Explain the lock and key hypothesis of enzyme function

To start, it is important to have a clear understanding of what an enzyme is. An enzyme is a biological molecule which speeds up the rate of a reaction without being changed or used up in the process. Each enzyme can only catalyse a certain reaction and this is determined by what is known as the active site. This is basicaly 3D shape within the enzyme that the molecules involved in the reaction fit into exactly. Any molecules which have a shape that dosen't match will not fit, so the reaction cannot be cataysed. 

A simplified way to think of this is as a lock and key. To fit into and opperate a lock, a key has to have a specidic shape that matches the lock. If the key is the wrong shape, it may still be able to get into the lock but will not open the door. You need the correct key and lock in just the shame way you need the correct enzyme and molecules.