Heat, applied through boiling and autoclaving is used to sterilize. But how does heat destroy cells and viruses? Here is a simple explanation of heat’s mode of action. Applying heat is one of the simplest ways to kill microbes, at least for controlling vegetative cells (active, metabolizing cells) and viruses. High temperatures do their damage by destroying organic molecules such as proteins, carbohydrates, lipid and nucleic acids. These molecules have important roles for proper functioning of cells and viruses.
* The Effect of Heat on Organic Molecules *
Heat generally has two main categories by which it disable and kill cells and viruses:
1. Alteration of Cell Walls and Membranes
2. Damage to Proteins and Nucleic Acids
* Alteration of Cell Walls and Membranes by Heat *
Cell Walls: Prokaryotic bacteria, eukaryotic plants and some fungi have cells that are surrounded by a protective cell wall composed mainly of structural carbohydrates and which helps maintain cell shape. Heat can disrupt the bonds within cell walls making them weak and structurally unsound.
Cell Membranes: Phospholipids, polar molecules that are soluble in both water and oil, are the main component of cell membranes and, in eukaryotic cells, phospholipids create an entire transport system for moving materials into, out of and around within the cell. At high temperatures, lipids become more fluid (Think about what happens to butter in the heat). Phospholipids also become more fluid when heat is applied, disrupting the integrity of the cellular membrane and making the cell leaky.
Viral Membranes: Some viruses, those that are considered to be enveloped, are surrounded by phospholipids that they steal from the cells that they parasitize. Enveloped viruses can be rendered harmless when their viral envelope is destroyed, because the virus no longer has the recognition sites necessary to identify and attach to host cells.
* How Heat Causes Damage to Proteins and Nucleic Acids *
Cellular Proteins: Proteins are made based on the instructions contained in genetic material (nucleic acids). These large three dimensional molecules are composed of amino acids linked together by peptide bonds. Heat denatures (changes the shape of) proteins, and the 3-D structure of a proteins is essential to its function. If a protein’s shape is irreversibly changed, the protein is no longer functional. An example of this in daily life is the cooking of an egg. Eggs are full of protein. As the egg cooks, the albumin protein in the egg white is denatured and the gooey fluid becomes a white solid. You can’t uncook an egg. Denaturation of protein is irreversible and can be deadly for a cell.
Nucleic Acids: Composed of linked nucleotides, nucleic acids, such as DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), are the genetic instructions of cells and viruses. They contain a code that allows for the building of protein molecules. Like proteins, nucleic acids are very heat sensitive. High temperatures can result in fatal mutations to DNA or can halt the process of protein synthesis, by damaging RNA.
* Sources *
Schauer, Cynthia (2007) Lab Manual to Microbiology for the Health Sciences, Kalamazoo Valley Community College.
Bauman, R. (2005) Microbiology. Pearson Benjamin Cummings.