Lessons 34–35 · 2 lessons · A. Zikiryayev, A. To‘xtayev, I. Azimov, N. Sonin. Biology. Basics of Cytology and Genetics, Grade 9, revised and expanded 5th edition. “Yangiyul Poligraph Service”, Tashkent, 2019
34
Chemosynthesis
Textbook: p. 87
GoalKnow the essence of chemosynthesis, chemosynthetic bacteria and their role in nature.
Chemosynthesis is the synthesis of organic matter from CO₂ by some bacteria using the energy of oxidation reactions of inorganic substances; they have no chlorophyll and need no light. This method was discovered in 1887–1890 by the Russian–Ukrainian microbiologist S. N. Winogradsky. Nitrifying bacteria oxidise ammonia to nitrite and nitrite to nitrate (nitrate is a nitrogen source that plants take up readily); iron bacteria change divalent iron to trivalent, and sulfur bacteria turn hydrogen sulfide into sulfur and sulfate. Chemosynthetic organisms are important in the cycling of elements such as nitrogen, sulfur and iron in nature and improve soil fertility. On the ocean floor around hot vents (hydrothermal vents), where sunlight does not reach, chemosynthetic bacteria are the primary food for a whole ecosystem (worms, molluscs); this was discovered in 1977.
Worked examples
NH₃ → NO₂⁻ → NO₃⁻: nitrification proceeds in two steps and each step gives the bacteria energy.
In photosynthesis the energy source is light, in chemosynthesis the oxidation of inorganic substances; in both, organic matter is made from CO₂.
Class activity
“Photosynthesis or chemosynthesis?” Cards list sources (sunlight, ammonia oxidation, H₂S oxidation, chlorophyll). Groups sort them into the two processes and find the common feature (organic matter from CO₂).
Practice
1
What is chemosynthesis?
Synthesis of organic matter from CO₂ using the energy of oxidation of inorganic substances.
2
Name two groups of bacteria that can carry out chemosynthesis.
Nitrifying, iron or sulfur bacteria.
3
State one common and one different feature of photosynthesis and chemosynthesis.
Common: organic matter is made from CO₂. Difference: the energy source is light versus oxidation of inorganic substances.
4
Why is life possible without sunlight around hydrothermal vents?
Chemosynthetic bacteria make organic matter with the energy from oxidising substances such as H₂S and feed other organisms.