Lessons 3–4 · 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
4
Prokaryotic cells
Textbook: pp. 13–16
GoalKnow the structure of prokaryotic cells, the features and importance of bacteria and blue-green algae.
New words
prokaryote · prokariotspore · sporaaerobe and anaerobe · aerob va anaerobnodule bacteria · tugunak bakteriyalar
Explanation
Prokaryotes have no formed nucleus: a circular DNA lies in the cytoplasm, and membrane organelles such as mitochondria and plastids are absent. They reproduce not by mitosis but by simple division into two (the textbook calls this amitosis). By shape bacteria are cocci (spherical), bacilli (rod-shaped) and vibrios and spirilla (curved); in unfavourable conditions some form resistant spores. Those living with oxygen are aerobes and those living without it are anaerobes. Bacteria are useful in decay and fermentation, in making dairy products, yoghurt and silage; nodule bacteria on the roots of legumes fix atmospheric nitrogen and enrich the soil. Parasitic bacteria cause diseases such as tuberculosis, cholera and anthrax; vaccination, clean water and hygiene protect against them. Blue-green algae (cyanobacteria) photosynthesise with chlorophyll and are also prokaryotes: Oscillatoria is filamentous, Nostoc forms colonies.
Worked examples
In making yoghurt, lactic acid bacteria ferment milk sugar: an example of the useful activity of bacteria.
Nodules on the roots of alfalfa or clover: the bacteria inside bind air nitrogen and enrich the soil with mineral nitrogen.
Class activity
“Bacteria class”. Draw three shapes (cocci, rods, spirals) on the board; students put cards with examples from life (yoghurt, anthrax, nodules) into a useful or harmful column. Do not experiment with microbes at home.
Practice
1
State the main difference between prokaryotes and eukaryotes.
Prokaryotes have no formed nucleus and no membrane organelles; DNA is circular in the cytoplasm.
2
Write the three shape groups of bacteria and describe what each looks like.
Cocci: spherical; bacilli: rod-shaped; vibrios and spirilla: curved.
3
Wheat is sown the next year on a field where legumes were grown. Why may the soil contain more nitrogen?
Nodule bacteria on legume roots bind atmospheric nitrogen and enrich the soil in nitrogen compounds.
4
Why are milk and meat kept in a refrigerator?
Bacteria multiply slowly in the cold, so food spoils later. But cold does not kill bacteria, so food cannot be kept in a refrigerator for ever.