What is the Difference Between Zonal and Isopycnic Centrifugation

Centrifugation is a separation method that involves rotating a sample around a fixed axis to produce a centrifugal force. This force drives particles down through a liquid medium, with their sedimentation rate varying primarily according to their density and size. Centrifugation is widely used in fields such as biology, biochemistry, and chemistry for the isolation and separation of mixtures of immiscible liquids.

What is the difference between zonal centrifugation and isopycnic centrifugation? Zonal centrifugation separates particles based on size, whereas isopycnic centrifugation separates particles based on density.

Key Areas Covered

1. What is Zonal Centrifugation 
      – Definition, Features 
2. What is Isopycnic Centrifugation
      – Definition, Features 
3. Similarities Between Zonal and Isopycnic Centrifugation
      – Outline of Common Features
4. Difference Between Zonal and Isopycnic Centrifugation
      – Comparison of Key Differences
5. FAQ: Zonal and Isopycnic Centrifugation
      – Answers to Frequently Asked Questions

Key Terms

Zonal Centrifugation, Isopycnic Centrifugation

What is Zonal Centrifugation

Zonal centrifugation fractionates particles based on both size and shape. The procedure involves layering a sample in a restricted zone on top of a pre-poured density gradient. During centrifugation, particles migrate into the density gradient, which has densities much lower than the densities of the particles being centrifuged. A small volume of a suspension is layered over a shallow density gradient, which stabilizes the sedimentation of the particles.

On centrifugation, particles move away from the starting zone with velocities determined by their size, shape, and the centrifugal force applied. After centrifugation for a certain period, particles will be found in a series of zones spaced according to their relative velocities. This method allows the separation of particles differing in sedimentation rate. This way, rate-zonal centrifugation complements differential centrifugation.

An advantage of rate-zonal centrifugation is that it avoids the problem of cross-contamination of particles with different sedimentation rates by layering the sample as a narrow zone on top of a density gradient. The speed at which particles sediment depends primarily on their size and mass instead of density. As the particles in the band move down through the density medium, zones containing particles of similar size form, with faster sedimenting particles moving ahead of slower ones.

What is Isopycnic Centrifugation

Isopycnic centrifugation separates sample components based on their density. A density gradient is established using equilibrium sedimentation, often with caesium ions. The sample components migrate to regions where their density matches that of the surrounding solution, forming distinct bands. This method is independent of time and relies solely on the actual buoyant density of the particles.

In isopycnic centrifugation, particles are separated solely based on their density. Particle size only affects the rate at which particles move until their density is the same as the surrounding gradient medium. The density of the gradient medium must be greater than the density of the particles to be separated. By this method, particles will never sediment to the bottom of the tube, regardless of the centrifugation time. Upon centrifugation, particles of specific density sediment reach the point where their density matches the gradient media’s density (the equilibrium position). The gradient is then said to be isopycnic, and the particles are separated according to their buoyancy.

The density of biological particles is sensitive to the osmotic pressure of the gradient, so isopycnic separation may vary significantly depending on the gradient medium used.

Similarities of Zonal and Isopycnic Centrifugation

1. Both zonal and isopycnic centrifugation are important techniques for separating cellular components.
2. These methods are widely used in various scientific fields, including biochemistry, molecular biology, and microbiology.
3. They are commonly used for sample preparation, purification, and analysis, as well as for studying the structure and function of biological molecules and organelles.

Difference Between Zonal and Isopycnic Centrifugation

Definition

• Zonal centrifugation is a centrifugation method that separates particles based on both size and shape, whereas isopycnic centrifugation is a centrifugation method that separates particles based solely on their buoyancy density.

Particle Movement

• In zonal centrifugation, particles travel through the gradient at different speeds depending on their properties. In isopycnic centrifugation, particles migrate through the gradient until they reach a region where their density equals the density of the surrounding medium, reaching equilibrium.

Density Gradient

• While zonal centrifugation uses a shallow density gradient with a relatively low viscosity, isopycnic centrifugation uses a steeper density gradient with a higher viscosity.

Applications

• Zonal centrifugation is useful for separating particles with similar densities but different sizes or shapes, like separating organelles in a cell. Meanwhile, isopycnic centrifugation is useful for separating particles with very slight density differences, like separating different types of viruses or proteins.

Conclusion

In conclusion, isopycnic centrifugation separates components based on their buoyant density, while zonal centrifugation separates components based on their sedimentation rate. This is the basic difference between zonal centrifugation and isopycnic centrifugation. Each technique has specific applications and is chosen based on the nature of the sample and the desired separation criteria. Isopycnic centrifugation is ideal for separating particles that differ in density but not in size, while zonal centrifugation is best suited for separating particles that differ in size but not in density. 

FAQ: Zonal and Isopycnic Centrifugation

1. What are the two types of centrifugation?

There are actually two main categories of centrifugation techniques used to separate mixtures based on their components’ properties. They are density gradient centrifugation and differential centrifugation.

2. How is differential centrifugation different from isopycnic centrifugation?

Differential centrifugation and isopycnic centrifugation are both centrifugation techniques used to separate particles in a mixture. But there are differences among them too. The main difference is that separation basis of differential centrifugation relies on sedimentation rate while separation basis of isopycnic centrifugation relies on particle density.

3. What is the difference between centrifugation and ultracentrifugation?

In centrifugation, speed typically ranges from a few hundred to 65,000 x g (relative centrifugal force) while in ultracentrifugation, speed reaches much higher speeds, ranging from 100,000 x g to 1,000,000 x g and beyond.

4. What is zonal sedimentation?

Zonal sedimentation, also known as rate-zonal centrifugation, is a specific type of density gradient centrifugation technique. It separates particles based on their sedimentation rate within a pre-formed density gradient.

5. What are the advantages of zonal centrifugation?

One advantage of zonal centrifugation is that it separates particles based on a combination of their size, shape, and sedimentation rate. In addition, the centrifugation speeds used in zonal centrifugation are typically lower compared to ultracentrifugation.

Reference:

1. “Zonal Centrifugation – An Overview.” Science Direct. 
2. “Isopycnic.” Wikipedia. Wikipedia Foundation. 

Image Courtesy:

1. “Purified Viruses” (CC BY 3.0) via Commons Wikimedia
2. “Zonal Centrifuge” By Jolozaga – Own work (CC BY-SA 3.0) via Commons Wikimedia