A level Biology exam revision resources written by A level Examiners


This technique is used to obtain pure samples of organelles for further study.

Live tissue is placed into an ice-cold isotonic solution. As this solution is isotonic it prevents distortion of cell organelles, and because it is ice-cold it slows any enzyme reaction occurring. The tissue is then ground up using a homogeniser. This ruptures the cells and releases the organelles into solution. Sometimes ultrasound can be used to rupture cells.

The resulting homogenate is then placed in a centrifuge, this is a device that spins the contents of a test tube at very high speeds, causing the cell fractions to settle out based on their mass.

Large organelles, such as chloroplasts and nuclei, can be separated by spinning the centrifuge at low speeds. If the remaining liquid (supernatant) is spun again at a higher speed then mitochondria and lysosomes can be separated. Finally fragmented endoplasmic reticulum with attached ribosomes can be sedimented if the supernatant is spun at very high speeds.

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