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.
ABA stress signaling involves activation of phospholipase C → IP3 generation → Ca2+ release from vacuoles → opening of anion channels (Cl-, malate) → efflux of anions and K+ → loss of turgor → stomatal closure. This is the canonical ABA-signaling pathway in guard cells.
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.
Transpiration pull is the tension/negative pressure developed in xylem vessels due to continuous water loss (transpiration) from leaves. This creates a suction force that pulls water upward from roots. It can occur both day and night through lenticels and cuticles, though primarily during day.
The Kok cycle involves S0→S1→S2→S3→S4 states. Four electrons are removed sequentially from the Mn4CaO5 cluster before O2 is released at the S4→S0 transition.
White patches result from chloroplasts lacking chlorophyll or being non-functional, typically due to mutations in chloroplast DNA (maternal inheritance). Green patches have functional chloroplasts.
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%.
Phytochrome exists in two interconvertible forms (Pr and Pfr) and acts as a red/far-red light sensor, measuring day length for photoperiodic flowering responses.
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.
