Category : 11th Class
In isotonic medium cells components are separated, it is two step process.
Homogenisation : Cell products are separated in isotonic medium (0.25 M sucrose solution) either with the help of homogeniser of ultrasonic vibrations kept at 0 – 4°C. A homogenised cell is called homogenate.
Differential centrifugation : Homogenisation product is rotated (centrifuged) at different speeds. The sediment or pellete of each speed is collected. e.g., nuclei at \[1000\times g\](g= force of gravity) for 10 minutes, chloroplast and mitochondria at \[10,000\times g\] for 15 minutes. The particle settle according to their sedimentation ratios. Sedimentation coefficient is expressed in svedberg unit ‘S’ related with molecular weight of the particles. For the detail study of mitochondria it is the best technique. 'S' is measured by analytical centrifugation.
The various cell organelles and macromolecules sediment in the following order.
\[Nucleus\to Chloroplast\to Mitochondria\to Ribosome\to DNA\to mRNA\to tRNA\]
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