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1.What is the function of heterocyst?

2.Where it is present?

A heterocyst I think may be a protecting organ in cells!

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    $\begingroup$ They help in nitrogen fixing . Ex nostoc of and anabaena $\endgroup$ – Ashish Aug 5 '17 at 6:07
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According to wikipedia :

Heterocysts are specialized, pale-yellow,thick-walled cells with disputed function nitrogen-fixing formed during nitrogen starvation by some filamentous cyanobacteria, such as Nostoc punctiforme...

Thus by definition these are not within the cells, but differentiated cells themselves.

This is a good paper that can give you details on nitrogen fixation in the cyanobacterium Anabaena variabilis.

In general: cyanobacteria are photosynthetic prokaryotes, and many of them are capable of fixing nitrogen (that is the ability to use and incorporate nitrogen from the air as N2 gas). The enzyme called nitrogenase is oxygen sensitive thus either temporal or spatial separation of nitrogen fixing must be done to avoid damage to the enzmye from oxygen produced by photosynthesis.

In Anabaena spp., aerobic nitrogen fixation is confined to differentiated cells called heterocysts that form in a semiregular pattern in a filament in response to nitrogen starvation. Fixed nitrogen in the heterocysts is transported to vegetative cells in the filament, while vegetative cells supply carbon and reductant to heterocysts

So as you can see spearate cells work together to supply each other with necessary nutrients. It is important to note that when sufficient fixed nitrogen source is available the number of such specialized formations is low due there is no need for extra nitrogen to be fixed

There are plenty of details on heterocysts and nitrogen fixation in the linked article. Also this paper hold details of regulation of nitrogen fixation and also has a thorough image of the process:

nitrogen fixation

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  • $\begingroup$ Though perhaps not relevant to the question, and certainly not true about any cyanobacteria that I know of, but just because something is a "cell" or single celled organism, does not mean it can't be contained within other cells. Mycoplasma, for example, do it all the time. Admittedly it doesn't normally end well for one group of cells, but it happens. +1 $\endgroup$ – Atl LED Sep 18 '15 at 2:07
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First of all the short and sweet definition of heterocyst are the specialized cells with homogeneous protoplast. Both the ends have thickenings called polar nodules. Polar nodules are provided with minute pores. Heterocyst help to fix the atmospheric nitrogen compounds. In some species of Nostoc like N.commune heterocysts function as resting pores. The protoplasm of the heterocysts become functional and germinate to form a new filament.

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  • $\begingroup$ Hi Christina. Welcome to Biology. Can you add some references to your answer. You can have a look at the other answer. In general, if you post an answer make sure that you are not exactly repeating the stuff already said in the other answers. You can of course provide a better answer than the previous ones by adding additional supportive references. That is not the case here. $\endgroup$ – WYSIWYG Sep 10 '15 at 14:06

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