So let's begin by discussing what we mean by the lock and key model. So lock and key simply means we have a key, we have a lock. And essentially when we place the key into the lock, we know that the shapes are complementary and that's why we have a perfect fit between our lock and between our keys. So in the lock and key model, the substrate fits precisely and perfectly into the active side due to their complementary shape. So even before they actually bind, what this model tells us is the active side of the enzyme. So the green structure is the enzyme, this is the active side and this is a substrate.