Nitrogen Metabolism in Plants

Nitrogen is one of the important elements found in living organisms. Supply of nitrogen from the atmosphere is maintained.

Nitrogen is an essential constituent of amino acids and hence the proteins cannot be absorbed by the plants directly from the atmosphere though it contains 79% of nitrogen. Plants can only absorb nitrogen in the form of salts of nitrogen.

Biological Nitrogen Fixation

Conversion of nitrogen into compounds is essential by combining with carbon, hydrogen and oxygen before it can be absorbed by the plants. This is known as nitrogen fixation. The micro-organisms which can fix atmospheric nitrogen can be classified into two main groups:

i) Symbiotic micro-organisms

ii) Free living micro-organisms

Symbiotic Micro-organisms

Some plants have an ability to fix nitrogen from the atmosphere through a process called symbiotic nitrogen fixation.

The symbiotic bacterium Rhizobium is found in association with the root nodules of the leguminous plants such as beans, peas, gram and groundnut. The association between the Rhizobium and the leguminous plants is symbiotic in nature.

Root nodules acts as a site of Nitrogen fixation. The root nodules contain a pigment called leghaemoglobin. It gives a pinkish colour as it is closely related to the haemoglobin present in RBC's. Like haemoglobin it also combines with oxygen. By combining with oxygen leghaemoglobin protects the enzyme nitrogenase, which functions only under anaerobic conditions. Nitrogenase is the only enzyme that can split nitrogen molecule for nitrogen fixation.

Formation of root nodules

Root nodules are the sites of nitrogen fixation.

a) When a root hair of a leguminous plant comes in contact with the bacterium -Rhizobium, it curls or becomes deformed.

b) At the site of curling, the rhizobia (bacteria) invade the root tissue.

c) Some of the bacteria within the root tissue enlarge to become membrane bound structures called bacteroids. These cannot divide, while some bacteria remain untransformed to facilitate further infection.

d) The plant responds to this invasion by forming an infection thread made up of plasma membrane that grows inward from the infected cell of the host, separating the infected from the rest of the plant.

e) Cell division now sets in, in the infected tissue leading to nodule formation. The nodule thus formed establishes a direct vascular connection with the host for the exchange of nutrients.

Nitrogen fixation

The different steps involved in nitrogen fixation are as follows:

1) Atmospheric nitrogen is reduced by the addition of hydrogen atoms.

2) As a result the there bonds between the two nitrogen atoms are broken down resulting in the formation of ammonia.

3) Nitrogen fixation thus requires the following three components:

i) A strong reducing agent

ii) ATP to transfer hydrogen atoms to dinitrogen.

iii) enzyme systems.

4) Ammonia thus formed as a result of nitrogen fixation is used for the synthesis of amino acids. Amino acids are the building blocks for the synthesis of proteins.

Synthesis of amino acids

Amino acids are synthesised by two main processes:

1) Reductive animation

In this process, ammonia reacts with a-ketoglutaric acid to form glutamic acid.

2) Transamination

Glutamic acid is the main amino acid from which other 17 amino acids are formed through transamination.

Each amino acid is made up of one carboxyl group (-COOH) and one or more amino groups (-NH₂). Transamination involves the transfer of amino group from one amino acid to the ketogroup of keto acid. The enzyme responsible for transamination is transaminase.

Amides

Amides contain more nitrogen than amino acids and form the structural part of most proteins. The two important amides found in plants are asparagine and glutamine. They are formed from two amino acids namely glutamic acid and aspartic acid. During amide formation, the hydroxyl part of the acid is replaced by another NH₂ radicle. This reaction takes place in the presence of the enzymes glutamine synthetase or asparagine synthetase.

Proteins synthesis

Proteins are made up of amino acids. Proteins are in the form of one or more chain called polypeptide chains. Within the polypeptide chains, the amino acids and amides are linked through peptide bonds, involving the carboxyl group of one amino acid and the amino group of the next. The number of amino acids varies greatly among proteins and thus differs the molecular weight of proteins also.

Free-living Micro-organisms

The micro-organisms like cyanobacteria (blue-green algae) and photosynthetic bacteria can also fix nitrogen. Some cyanobacteria also act as symbionts and are present in association with lichens; liverwort - Anthoceros; fronds of water fern - Azolla and roots of a gymnosperm - Cycas.

Process of Biological Nitrogen Fixation

During N₂ fixation, atmospheric N₂ is fixed into organic compounds (like amino acids, proteins, nucleic acids) in living organisms, via inorganic forms such as NH₄⁺.

In nitrogen fixation a large amount of energy is required to separate the two nitrogen atoms of the nitrogen molecule. This energy is derived from the ATP molecules in the micro-organisms. ATP molecules are provided either by respiration or photosynthesis.

The nitrogen fixation takes places in the presence of enzyme nitrogenase only, which reduces the nitrogen molecule (dinitrogen) into ammonia.

Ammonia formed as a result of nitrogen fixation is ultimately utilised for the synthesis of amino acids. The amino acids are used to make organic compounds mainly proteins. The sequence of events that occur during nitrogen fixation are shown in the following figure:

A schematic diagram showing progressive reduction of one molecule of nitrogen in the presence of enzyme nitrogenase to yield two molecules of ammonia

The plants pick up nitrogen from the soil in the form of ammonium ions (NH₄⁺) or nitrate ions (NO₃^-), ammonia being the main product of biological nitrogen fixation. It is converted to nitrates by a number of soil bacteria (nitrate being the major source of nitrogen for plants)

Ammonification

The dead remains of plants and animals are decomposed through microbial activities to produce ammonia. This process is known as ammonification.

Nitrification

It is the process of conversion of ammonia to nitrates. It is accomplished by nitrifying bacteria like nitrosomonas, nitrosococcus and nitrobacter.

Ammonia is first converted to nitrite ions and then to nitrate ions as given below:

The nitrifying bacteria mentioned above are chemoautotrophs as they derive energy by oxidising inorganic materials like ammonia and nitrite. The bacteria are aerobic and oxygen is an electron acceptor.

The nitrate ion thus obtained, is either made available to the plant or converted to nitrogen gas by a process of denitrification.

Nitrate Assimilation in Plants

After nitrates have been absorbed by the plant, they are reduced to ammonia with the help of two enzymes nitrate reductase and nitrite reductase.

Nitrate reductase

Reduces nitrate to nitrite. This reaction can take place in any part of the plant body. Nitrate reductase is a flavoprotein and contains molybdenum.

Nitrite reductase

Reduces the nitrite ions to ammonium ions. Nitrite reductase does not require molybdenum and may contain copper and iron. Since ferredoxin is the direct source of electrons, this reaction takes place in the leaves and the nitrite ions formed elsewhere are also transported to leaves for reduction.