MediumBiologyHigh School (11–12)Diversity of Living Organisms › Kingdom MoneraConceptualNEETBoard Exam
In some rice fields, farmers grow the small water fern Azolla.
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In some rice fields, farmers grow the small water fern Azolla. Inside its leaves lives the cyanobacterium Anabaena. What are heterocysts, and what do they do? Why does this partnership help the rice crop?
Need a reminder? The key ideaShow
Cyanobacteria are photosynthetic autotrophs; some, such as Nostoc and Anabaena, fix nitrogen in heterocysts. Chemosynthetic bacteria oxidise inorganic substances (nitrates, nitrites, ammonia) for energy and recycle nutrients. Heterotrophic bacteria are the most abundant: decomposers, makers of curd and antibiotics, nitrogen fixers in legume roots, and pathogens (cholera, typhoid, tetanus, citrus canker). Mycoplasma are the smallest cells and have no cell wall.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Heterocysts. They are special cells of some cyanobacteria, such as Anabaena and Nostoc.
Step 2. Their work. They fix nitrogen: they turn the nitrogen gas of the air into compounds that plants can use.
Step 3. The crop. When the fern and the cyanobacteria decay, the fixed nitrogen enriches the water and the soil. Rice needs nitrogen for its proteins, so less fertiliser is needed.
Answer: heterocysts are nitrogen-fixing cells of cyanobacteria; they enrich the field with nitrogen for the rice.
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