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Nitrogen cycle - class-X

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Reaction carried out by $N _2$ fixing microbes include
$2NH _3 + 3O _2 \rightarrow 2NO _2^- + 2H^+ + 2H _2O$            (i)
$2NO _2^- + O _2 \rightarrow 2NO _3^-$                                       (ii)
Which of the following statements about these equations is not true?

  1. Step (i) is carried out by Nitrosomonas or Nitrosococcus

  2. Step (ii) is carried out by Nitrobacter

  3. Both step (i) and (ii) can be called nitrification.

  4. Bacteria carrying out these steps are usually photoautotrophs.


Correct Option: D
Explanation:
Phototrophic bacteria are a group of bacteria, whose energy for growth is derived from sunlight and their source of carbon comes from carbon dioxide or organic carbon.
So, the correct option is 'Bacteria carrying out these steps are usually photoautotrophs'.

A farmer adds Azotobacter culture to soil before sowing maize. Which mineral element will be replenished by doing so?

  1. N

  2. P

  3. K

  4. S


Correct Option: A
Explanation:
Azotobacter is a free living bacteria in the soil. It helps some cereal crops like maize (Zea mays) in nitrogen fixation. The farmer adds Azotobacter culture to the maize field for enhancing the nitrogen element in the soil by the process of biological nitrogen-fixation. 
Thus, the correct answer is 'N.'

Which of the following is a non-symbiotic nitrogen fixing prokaryote?

  1. Azotobacter

  2. Clostridium

  3. Beijerinckia

  4. All of these


Correct Option: D
Explanation:

Two kinds of nitrogen-fixing bacteria are recognized. The first kind, the free-living (non-symbiotic) bacteria, includes the cyanobacteria (or blue-green algae), Anabaena and Nostoc and genera such as Azotobacter, Beijerinckia, and Clostridium

Thus, the correct answers is 'All of these.'

Which of the following is a free living aerobic nitrogen fixing bacterium present in the soil?

  1. Nitrosomonas

  2. Rhizobium

  3. Azotobacter

  4. Pseudomonas


Correct Option: C
Explanation:

Azotobacter and Beijerinckia  are aerobic free living saprophytic nitrogen fixing bacteria, while Clostridium is anaerobic free living saprotrophic nitrogen fixing bacterium.

Mark the incorrect match.

  1. Rhodospirillum-anaerobic nitrogen fixer

  2. Azotobacter- aerobic nitrogen fixer

  3. Bacillus vulgaris-denitrifying bacteria

  4. Rhizobium- $N _2$ fixation in alfalfa


Correct Option: A

Which one of the following bacteria is not connected with nitrogen fixation?

  1. Azotobacter

  2. Nitrosomonas

  3. Frankia

  4. Rhizobium


Correct Option: B
Explanation:

Nitrogen fixation is referred to as conversion of atmospheric nitrogen into ammonia, whereas the Nitrosomonas is a genus of Gram-negative, chemoautotrophic bacteria. This organism oxidizes ammonia into nitrite as a metabolic process. 

 So, the correct option is 'Nitrosomonas'.

Which of the following is a flowering plant with nodules containing filamentous nitrogen-fixing microorganism?

  1. Cicer arietinum

  2. Casuarina equisetifolia

  3. Crotalaria juncea

  4. Cycas revolute


Correct Option: B
Explanation:
Answer :
i) All the casuarinas are nitrogen-fixing. 
ii) Casuarinas support an actinorhiza symbiont in their root nodules, as opposes to the rhizobium symbiont found in the root nodules of leguminous trees that fix N2. 

Therefore correct answer is option B.Casuarina equisetifolia.

Nitrifying bacteria convert the________________

  1. Nitrates into nitrites

  2. Ammonium salts into nitrates

  3. Ammonium salts into amino acids

  4. All of the above


Correct Option: B
Explanation:

Nitrifying bacteria are chemoautotrophic or chemolithotrophs depending on the genera (Nitrosomonas, Nitrosococcus, Nitrobacter, Nitrococcus) bacteria that grow by consuming inorganic nitrogen compounds. Many species of nitrifying bacteria have complex internal membrane systems that are the location for key enzymes in nitrification: ammonia monooxygenase, which oxidizes ammonia to hydroxylamine, and nitrite oxidoreductase, which oxidizes nitrite to nitrate. Nitrifying bacteria are a narrow species in the environment and are found in the lowest numbers, where considerable amounts of ammonia are present (areas with extensive protein decomposition, and sewage treatment plants). Nitrifying bacteria thrive in lakes and rivers streams with high inputs and outputs of sewage and wastewater and freshwater because of the low ammonia content.

An organism used as a biotertilizer for raising soyabean crops is ______________.

  1. Azotobacter

  2. Azospirillum

  3. Rhizobium

  4. Nostoc


Correct Option: C

Nitrifying bacteria _________________.

  1. Oxidize.2riimonia to nitrates

  2. Convert free nitrogen to nitrogen compounds

  3. Convert proteins into ammonia

  4. Reduce nitrates to free nitrogen


Correct Option: A

Nitrifying bacteria are able to 

  1. Convert atmospheric nitrogen into soluble forms

  2. Convert ammonia to nitrate

  3. Ammonia to nitrogen

  4. Nitrate to nitrogen


Correct Option: B
Explanation:

Nitrification is an important process in nitrogen cycle in nature, which is brought about by nitrifying bacteria. In warm moist soils having a temperature of $30-35^0$C and neutral pH, ammonia gets oxidized to nitrite ($NO _2^-$) and then nitrate ($NO _3^-$) by the process of nitrification. 

Nitrifying bacteria like Nitrosomonas convert ammonia to nitrite and another bacterium, called as Nitrobacter converts nitrite to nitrate.

The common nitrogen-fixer in paddy field is _______________.

  1. Azospirillum

  2. Oscillatoria

  3. Frankia

  4. Rhizobium


Correct Option: A
Explanation:

The common Nitrogen fixer in the paddy field is Azospirillum. This is a soil and root bacteria. It is a nitrogen-fixing bacteria when added to the soil it multiplies and supplies 30-40Kg nitrogen per hectare per season. It also produces Indole acetic acid which helps in growth.


Hence the correct option is A.

Which of the following can act as Nitrogen fixers?

  1. Blue green algae

  2. Bacteria

  3. Both A and B

  4. None of these


Correct Option: C
Explanation:

Some bacteria and cyanobacteria (blue-green algae) are able to fix nitrogen into usable forms of salts of nitrogen for plants.

The microorganisms that play an important role in maintaining the balance of nitrogen gas in the atmosphere are ________

  1. Bacteria

  2. Algae

  3. Virus

  4. Protozoa


Correct Option: A
Explanation:

Rhizobium and Cynobacteria are involved in nitrogen fixation. There are many more bacteria like Nitrobacter, Nitrosomanas etc involve in maintaining the balance of nitrogen gas in the atmosphere.

The bacteria present in root nodules of leguminous plants that fix the atmospheric nitrogen is

  1. Rhizobium

  2. Blue-green algae

  3. Nitrifying bacteria

  4. Paramecium


Correct Option: A
Explanation:

Rhizobium is a bacterium found in the root nodules of leguminous plants. It induces the formation of special structures called nodules inside which it fixes atmospheric nitrogen. It does this with the help of nitrogenase enzyme that converts of nitrogen to ammonium that is an important factor required for plant growth. In turn, the bacterium receives shelter from the plant. Thus the correct answer is option A.

Oxidation of nitrite into nitrate is done by

  1. E. coli

  2. Nitrobacter

  3. Pseudomonas

  4. All of the above


Correct Option: B
Explanation:

The second step (oxidation of nitrite into nitrate) is done mainly by bacteria of the genus Nitrosomonas and Nitrobacter.

Oxidation of Ammonia into nitrite is performed by


(a) Nitrosomonas (b) Nitrosococcus (c) Nitrobacter (d) Nitrospira

  1. a, b

  2. b, c

  3. c, d

  4. a, d


Correct Option: A
Explanation:

Oxidation of Ammonia to nitrite is performed by two groups of bacteria. Nitrosomonas and Nitrosocossus are examples of bacterias which perform the oxidation of ammonia to nitrite.

Anaerobic bacteria are poisoned by which element ?

  1. Oxygen

  2. Nitrogen

  3. Carbon dioxide

  4. Carbon


Correct Option: A
Explanation:

Anaerobic bacteria do not need oxygen. This is because they don't  use oxygen in any of their processes. Thus bacteria are poisoned by oxygen.

Nitrifying bacteria convert the

  1. Nitrates into nitrites

  2. Nitrites into nitrates

  3. Ammonium salts into nitrates

  4. Ammonium salts into amino acids


Correct Option: C
Explanation:

Nitrification is a process of conversion of ammonium salts to nitrites and nitrites to nitrates. The bacteria that carry out this process are called as nitrifying bacteria. Nitrosomonas, Nitrosospira, Nitrosococcus convert ammonia into nitrites. Nitrobacter, Nitrospina, Nitrococcus convert nitrites into nitrates.

Thus, the correct answer is 'Ammonium salts into nitrates.'

In excess, $C{O} _{2}$

  1. Accelerates growth

  2. Does not influence growth

  3. Inhibits growth

  4. Slows down growth


Correct Option: C
Explanation:

Atmospheric carbon dioxide is used by plants to produced food. Due to industrialization, large amount of carbon dioxide is released in the atmosphere. This CO$ _2$ provides nutrient source to certain plants. However, excess amount of CO$ _2$ inhibits growth in plants. It inhibits nitrate assimilation in certain plants. It basically inhibits absorption of nitrogen.

Thus, the correct answer is option C.

Nitrosomonas and Nitrobacter convert

  1. Carbon dioxide to carbohydrates

  2. Ammonium ion into nitrate

  3. Nitrate ion into nitrogen molecule

  4. Nitrogen into nitrate


Correct Option: B
Explanation:

Nitrification is a process of conversion of ammonium ions to nitrites and nitrites to nitrates. The bacteria that carry out this process are called as nitrifying bacteria. Nitrosomonas, Nitrosospira, Nitrosococcus convert ammonia into nitrites. Nitrobacter, Nitrospina, Nitrococcus convert nitrites into nitrates.

Thus, the correct answer is 'Ammonium ion into nitrate.' 

The common nitrogen fixer organism in paddy fields is

  1. Azospirillum

  2. Oscillatoria

  3. Frankia

  4. Rhizobium


Correct Option: A
Explanation:

Atmospheric nitrogen is not available to plants. There are certain nitrogen fixing bacteria which converts atmospheric nitrogen into ammonium compounds and make it available to the plants. Examples include Azotobacter, Frankia, Rhizobium, Azospirillum. Out of these, Azospirillum is widely used as nitrogen fixer organism in paddy fields. It is used as biofertilizer in rice fields. It promotes the plant growth.

Thus, the correct answer is 'Azospirillum.'

Nitrite is changed to nitrate by

  1. Nitrobacter

  2. Nitrosomonas

  3. Clostridium

  4. Pseudomonas


Correct Option: A
Explanation:

There are two steps in nitrification process by which ammonia is converted into nitrite and nitrite into nitrate. The bacteria that convert ammonia to nitrites include NitrosomonasNitrosospira and the bacteria that convert nitrites to nitrates include NitrobacterNitrospina

Thus, the correct answer is 'Nitrobacter.'

Carefully read the following reactions carried out by nitrifying bacteria. Identify the statement about these equations which is not true.
2NH$ _3$+3O$ _2$ $\rightarrow$ 2NO$^- _2$ +2H$^+$ +2H$ _2O$ ---- (I)
2NO$^- _2$+O$ _2$ $\rightarrow$ 2NO$^- _3$ ---- (II)

  1. Step I is carried out by Nitrosomonas or Nitrosococcus

  2. Step II is carried out by Nitrobacter

  3. Both the steps I and II can be called nitrification

  4. Both the steps occur only in photoautotrophs


Correct Option: D
Explanation:
I) In the first step, ammonia is oxidised to nitrites by the action of nitrosomonas and nitrococcus bacteria.
II) In the second step, nitrate is further oxidised to nitrate with the help of nitrobacter and nitrocystis.
Both the reaction is involved in the process of nitrification. These nitrifying bacteria involved are called chemoautotrophs.
So the correct answer is 'Both the steps occur only in photoautotrophs'.

Which one is not a free living nitrogen fixer?

  1. Rhodospirillum

  2. Azotobacter

  3. Rhizobium

  4. Bacillus


Correct Option: C
Explanation:

The organisms involved in nitrogen fixation are termed as nitrogen-fixing organisms. They convert free nitrogen (gaseous form dinitrogen, N2) into forms such as nitrates and nitrites that can be utilized by plants. These organisms could either be free-living or found in symbiotic, mutualistic, or commensalistic relationships with other plants etc. Rhizobium is found in a symbiotic relationship with plant roots. 

So, the correct answer is 'Rhizobium'.

Which of the following is free-living bacteria responsible for nitrogen fixation 

  1. Pseudomonas

  2. Rhodospirillum

  3. Azotobacter

  4. All of these


Correct Option: D
Explanation:

Nitrogen-fixing microorganisms are recognized: free-living (non-symbiotic) bacteria, including the cyanobacteria. Anabaena and Nostoc and genera such as Azotobacter, Beijerinckia, and Clostridium and mutualistic bacteria such as Rhizobium, associated with leguminous plants.

So, the correct option is 'All of these'.

In the process of nitrification, ammonia is first oxidized to nitrite by

  1. Nitrosomonas and Nitrosococcus

  2. Nitrosomonas and Nitrobacter

  3. Pseudomonas and Nitrosococcus

  4. Pseudomonas and Thiobacillus


Correct Option: A
Explanation:

First step, ammonium ions are oxidised to nitrites. Nitrosomonas  are the  most important agents of oxidation of ammonia to nitrite in soil. Certain other bacteria are Nitrosococcus , Nitrosolobus ,etc.

Second step , oxidation of nitrite to nitrate takes place by Nitrobacter. 
So the correct option is "Nitrosomonas and Nitrococcus".

Which of the following is a symbiotic nitrogen fixer ?

  1. Azolla

  2. Glomus

  3. Azotobacter

  4. Frankia


Correct Option: D
Explanation:
Symbiotic nitrogen fixation is part of a mutualistic relationship in which plants provide a niche and fixed carbon to bacteria in exchange for fixed nitrogen. Frankia is a nitrogen-fixing bacterium that lives in the soil and has a symbiotic relationship with many plants. Frankia is a genus of nitrogen-fixing, filamentous bacteria that live in symbiosis with actinorhizal plants, similar to the Rhizobium bacteria found in the root nodules of legumes in the family Fabaceae.

So, the correct answer is 'Frankia'.

Phosphorus is required for better nitrogen fixation by Rhizobium

  1. True

  2. False


Correct Option: A
Explanation:
Phosphorous plays an important role in the symbiotic association for the natural fixation of the nitrogen in the plants .
Hence the given statement is true .

A nitrogen-fixing bacterium, which forms a loose association with the roots of crop plants (grasses) is

  1. Azotobacter

  2. Bacillus polymyxa

  3. Clostridium

  4. Azospirillum


Correct Option: D
Explanation:
Azospirillum is a free-living nitrogen-fixing bacteria closely associated with crops of grass family
So, the correct answer is 'Azospirillum'

How many organisms In the list given below are $N _2$ fixer autotrophs?
Azotobacter, Chromobum, Azospirillum, Rhodospirillum, Rhizobium, Nitrobacter, Aulosira, Nostoc, Osallatona

  1. Three

  2. Four

  3. Five 

  4. Six


Correct Option: C
Explanation:

Nitrogen-fixing bacteria are capable of fixing atmospheric nitrogen to inorganic substances usable by plants. Example from the given list are Azetobactor, Azospirillum, Rhizobium, Nitrobactor and Nostoc.

So the correct answer is 'five'.

Which of the following is a free living aerobic non - photosynthetic nitrogen-fixer ?

  1. $Rhizobium$

  2. $Azotobacter$

  3. $Azospirillum$

  4. $Nostoc$


Correct Option: B
Explanation:

Azotobacter is a free-living nitrogen-fixing bacterium, which is used as a biofertilizer in the cultivation of most crops. Azotobacter respires aerobically, receiving energy from redox reactions, using organic compounds as electron donors. Azotobacter can use a variety of carbohydrates, alcohols, and salts of organic acids as sources of carbon. 

Rhizobium fixes nitrogen by symbiotic association. Nostoc is photosynthetic. Azospirillum is anaerobic. 
So, the correct option is 'Azotobacter'.

All of the following statements concerning the actinomycetous filamentous soil bacterium Frankia are correct except that Frankia

  1. Can induce root nodules on many plant species

  2. Can fix nitrogen in the free-living state

  3. Like Rhizobium, it usually infects its host plant through root hair deformation and stimulates cell proliferation in the host's cortex

  4. Forms specialized vesicles in which the nitrogenase is protected from oxygen by a chemical barrier involving triterpene hopanoids


Correct Option: B
Explanation:

Frankia is a bacterium which grows in a symbiotic association with the roots of non-leguminous plants.  It can fix atmospheric nitrogen. It cannot fix nitrogen in free-living state.

So, the correct option is 'Can fix nitrogen in free-living state'.

Bacteria involved in two-step conversion ammonia into nitrate are

  1. Azotobacter and Nitrosomonas

  2. Pseudomonas and Nitrobacter

  3. Azotobacter and Achromobacter

  4. Nitrosomonas and Nitrobacter


Correct Option: D
Explanation:

Nitrification is a process of conversion of ammonium ions to nitrites and nitrites to nitrates. It is a two-step process. The bacteria that carry out this process are called as nitrifying bacteria. Nitrosomonas, Nitrosospira, Nitrosococcus convert ammonia into nitrites. Nitrobacter, Nitrospina, Nitrococcus convert nitrites into nitrates.

Thus, the correct answer is 'Nitrosomonas and Nitrobacter.' 

Which of the following is a symbiotic nitrogen fixer?

  1. Frankia

  2. Anabaena azollae

  3. Glomus

  4. Both A and B


Correct Option: D

Which gene cluster is responsible for nitrogen fixation in bacteria?

  1. Nod, Nif, Nfx

  2. Nod, Nif, Fix

  3. Nod, Ndf, Nfx

  4. Ndx, Nif, Fix

  5. Ndx, Nif, nix


Correct Option: B
Explanation:
Nitrogen fixation is a process by which nitrogen present in the atmosphere is converted into ammonia. It is essential to life because fixed inorganic nitrogen compounds are required for the biosynthesis of all nitrogen-containing compounds. This process is done with the help of nitrogen-fixing bacteria that have Nod, Nif and Fix gene clusters responsible for nitrogen fixation.
So the correct answer is 'Nod, Nif, Fix'.

Match the columns and choose the correct option.

I II
(a) Free-living aerobic nitrogen fixers 1. Anabaena and Nostoc
(b) Anaerobic nitrogen fixers 2. Pseudomonas and Thiobacillus
(c) Nitrogen-fixing cyanobacteria 3. Nitrosomonas and Nitrosococcus
(d) Denitrifying bacteria 4. Azotobacter and Beijerinckia
(e) Nitrifying bacteria 5. Rhodospirillum
  1. a - 4, b - 5, c - 1, d - 2, e - 3 

  2. a - 5, b - 4, c - 1, d - 3, e - 2

  3. a - 4, b - 5, c - 2, d - 3, e - 1

  4. a - 4, b - 3, c - 1, d - 2, e - 5

  5. a - 4, b - 5, c - 3, d - 2, e - 1


Correct Option: A
Explanation:
a) Free-living aerobic nitrogen fixers - Azotobacter and Beijernikia
b) Anaerobic nitrogen fixers - Rhodospirillum
c) Nitrogen-fixing cyanobacteria - Anabaena and Nostoc
d) Denitrifying bacteria - Pseudomonas and Thiobacillus
e) Nitrifying bacteria - Nitrosomonas and Nitosococcus
So the correct answer is 'a-4, b-5, c-1, d-2, e-3'

Which one helps in absorption of phosphorus from soil by plants?

  1. Frankia

  2. Anabaena

  3. Glomus

  4. Rhizobium


Correct Option: C
Explanation:

Frankia, Anabaena and Rhizobium are nitrogen-fixing bacteria that absorbs nitrogen from the soil. Glomus is a myccorhizal fungi. The fungal symbiont in these associations absorbs phosphorus from soil and passes it to the plant.
Thus, the correct answer is option (C). 

Which of the following is the function of nitrifying bacteria?

  1. Oxidise $NH _3$ to $NO^- _3$

  2. Oxidise $NH _3$ to $NH^+ _4$

  3. Convert $NO^- _3$ to $NH _3$

  4. Convert $NO^- _3$ to $N _2$


Correct Option: A
Explanation:
Nitrifying bacteria is an aerobic bacteria that use inorganic chemicals as an energy source. They are important in the nitrogen cycle as converters of soil ammonia to nitrates by the process of oxidation.
So the correct answer is 'oxidise NH$ _3$ to NO$ _3$-'.
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