Cells are the basic units of life, as they are the simplest distinct repeating biological "objects" that bear the major properties associated with life, such as reproduction and metabolism. As self-contained entities, they have a well-defined physical form, and just as with everyday plants and animals, sufficient physical disruption to this "vessel" can quickly lead to loss of life for the organism in question.
The membrane surrounding cells does its job extremely well, having maintained its same basic form in all life on Earth for hundreds of millions of years. But it is not a magical barrier and it can be lethally disrupted by various kinds of forces, leading to the breaking apart of the cell and its contents in the same way as, say, a rubber balloon that is overfilled with juice and fruit and then pops.
Cell lysis is this breaking apart of a cell by some external force. While it is fatal to the cell, there are certain situations in which human scientists want to lyse a cell or cells to get at the contents without destroying them. (Think old bankrobber movies where the bad guys try to blow up a vault without burning up the money inside.) A lysis solution, also called a lysis buffer, is one of many ways to accomplish this.
Components of Cells: What's to Lyse?
Cells come in two basic types, reflecting the two taxonomic domains at the "root" of the branching tree of life: prokaryotic and eukaryotic, with the corresponding domains being Prokaryota (bacteria and other single-celled, or unicellular, organisms) and Eukaryota (plants, animals, protists and fungi, very few of them unicellular).
Prokaryotic cells usually have little more to them than the four elements common to all living cells: a cell membrane, a cytoplasm (the "goo" making up most of the cell interior), genetic material in the form of DNA (deoxyribonucleic acid) and ribosomes for making proteins. Eukaryotic cells, on the other hand, contain a lot of other features, including a nucleus around their DNA.
The main characteristic separating eukaryotic cells from prokaryotic cells is that eukaryotic cells have membrane-bound organelles. The plasma membrane around these structures is virtually identical to that around the cell as a whole, and thus they are vulnerable to the same kinds of physical and chemical threats.
In fact, one kind of organelle, called a lysosome, has the sole purpose of dissolving the waste products of cell metabolism to get rid of them.
Cell Lysis Basics
Cell lysis, in the context of this article, will refer to the purposeful lysis of cells by humans so that the contents can be obtained intact, not to just the physical or chemical event of lysis. What are some of the things inside cells that scientists and others might want access to?
If you can't think of a reason off the top of your head, consider the part of a cell that you see as functioning more or less as its brain. That would be the nucleus (in eukaryotes) of the agglomeration of DNA somewhat resembling a membrane-free, diffuse nucleus (in prokaryotes).
The genetic material has "memory" in a real sense, as it preserves information much like your mind does, though using different processes. DNA is therefore an invaluable target of science workers who need to extract it from cells intact using a lysis method.
Cells contain a variety of other substances of interest to medical and other researchers and lab workers, including DNA's sibling RNA (ribonucleic acid) and a variety of proteins, hormones and other macromolecules. Protein extraction specifically is discussed below.
Cell Lysis Definition and Types
Lysis is simply the process of breaking something apart at the microscopic level. It means essentially the same thing as "dissolving," except that you can't see it happening with your unaided eye. Scientists and others now have a variety of ways to lyse cells for strategic purpose.
(Remember, while a cell dies when it is lysed, this does not mean that "lyse" is equivalent to "destroy.")
Generally, these methods of cell lysis include mechanical and non-mechanical lysing methods, with the latter three including physical, chemical and biological means of bringing about cell lysis. Using a cell lysis buffer solution qualifies as a chemical method.
Mechanical Forms of Cell Lysis
Mechanical disruption of the cell may take the form of a bead mill, in which small glass, metal or ceramic spheres are shaken at high speed along with a liquid mixture of the cells of interest. In this method, the beads simply break the cells open.
Alternatively, sonication, or the use of sound waves, provides a different type of effective cell-membrane disruption via a mechanical apparatus that can be effective. These sounds waves have a frequency of about 20 to 50 kHz, or 20,000 to 50,000 beats per second. The method is noisy and also creates enough heat to make this method troublesome for especially heat-sensitive materials.
Other Forms of Cell Lysis
Physical lysis: Osmotic shock is one way to lyse cells; it lowers the ionic "pull" of the medium the cells are in, which can cause water to leave the medium and flow into cells. This in turn may cause cells to swell and burst. Surfactants are a kind of detergent that can be used to disrupt cell membranes in this process.
Most bacteria, yeast and plant tissues, however, are resistant to osmotic shocks thanks to their cell walls, which eukaryotic cells as a rule lack. As a result stronger disruption techniques are usually required.
A cell bomb is another physical means of disrupting cells. Here, cells are placed under very high pressure (up to 25,000 pounds per square inch, or about 170 million Pascals). When the pressure is rapidly released, the sudden pressure change causes gases that have dissolved in cells to be released as bubbles. This in turn breaks open the cells.
Biological lysis: Enzymes may be useful in helping to degrade the cell walls of bacteria. Lysozyme, for example, is very useful for breaking down the cell wall of bacteria, which is a sturdier barrier than the cell membrane. Other enzymes commonly employed include cellulase (which degrades starch) and proteases (which degrade proteins).
Chemical lysis: Detergents, as noted, are used during the osmotic-shock method of cell lysis, but can also be used in stand-alone cell lysis through the use of a chemical solution alone. These detergents work simply by making the proteins embedded in the cell membrane (which is mostly phosphate and lipids) more soluble, making it easier for the membrane as a whole to be degraded.
What Is in a Lysis Buffer?
The term "cell lysis solution" is sometimes, though not always, used interchangeably with "lysis buffer." So it is useful to know the specific ingredients of a chemical cocktail designed specifically to break down the cell membrane without compromising the integrity of the cell contents.
A typical lysis buffer might contain a mixture of buffering salts, such as the following:
- 50 mM Tris-HCl pH 7.5 (an industrial buffer with a slightly alkaline, or basic, pH or hydrogen ion level)
- 100 mM NaCl (table salt)
- 1 mM DTT (specifically for proteins)
- 5% glycerol (a sugar alcohol and the "backbone" of lipids)
Protein Extraction Technique
Protein extraction is a simple enough process, at least in principle. First, the cells from which a specific protein will be taken are lysed. Whichever of the above-described methods is selected, once the protein has been collected, it will usually need to be separated from a lot of background matter that, at least for present purposes, is unwanted.
For example, nucleic acids (DNA and RNA) almost always make their way into the lysate, or the solution containing the freed cell contents. Special chemical preparations can be used to "wash" the nucleic acid from the solution and leave mostly protein behind. Additional chemical and physical steps will lead to greater and greater purity in the protein being collected.
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.