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 exist in a non-living environment, which is basically hostile and can easily do harm. Fortunately, every 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, it must repair itself in order to be able to function. How does repair happen? There are two levels of biological repair: 1) cellular repair and 2) tissue repair. These two types of repair take place through different mechanisms.

Cells are the basic building blocks of living things. They are tiny, microscopic units surrounded by membranes, which separate what’s inside from the surroundings. Cells contain even tinier organelles that carry out the work of keeping each cell alive. Human cells are surrounded by extracellular fluid, mostly coming from blood.

Cells can 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 really a toxic molecule. It tends to react with other 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 food we eat and in 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 few 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 biological molecules. The small subunits are then re-used to produce new molecules 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 using information from a gene to manufacture proteins. These complicated biochemical processes occur continually in all cells except red blood cells.

So, the bottom line is: many cells are able to survive for a long time because they can  repair moderate damage and produce new molecules–if they have adequate nutrition and energy. Important exceptions include cells of the skin and digestive tract, which are damaged by friction, bacterial invasion, chemicals and enzymes in the gut. Thus, their lifetimes are 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 when it has been damaged.