Cell lysis is the process of breaking open a cell, typically by disrupting its cell membrane, to access its internal components. Cell lysis solutions are fundamental to many laboratory procedures.
The Cell Membrane’s Structure
The cell membrane, also known as the plasma membrane, forms the protective outer barrier of every cell. It primarily consists of a phospholipid bilayer, a double layer of lipid molecules.
Each phospholipid molecule has a hydrophilic, or water-attracting, head and two hydrophobic, or water-repelling, tails. These molecules spontaneously arrange themselves in water, with the tails facing inward to form a water-free interior and the heads facing outward to interact with the aqueous environment.
Embedded within this lipid bilayer are various proteins that perform a multitude of functions, including transport and structural support. Cholesterol, another lipid, is also interspersed within the bilayer in animal cells, helping to regulate membrane fluidity and stability across different temperatures. This intricate arrangement creates a selectively permeable barrier, controlling the movement of substances into and out of the cell.
Components of Cell Lysis Solutions
A common component found in these solutions is detergents, which are amphipathic molecules possessing both water-attracting and water-repelling properties. Detergents can be classified as ionic, non-ionic, or zwitterionic, with examples like sodium dodecyl sulfate (SDS) being ionic and Triton X-100 or NP-40 being non-ionic.
These solutions also typically contain buffering salts, such as Tris-HCl, to maintain a stable pH, which is important for the stability of cellular molecules. Ionic salts like sodium chloride (NaCl) and potassium chloride (KCl) are included to establish and maintain the ionic strength of the buffer, which influences protein solubility and prevents unwanted aggregation.
Enzyme inhibitors, such as protease inhibitors and phosphatase inhibitors, are often added to prevent the degradation of proteins and other biomolecules by enzymes released during lysis. Chelating agents like EDTA may also be present to bind metal ions, which can inhibit certain enzymes or interfere with the isolation process.
How Lysis Solutions Disrupt Membranes
Detergents are the primary agents in lysis solutions that directly interact with and disrupt the cell membrane. Being amphipathic, detergents insert their hydrophobic parts into the lipid bilayer, interacting with the fatty acid tails of phospholipids.
As detergent concentration increases, they begin to solubilize the membrane lipids and proteins by forming mixed micelles, which are small, spherical structures containing both detergent and membrane components. This process effectively disintegrates the membrane’s structure, causing it to break apart.
Ionic detergents, like SDS, are particularly strong; they not only disrupt the lipid bilayer but also denature and unfold membrane proteins by breaking protein-protein interactions. Non-ionic detergents, such as Triton X-100 and NP-40, are milder and primarily solubilize the lipid bilayer and membrane proteins while often preserving the native structure and interactions of proteins. Zwitterionic detergents like CHAPS offer a balance, being harsher than non-ionic but milder than ionic, allowing for protein solubilization while retaining some protein interactions.
Beyond detergents, some lysis solutions employ osmotic lysis, particularly for cells without rigid cell walls, such as mammalian cells. This method involves placing cells in a hypotonic solution, which has a lower solute concentration than the cell’s interior. Water then rushes into the cell by osmosis, causing it to swell. The cell membrane stretches under this increasing internal pressure until it ruptures, releasing the cellular contents.
Why Cells Are Lysed
This process is a necessary initial step in extracting biomolecules such as DNA, RNA, and proteins. Researchers can then analyze these molecules to understand cellular functions, gene expression, and protein interactions.
Cell lysis is also fundamental for diagnostic purposes, including the molecular diagnostics of pathogens and cancer diagnostics. For example, DNA extraction for genetic analysis, such as PCR or sequencing, requires cells to be lysed to release their genetic material. Similarly, protein purification for studying protein structure and function, or for use in assays like Western blotting, begins with cell lysis to obtain a crude lysate containing the desired proteins.