12. In a sucrose density gradient, what is the order of organelle sedimentation from
lower to higher concentrations of sucrose?
a. Golgi, smooth endoplasmic reticulum, rough endoplasmic reticulum
b. smooth endoplasmic reticulum, Golgi, rough endoplasmic reticulum
c. rough endoplasmic reticulum , smooth endoplasmic reticulum, Golgi
d. Golgi, rough endoplasmic reticulum, smooth endoplasmic reticulum
Sucrose Density Gradient Organelle Sedimentation Order: CSIR NET Solved
In sucrose density gradient centrifugation, organelles separate based on buoyant density, with lower-density components sedimenting first in regions of lower sucrose concentration, followed by higher-density ones deeper in the gradient. The correct order from lower to higher sucrose concentrations is Golgi, smooth endoplasmic reticulum, rough endoplasmic reticulum. This matches option a.
Option Analysis
Sucrose density gradients form increasing concentrations from top (low density) to bottom (high density), so lighter organelles band higher up, while denser ones pellet lower.
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Option a (Golgi, smooth ER, rough ER): Correct. Golgi has the lowest density due to large, less compact vesicles. Smooth ER, tubular and ribosome-free, is intermediate. Rough ER is densest from ribosome attachment.
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Option b (smooth ER, Golgi, rough ER): Incorrect. Smooth ER is denser than Golgi due to structural differences, so Golgi sediments first.
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Option c (rough ER, smooth ER, Golgi): Incorrect. Rough ER, being heaviest, sediments last, not first; this reverses the density order.
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Option d (Golgi, rough ER, smooth ER): Incorrect. Rough ER exceeds smooth ER in density because of ribosomes, placing it after smooth ER.
Density Factors
Golgi’s stacked cisternae and vesicles yield low buoyant density (~1.10-1.15 g/cm³). Smooth ER’s tubular form lacks ribosomes, giving moderate density (~1.15-1.20 g/cm³). Rough ER’s ribosome coating increases density (~1.20-1.25 g/cm³), confirmed in cell fractionation studies.
In cell biology, the sucrose density gradient organelles sedimentation order determines how Golgi, smooth endoplasmic reticulum (ER), and rough ER separate during centrifugation, crucial for CSIR NET Life Sciences preparation. This technique bands organelles by buoyant density in a sucrose gradient, from low concentration (top) to high (bottom). Understanding this order—Golgi, smooth ER, rough ER—helps analyze subcellular fractionation.
Why This Order Occurs
Centrifugation drives organelles to sucrose regions matching their density. Golgi floats highest with low density from vesicular structure. Smooth ER follows, lighter without ribosomes. Rough ER sinks deepest, densest due to ribosome-studded membranes. Rates depend on size, shape, and density per Svedberg units.
CSIR NET Exam Relevance
This question tests organelle properties and centrifugation principles, common in Unit 2 (Cell Biology). Ribosomes add ~20-30% density to rough ER versus smooth. Practice with homogenates shows clear banding: Golgi at 20-30% sucrose, smooth ER at 30-40%, rough ER at 40-50%.
Practical Applications
Used for isolating pure organelles to study functions, like protein synthesis in rough ER. Enhances research in biochemistry and biotech for CSIR NET aspirants.
