Perhaps the most amazing feature of living beings (organisms) is that they are often able to repair injuries inflicted by the environment. That’s lucky for us, because our bodies sustain a lot of damage as we move through life, and the body needs to last a lifetime.
Living organisms exists in a non-living environment, whichis basically hostile and can easily do harm. Fortunately, very life form is endowed with protective strategies against invasion from outside; in humans, the most obvious protective shield would be the skin. But protective strategies do not always work, and all organisms are subject to damage.
Damage can be lethal, or an organism may survive the damage. If it survives, an organism must repair itself in order to be able to function properly. How does the repair process occur? There are two levels of biological repair: 1) cellular repair and 2) tissue repair. These two types of repair are accomplished through different mechanisms.
Cells are the basic building blocks of living things. They are tiny, microscopic units surrounded by membranes separating them from their surroundings. Cells contain even tinier organelles that carry out the work of keeping each cell alive. Human cells are surrounded by extracellular fluid, mostly derived from blood. (This discussion, and this site in general, will focus on human biology.)
Cells may be damaged by chemicals or by radiation, or they may be invaded by viruses. Cellular damage occurs even during normal, everyday activity. For one thing, cells need oxygen to fuel the process of energy production, but oxygen is a highly toxic molecule. It tends to react with nearby molecules, causing them to change structure and lose function. Special cellular organelles, called peroxisomes, help minimize oxygen damage to cells. Many other kinds of molecules can also cause cellular damage, including toxins in the food we eat and the air we breathe. Examples include alcohol, smoke, pesticides, and many medicines.
Thus, the survival time of most cellular proteins is shorter than you might think – ranging from about half an hour to a day or so. A few proteins, well protected inside cellular organelles, can last longer, perhaps a couple of weeks.
During the repair process, a cell must clear out damaged proteins and produce new functional molecules. Special organelles, called lysosomes, contain enzymes capable of breaking down proteins. Damaged or misfolded proteins usually find their way into lysosomes and are broken down into amino acids, the building blocks of proteins. These small subunits are then re-used to produce new proteins in a fine example of cellular recycling.
New proteins are constantly being manufactured by cells—in part to replace damaged proteins, and in part to perform new functions that a cell may be called on to perform. This process takes place through gene expression, which involves nuclear transcription (modeling RNA on a DNA template) and cytoplasmic translation (forging proteins using RNA “instructions”). These are complicated biochemical processes that occur continually in all cells except red blood cells.
So, the bottom line is: most cells survive for a fairly long time because they are able to repair moderate damage and to produce new molecules, if they have adequate nutrition and energy supply. Again, a major exception is red blood cells, which have a half-life of about three months and must be constantly replenished in the bone marrow. Other important exceptions include cells of the skin and digestive tract, which are damaged by the friction, bacterial invasion, damaging chemicals and enzymes they are exposed to. Thus, their half-life is measured in days or weeks rather than months.
The topic for the next entry will be tissue repair, the other major strategy for fixing the body after it has been damaged.