Enzyme action can be intracellular - within cells, or extracellular - outside cells.

Enzymes have an active site, which has a specific shape. The active site is where the substrate binds to creating the enzyme-substrate complex

They can affect structures in an organism as well as functions.

Enzymes are highly specific due to their tertiary structure.

They catalyse metabolic reactions at a cellular level (respiration) and for the organism as a whole (digestion).

1. If two substrate molecules need to be joined, being attached to the enzyme holds them close together, reducing any repulsion between the molecules so they can bond more easily.

Energy needs to be supplied before the reaction can start, this is called activation energy. Enzymes lower the amount of activation energy needed, this causes reactions to happen at a lower temperature. This speeds up the rate of reaction.

2. If the enzyme is catalysing a breakdown reaction, fitting into the active site puts a strain on bonds in the substrate, so the substrate molecule breaks up more easily.

Early scientists believed substrates perfectly fitted their active site this is when they can up with the lock and key analogy.
They soon realised this model did not give the full story. New evidence came to light and proved the enzyme-substrate complex changed shape slightly. This locks the enzyme even more tightly to the enzyme. This was labelled the induced fit model.

The induced fit model helps to explain why the enzymes are so specific. The substrate doesn't always have to be the right shape as the active can sightly change shape to ensure the enzyme-substrate complex locks in more tightly.