Proteins are large complex molecules made up of amino acids that perform many diverse and essential functions within living cells; enzymes, for example, enable chemical reactions to take place. Their function is directly a consequence of the 3D arrangement of their amino acid chains in space. Physical and chemical stress factors interact with proteins to alter their structure and consequently, their function.
Whether cells can isolate themselves from their environment and protect themselves depends on the type of conditions they are exposed to as well as the properties of their cell membranes. Extreme environments that are mediated through chemicals (pH and salty environments) can be manipulated by cells so that their internal biochemistry need not be rewritten. For example, there is usually a difference of 2 pH units between the outside and inside of cells in an acid or alkali environment. Cells can achieve this by actively pumping ions out of the cell through specialised membrane protein channels.
Temperature and pressure cannot be restricted to the outside of the cell; consequently, organisms that live in extreme conditions must adapt their biochemistry. Some of the most common adaptations to extreme environments are made to membranes and enzymes to ensure their continued function.