Cell Fractionation: Definition, Steps & Methods

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  • 0:01 What Is Cell Fractionation?
  • 0:43 Step One: Breaking Your Cells
  • 1:57 Step Two: Separation
  • 3:12 Step Three: Collection
  • 4:25 Checking Your Cell Fractions
  • 5:32 Lesson Summary
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Lesson Transcript
Instructor: Erika Steele

Erika has taught college Biology, Microbiology, and Environmental Science. She has a PhD in Science Education.

Cells' structure and chemical composition are extremely complex. Cell fractionation allows scientists to study various parts of cells in order to determine their function and biochemical composition.

What Is Cell Fractionation?

Have you ever wondered how scientists were able to determine that mitochondria are the powerhouses of the cell or how they figured out that the endoplasmic reticulum has a different function from the Golgi apparatus? Scientists were able to discern the functions of organelles by separating them in a process called cell fractionation.

The process is pretty simple; you take some cells, throw them in a blender, and then centrifuge them to separate the organelles, as shown in this figure.

Cell fractionation allows you to study the different parts of a cell in isolation. When the organelles have been isolated, you can see which organelles make the most energy. Let's look further into the steps involved in this process.

Step One: Breaking Your Cells

To get to the content inside of something, you have to open it. Cells are no different. Detergents allow the cell membrane to be opened so that the contents inside the cell can be obtained. Detergents disrupt cellular membranes because they can interact with both membranes and parts of the cell that are soluble in water. Since detergents can interact with both lipid (membrane) and soluble (cytoplasmic) parts of the cell, they allow cellular components to be mixed or homogenized.

When the cellular components are mixed, a cell homogenate, or cell lysate, is formed. In fact, the reason detergents and soaps wash away oil and grease is because they allow things that don't normally mix with water to be dissolved in water and washed away. Lysing, or opening the cells, in detergent is usually combined with a physical method that breaks the cell further, such as machines that are similar to blenders, glass beads, or breaking the cell using sound energy. Using physical methods in combination with detergents ensures that all of the cells in the sample get broken, and you can isolate as much of your cellular fraction as possible.

Step Two: Separation

Cell homogenates are separated into fractions by spinning them super-fast in a process called centrifugation. If you have ever ridden an anti-gravity carnival ride, then you understand how centrifugation works. These rides eventually reach enough speed that you are pushed against the walls and do not need a seat belt to stay up. Similarly, centrifugation produces forces that are thousands of times higher than gravity, and cellular components are pushed toward the bottom of the container they are in.

Centrifugation applies enough force to cell homogenates to allow different cellular fractions to separate based on properties such as mass, density, and shape, as shown here in the figure here. Centrifugation causes components that are too heavy to resist the force of gravity to move to the bottom of the tube, as seen in this figure.

The smaller components stay homogenized in the liquid (labeled the supernatant in the image) and the larger components will move to the bottom. Repeating the centrifugation with increasing force allows smaller cellular components to be separated as seen in this figure.

Step Three: Collection

The way cell fractions are collected will depend on the liquid in which the cell fractions are centrifuged. Cell fractions are usually centrifuged in a medium, or liquid, that provides osmotic support such as sucrose or Percoll. The liquids aid in the separation of cellular components based on density and size. If only one concentration of sucrose or Percoll is used, it is called differential centrifugation because the different fractions will be collected by centrifuging the sample several times, as shown in this figure.

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