized.

2H₂S + O₂

2Н₂O + 2S + Е;

2S + 3O₂ + 2Н₂O

2H₂SO₄ + Е.

Large populations of sulfur bacteria inhabit the Black Sea, whose waters are rich in hydrogen sulfide.

3. Nitrifying bacteria oxidize ammonia into nitrous acid and subsequently into nitric acid:

2NH₃ + 3O₂

2HNO, + 2Н₂O + Е;

2HNO₂ + O₂

2HNO₃ + E.

All of the listed bacteria are aerobic, meaning the oxidation of inorganic substances occurs in the presence of oxygen. The energy released in the process is stored in ATP molecules, which are subsequently used to synthesize organic compounds:

ADP + P_i

ATP.

Anaerobic chemosynthesizers also participate in oxidation processes, but without the involvement of oxygen. Under anaerobic conditions, certain sulfate-reducing bacteria reduce sulfates and extract hydrogen from them. Denitrifying bacteria, which reduce free nitrogen, are also anaerobes.

Chemosynthesizing bacteria play a vital role in the biosphere by driving the elemental cycles in nature. The majority of these bacteria reside in the soil, where their activity directly contributes to soil fertility and natural water purification. Typical Examples of nitrifying bacteria include the ROOT-nodule bacteria found on the roots of legumes.

Questions and Review Exercises

1. What method of organic matter synthesis, other than photosynthesis, exists on Earth?

2. How do chemosynthesizing bacteria differ from photosynthetic organisms?

3. In your opinion, why did photosynthetic organisms achieve evolutionary dominance over chemosynthesizers?