Govt. Exams
Entrance Exams
While plant cells lack true centrosomes with centrioles, plasmodesmata serve similar intercellular communication functions. However, most plant cells lack a distinct MTOC (microtubule-organizing center).
# Transpiration Coefficient in Different Plant Types
The transpiration coefficient measures water loss efficiency relative to biomass production, with lower values indicating more water-efficient plants.
Step 1: Understanding Transpiration Coefficient
The transpiration coefficient is defined as the ratio of water transpired to dry matter produced by a plant.
Step 2: Comparing Photosynthetic Pathways
C4 plants (like maize, sugarcane) have evolved a specialized two-stage carbon fixation process using phosphoenolpyruvate carboxylase, which concentrates CO₂ around RuBisCO enzyme. This reduces photorespiration and allows stomata to remain partially closed, minimizing water loss while maintaining photosynthetic efficiency.
- C3 plants (wheat, rice): Open stomata longer, higher transpiration coefficient (~500-600)
- C4 plants (maize, sugarcane): Efficient CO₂ fixation, lower transpiration coefficient (~200-250)
- CAM plants (cactus): Open stomata at night only, but produce less dry matter per unit time
- Hydrophytes (aquatic plants): Abundant water, no water conservation mechanism
The transpiration coefficient is lowest in C4 plants because they maximize photosynthetic efficiency while minimizing water loss through specialized leaf anatomy and biochemical pathways.
[Final Answer: (B) C4 plants]
The Z-scheme illustrates electron flow from PSII through the cytochrome b6f complex to PSI, showing how electrons are elevated twice to reach NADP+ reduction.
The Casparian strip contains suberin (waxy substance) and lignin, which prevents apoplastic (cell wall) movement of water and minerals, forcing symplastic transport.
For every 3 turns of Calvin cycle (fixing 3 CO2), 9 ATP and 6 NADPH are used. This gives a ratio of 3:2 (ATP:NADPH).
Water photolysis occurs in the thylakoid lumen as part of Photosystem II, releasing O2, H+, and electrons that drive the electron transport chain.
PSII catalyzes photolysis of water at the oxygen-evolving complex, transferring electrons to plastoquinone in the thylakoid membrane.
Stomatal conductance (gs) is the rate of gas diffusion through stomata, dependent on aperture width and inversely on distance (g = A/L).
Photosystem I is an integral membrane protein complex with multiple transmembrane domains, making it hydrophobic. Rubisco is soluble, while ferredoxin and plastocyanin are soluble electron carriers. This biochemical distinction is important for understanding photosynthetic mechanisms.
Protons accumulate in the thylakoid lumen (pH 4-5) due to water photolysis and electron transport, creating a chemiosmotic gradient for ATP synthesis. The stroma maintains pH ~8. This proton gradient is essential for understanding light reactions.
