How plant cells take up and transform nitrate

Plants Require Fixed Nitrogen
Nitrogen is frequently limiting to plant growth in nature and in crop fields, because large amounts of it are required for plant synthesis of amino acids, nucleotides, and alkaloids, among many other cellular constituents. Nitrogen is the largest component of plants by mass after carbon, oxygen, and hydrogen. Although Earth's atmosphere is 78% nitrogen gas (N2), plants cannot use nitrogen in this form.

To be of use to plants, soil nitrogen must occur in a combined form, such as ammonia (NH3), ammonium ion (NH4+), or nitrate ion (NO3+ Combined forms of nitrogen are also known as fixed nitrogen. Ammonia and its dissolved form—ammonium ion—can be used directly for amino acid production by plants, explaining why ammonia is often applied as a fertilizer to farm fields in spring. However, in oxygen-rich soils, microorganisms oxidize much of the ammonium to nitrate, so nitrate may be the form in which fixed nitrogen enters most plants.

Nitrate is imported into root cells by means of plasma membrane transporter proteins (Figure 35.12). Once inside the plants, the nitrate can be transported in the xylem for use elsewhere, stored for later use, or assimilated for immediate use. Cells store nitrate by transporting it into vacuoles. To assimilate nitrate, plant cells must reduce it to ammonium ion in a two-step process. First, the cytosolic enzyme nitrate reductase reduces nitrate (NO3?) to nitrite (NO2?) Second, in plastids, nitrite is reduced to ammonium ion by nitrite reductase. Another plastid enzyme, glutamine synthetase, uses ammonium ion to produce the amino acid glutamine.

Atmospheric Nitrogen Is Fixed by Natural and Industrial Processes
Much of the fixed nitrogen in soils has been recycled from compounds previously used by other organisms. New fixed nitrogen can be added to soils by the action of lightning, fire, and air pollution, as well as biological and industrial nitrogen fixation. Nitrogen fixation is the process by which atmospheric nitrogen gas is combined with hydrogen to produce ammonia. Most of the fixed nitrogen in soils is produced by biological nitrogen fixation, which is performed in nature only by certain prokaryotes. Nitrogen fertilizers applied to crops are produced by industrial nitrogen fixation, a human activity.