Heat and kinetic energyThe molecules in a gas or liquid move around continually. They have natural kinetic energy and move continously and randomly. These movements mean that molecules continually collide with each other. If a gas or liquid is heated, the kinetic energy of the molecules is increased and they move around more quickly. This makes molecules collide with each other much more often. It also means that as they collide, as they are moving at a greater speed, they collide with a greater force.Enzymes and collisionsAn enzyme can catalyse a reaction only if a substrate molecule collides with the active site of the enzymes so that an enzyme-substrate complex is formed. If the kinetic energy of both enzymes molecules and substrate molecules is increased by heating, there will be an increased number of collisions between enzyme and substrates. More collisions between enzymes and substrate mean an increased rate of reaction.Heat, vibration, breaking bonds and denaturation Applying heat to molecules not only makes them move faster if they are in a liquid or gas, it also makes the molecules vibrate. These vibrations put a strain on the bonds that hold the molecules together. In large molecules such as enzymes, the vibration of the molecules can break the weaker bonds such as hydrogen bonds and ionic bonds. These weaker bonds occur in large numbers in an enzyme molecule and are responsibe for holding the tertiary structure together. These bonds mainain the active site in its correct shape. As heat is increased, more bonds are broken. The tertiary structure is held less and less in the shape needed to keep the active site in its working form. This means the rate of reaction will decrease. If enough of these bonds are broken, the whole tertiary structure will unravel and the enzyme will stop working. As with all proteins, if the tertiary structure of an enzyme is changed significantly it cannot function. This is not reversible - even if the temperature is lowered again, the tertiary structure is too damaged to rebuild. This is called denaturation. Optimum Temperature Increasing the temperature increases the rate of reaction of an enzyme-controlled reaction at first, but as the temperature increases further, the rate of reaction decreases. Eventually the enzyme will stop working. The temperature that gives the maximum rate of reaction is the enzyme's optimum temperature. It is a balance between increasing kinetic energy and increasing vibration og the enzyme molecule (which will break the bonds). Measuring the effect of temperature on enzyme action usually involves carrying out enzymes-controlled reactions at different temperatures, using a water bath controlled by a thermostat. We can measure the production of product or the disappearance of substrate in a variety of ways.Optimum temperatures vary between enzymesMany enzymes have an optimum temperature somewhere between 40 and 50 degrees. But there are some organisms for which such enzymes would be useless. The temperatures at which these organisms live means that they must have heat resistant enzymes. In modern genetic engineering, heat-resistant enzymes are used to catalyse reactions involved in making many copies of DNA segments. The process is known as the polymerase chain reaction and is very similar to DNA replication.