Q.46 Given below are two statements: one is labelled as Assertion-A and the other is labelled as Reason-R
Assertion-A: Removal of apical bud (decapitation) causes loss of apical dominance.
Reason-R: Apical dominance is due to auxins and apical buds are site of auxin synthesis. Decapitation causes loss of auxin required for apical dominance.
In the light of the above statements, choose the correct answer from the options given below:
- Both A and R are true and R is the correct explanation of A
- Both A and R are true but R is NOT the correct explanation of A
- A is true but R is false
- A is false but R is true
Both A and R are true and R is the correct explanation of A. Lateral heterogeneity refers to the non-random distribution of photosynthetic complexes across thylakoid domains, with PSII predominantly in appressed grana regions.
Question Analysis
Thylakoid membranes exhibit lateral heterogeneity where protein complexes segregate into appressed (grana) vs non-appressed (stroma lamellae) regions. Assertion A defines this phenomenon; Reason R provides the classic example of PSII enrichment in stacked membranes, directly exemplifying A.
Option Breakdown
-
Both true, R explains A: Correct. PSII’s exclusive appressed localization demonstrates the unequal complex distribution that defines lateral heterogeneity.
-
Both true, R NOT explanation: Incorrect. R specifically illustrates A’s principle through the canonical PSII distribution pattern.
-
A true, R false: Incorrect. PSII is indeed primarily (though not 100% exclusively) in appressed regions; minor stroma thylakoid presence exists during turnover.
-
A false, R true: Incorrect. Lateral heterogeneity is established thylakoid architecture principle since Andersson & Anderson (1980).
Correct Answer: Both A and R are true and R is the correct explanation of A.
Lateral heterogeneity thylakoid complexes PS-II appressed regions creates domain-specific photosynthesis efficiency. Grana stacking excludes bulky PSI/ATP synthase while concentrating dimeric PSII for optimal excitation energy transfer.
Domain Organization
Appressed (Grana): PSII-LHCII supercomplexes; high protein density (70% membrane area), cation-induced stacking via LHCII interactions.
Non-appressed (Stroma lamellae): PSI-LHCI, ATP synthase, Calvin cycle enzymes; exposed stromal surface for NADP+ reduction, proton access.
Statement Truth Value Domain Distribution Assertion A True Unequal complex distribution Reason R True PSII→grana, PSI→stroma lamellae R explains A Yes Classic heterogeneity example Functional Significance
This architecture optimizes linear electron flow: PSII water oxidation in grana → plastoquinone pool diffusion → cyt b6f → PSI reduction in stroma lamellae. State transitions dynamically adjust LHCII position. Essential for CSIR-NET photosynthesis questions.
-
