Understanding Cell Fractionation and Centrifugation

This question relates to the process of cell fractionation, a technique used in cell biology to separate different cellular components based on their size and density. This is typically achieved through differential centrifugation.

Differential Centrifugation Explained

Differential centrifugation involves centrifuging a cell homogenate (a mixture of broken cells) at progressively higher speeds. Each speed step causes components of a certain size and density range to settle at the bottom of the centrifuge tube, forming a pellet. The liquid above the pellet, called the supernatant, is then transferred to a new tube and centrifuged at a higher speed.

Analyzing the Centrifugation Speed

The specific conditions mentioned are centrifugation at 1000g for 10 minutes after tissue cell homogenization. The 'g' stands for the relative centrifugal force (RCF), which indicates the force applied relative to gravity. The speed and duration determine which components will sediment:

• Low speeds (like 1000g) sediment the largest and densest components.
• Higher speeds sediment progressively smaller and less dense components.

Component Sedimentation at 1000g

Based on the typical order of sedimentation during differential centrifugation:

• Nuclei: These are the largest and densest organelles in the cell. They sediment readily at low speeds.
• Mitochondria: These are smaller and less dense than nuclei. They require moderately higher speeds (e.g., 10,000g) to pellet.
• Microsomes: These are fragments derived from the endoplasmic reticulum and Golgi apparatus. They are even smaller and require high speeds (e.g., 100,000g) to pellet.
• Ribosomes: These are the smallest components (along with macromolecular complexes) and require very high speeds (ultracentrifugation, >100,000g) to pellet.

Identifying the Pellet Component

Given the centrifugation force of 1000g, the largest and heaviest cellular components will form the pellet. In a typical cell homogenate, these components are the nuclei, along with any unbroken cells and larger cell debris.

Therefore, when tissue cells are homogenized and centrifuged at 1000g for 10 min, the Nuclei will be part of the pellet.