Photosynthesis Dark Reactions: Carbon & Nitrogen Assimilation

NADP⁺ acts as a powerful reducing agent, carrying hydrogen atoms essential for reducing carbon compounds and facilitating the formation of ATP and carbohydrates. NADP⁺ and ATP are also crucial for nitrogen assimilation during photosynthesis, leading to the synthesis of amino acids. Nitrogen, assimilated as nitrates and nitrites dissolved in the crude sap, is transported to plant tissues. Similarly, sulfur, obtained from sulfates and sulfites, is fixed to form the sulfhydryl (-SH) group of amino acids like cysteine. This process of incorporating bioelements, including phosphorus, into organic molecules is fundamental for plant life.

Carbon Fixation in the Dark Phase (Fig. 12, p. 184)

The fixation of carbon from CO₂ follows a cyclic pathway known as the Calvin-Benson cycle. From the perspective of carbon atoms entering and exiting this cycle, and following the general scheme of carbon assimilation in photosynthesis to produce glucose, the process can be summarized:

Six carbon atoms enter as CO₂ and exit as six carbon atoms in the form of C₆H₁₂O₆. These CO₂ molecules are captured by six molecules of ribulose 1,5-bisphosphate (RuBP), accounting for a total of 30 carbon atoms.

Calvin-Benson Cycle Equation

6 CO₂ + 12 (NADPH + H⁺) + 18 ATP → C₆H₁₂O₆ + 6 H₂O + 12 NAD⁺ + 18 (ADP + P_i)

The cycle concludes with the regeneration of the six molecules of ribulose 1,5-bisphosphate. This regeneration occurs as ribulose 5-phosphate is phosphorylated by a kinase, incorporating phosphate derived from the hydrolysis of 6 ATP molecules (assuming a cycle involving the addition of 6 molecules of CO₂).

Nitrogen Fixation in the Dark Phase

Similar to carbon fixation, nitrogen fixation utilizes the NADPH + H⁺ and ATP generated during the light-dependent reactions of photosynthesis.

Nitrogen is incorporated into the plant primarily as nitrate (NO₃^-) and nitrite (NO₂^-) ions. The process unfolds as follows:

Nitrate Reduction

Within the chloroplast envelope, nitrate ions are reduced to nitrite by the enzyme nitrate reductase, which transfers electrons from NADPH + H⁺ to nitrate.

Nitrite Reduction

Subsequently, nitrite ions are further reduced to ammonium ions (NH₄⁺) by the enzyme nitrite reductase, which also transfers electrons from NADPH + H⁺ to nitrite.

The ammonium ion (NH₄⁺) can then combine with alpha-ketoglutaric acid, a component of the Krebs cycle found in the mitochondrial matrix. This complex can then move into the hyaloplasm (cytosol).

In the hyaloplasm, the ammonium ion is incorporated into alpha-ketoglutaric acid, transforming it into the amino acid glutamic acid by adding an amino group. Through subsequent amino group transfer to other keto acids, the remaining amino acids required by the plant are synthesized.