Free radicals have many diverse effects, but it's helpful to concentrate on the three main reaction types of relevance to organisms. You will note the term 'oxidative damage', a term which might need a little explaining for those (like me!) who aren't in any way chemists. Recall that free radicals have a free electron in their outer orbital, a state that is highly unstable. One solution to this problem is to grab an electron from surrounding molecules. The surrounding molecule that loses the electron is said to be oxidised - hence the 'oxidative' part.
- Oxidative modification of proteins - Free radicals damage proteins in numerous ways, such as by altering their side chains, forming cross-links between different proteins, and causing fragmentation of the protein's backbone. To make matters worse, the body's proteosomes sense these proteins as being damaged, and simply degrade the proteins altogether.
- Peroxidation of lipids - 'Peroxidation' is the oxidative degradation of lipids. In this case, however, the reaction can be 'autocatalytic' - the free radical creates a fatty acid free radical, which goes ahead and creates more fatty acid free radicals... and so on, in a chain reaction. Since a major source of lipids are cell membranes, free radicals can cause serious damage to the body.
- DNA changes - These are multiple, and include altering the nitrogenous bases and even breaks in the DNA strand.
So, unregulated, free radicals are quite destructive things. Yet this seeming disadvantage can prove decidedly advantageous if managed tightly. That'll be what we discuss next: the biological functions of free radicals.
Main source: Robbins Pathologic Basis of Disease; 6th edn., Cotran, Kumar, Collins