Cell Fractionation and Ultracentrifugation for A-Level Biology

Introduction

• Purpose: Study internal structure and organelles of cells
• Methods: Cell fractionation, ultracentrifugation, and microscopes

Cell Fractionation

• Objective: Remove and isolate organelles from cells
• Process: Break open cells, separate organelles

Properties of the Solution

• Cold: Reduces enzyme activity to prevent damage to organelles
• Isotonic: Prevents osmosis, stabilizing organelle size and integrity
• Buffered: Maintains constant pH, protects organelles

Common Errors

• Emphasize "organelles" instead of "cells" in exam questions about solution properties

Homogenization

• Definition: Breaking open cells using a blender
• Example: Spinach leaves blended in a cold, isotonic, buffered solution
• Post-Homogenization: Filter solution to remove large debris, leaving a liquid with organelles

Ultracentrifugation

• Purpose: Separate individual organelle types
• Mechanism:
  • Spin at different speeds to separate organelles based on density
  • Use differential centrifugation to form pellets
  • Most dense organelles form pellets first

Differential Centrifugation Process

1. Initial Spin: Low speed to form the first pellet (most dense organelles)
2. Supernatant: Remove liquid containing remaining organelles
3. Repeat: Spin at increasing speeds to isolate subsequent organelles

Order of Organelles by Density

1. First Pellet: Nucleus
2. Second Pellet: Chloroplasts (plants) or Mitochondria
3. Third Pellet: Lysosomes
4. Fourth Pellet: Endoplasmic Reticulum
5. Last Pellet: Ribosomes

Exam Question Tip

• Know the order of organelles in pellets; e.g., chloroplasts in the second pellet for plant cells

Conclusion

• Cell fractionation and ultracentrifugation are crucial for studying cell organelles
• Ensure correct terminology and understanding of processes when answering exam questions

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