Introduce the idea of enzymes as biological catalysts through the use of a character conversation of two students eating their lunch - one could be having a cheese and ham sandwich and the other a pasta salad

Include:

• the definition of an enzyme 
• Enzymes are proteins,
• only work with specifc reactants
• under limited conditions of temperature and pH etc etc -
• provide some examples eg catalase, amylase from the what the st8udents are eating

Good reference source  - http://resources.schoolscience.co.uk/JohnsonMatthey/page39.htm

Enzymes work by a lock and key mechanism.

Use enzyme  animation in maths skills course showinging lock and key mechnanism

https://www.rsc.org/cpd/resource/RES00001512/geometry/RES00001503#!cmpid=CMP00005096