Question

Hydroponics have been shown to be a successful technique for growing of plants. Yet most of the crops are still grown on land. Why?

Answer 1

The nodule contains all the necessary biochemical components, such as the enzyme nitrogenase and leghaemoglobin. The enzyme nitrogenase is a Mo – Fe protein and catalyses the conversion of atmospheric nitrogen to ammonia, the first stable product of nitrogen fixation.

N₂ + 8e^– + 8H⁺ + 16ATP → 2NH₃ + H₂ + 16ADP + 16P₁

• In the above reaction, the ammonia synthesis by nitrogenase requires a very high input of energy (8 ATP for each NH₃ produced). The energy required, thus, is obtained from the respiration of the host cells.

The fate of NH₃: NH₃ produced through N, fixation is incorporated into amino acids as the aniino group. At physiological pH the NH₃ is protonated to form \[\ce{NH^{+}4}\] (ammonium) ion. While most of the plants can assimilate \[\ce{NH^{-}3}\] as well as \[\ce{NH^{+}4}\], lions. But \[\ce{NH^{+}4}\] ion is quite toxic to plants, hence cannot accumulate in them

• There are two main ways in which \[\ce{NH^{+}4}\] ion is used to synthesise amino acids in plants:

(I) Reductive amination: In these processes, ammonia reacts with α-ketoglutaric acid and forms glutamic acid as indicated in the equation given below:

\[\begin{array}{cc}
\ce{\underset{↓ Glutamate dehydrogenase}{α-ketoglutaric acid  + NH^{+}4} + NADPH}\\
\ce{Glutamate + H2O + NADP}
\end{array}\]

(II) Transamination: Transfer of amino group from one amino acid to the keto group of keto acid.

\[\begin{array}{cc}
\phantom{..}\ce{H}\phantom{.........................................................}\ce{H}\phantom{......}\\
\phantom{..}|\phantom{..........................................................}|\phantom{......}\\
\ce{R_1 - C - COO^{-} +  R_2 - C - COO^{-} ⇌ R_1 - C - COO^{-} + R_2 - C - COO^{-}}\\
\phantom{................}|\phantom{..................}||\phantom{.........................}||\phantom{..............}|\phantom{....................}\\
\phantom{...................}\ce{\underset{Amino-donor}{NH^{+}3}\phantom{.......}\ce{\underset{Amino-acceptor}{O}}\phantom{...................}\ce{O}\phantom{..............}\ce{NH^{+}3}}\phantom{......................}
\end{array}\]

• Glutamic acid is the main amino acid from which transfer of NH₂ (amino group) takes place and other amino acids are formed through transamination. Enzyme transaminase catalyses all such reactions. For example, the two most important amides (asparagine and glutamine) found in plants are a structural part of proteins.
• Asparagine is formed from aspartic acid and glutamine is formed from glutamic acid by the addition of amino groups to each. The hydroxyl part of the acid is replaced by another NH₂ radicle.