Enzymes act as biological catalysts, increasing the rate of chemical reactions.

Enzymes lower the activation energy required for a chemical reaction.

An enzyme-substrate complex is the complex formed when a substrate(s) binds to an enzyme. 

Substrates bind to an enzyme's active site. 

Enzymes are specific to their substrate(s) because they have unique tertiary structures that make their active sites complementary to a specific substrate or substrates. 

Enzymes lower the activation energy as they form temporary bonds between the R groups in an enzyme's active site and the substrate(s). 

During an enzyme-catalysed reaction, substrates are broken down or joined together to form products. 

The enzyme releases the product(s) from the active site and is then used again to bind to another substrate or substrates. 

Intracellular enzymes act within the cells that produce them.

Extracellular enzymes act outside the cells that produce and secrete them.

Activation energy is the energy required to start a chemical reaction.

The lock and key model suggests that enzymes have a specific active site that perfectly fits the substrate(s).

The induced-fit model proposes that the substrate or substrates do not fit perfectly into an enzyme's active site.

Instead, the enzyme's active site changes shape when the substrate(s) bind, putting a strain on the substrate's bonds and lowering the activation energy.