Govt. Exams
Entrance Exams
GA3 is the primary hormone regulating α-amylase synthesis in cereal aleurone cells during germination. GA3 inactivates DELLA repressor proteins, allowing transcription factors (GAMYB) to activate amylase genes. This hormone controls starch hydrolysis for energy mobilization in growing seedlings.
C4 photosynthesis concentrates CO2 around RuBisCO through the C4 pathway, minimizing photorespiration even at high temperatures and low CO2 availability. This is advantageous in tropical/arid regions where stomata must close frequently to conserve water, improving water-use efficiency (WUE).
Red light converts phytochrome to its active Pfr (far-red absorbing) form, which inhibits stem elongation and promotes photomorphogenesis. Continuous red light maintains high Pfr levels, suppressing etiolation. Cryptochrome responds to blue light, and phototropin detects directional light.
Photorespiration recycles glycolate back to useful 3-PG, but this process consumes ATP and reduces RuBP regeneration efficiency, decreasing net photosynthetic yield by 20-50%.
Under stress (high light, closed stomata reducing CO2), cyclic photophosphorylation increases, generating ATP without NADPH consumption. This helps dissipate excess light energy and prevent photoinhibition.
Wilting occurs when transpiration demand exceeds water uptake capacity. Root pressure alone (typically 2-10 atm) cannot overcome strong transpiration pull, leading to xylem tension and leaf wilting.
The quantum requirement is about 8 photons per CO2 fixed (4 for PSII and 4 for PSI to generate sufficient ATP and NADPH for the Calvin cycle).
While red light activates phytochrome and initiates de-etiolation, it alone is insufficient for complete chloroplast development and normal growth. Blue light and other wavelengths are also required.
The Z-scheme produces approximately 1.5 ATP per NADPH (or 3 ATP per 2 NADPH) due to the proton gradient and energy requirements in the electron transport chain.
RuBisCO has dual specificity. When O2/CO2 ratio is high (low CO2, high O2), the oxygenase activity dominates, leading to photorespiration.
