Answer:
Alveoli
Explanation:
The smallest branches are called bronchioles and at the end of these are your air sacs (alveoli).
The active site is complementary to its substrate due to its specific 3D structure. The substrate fits in to one specific active site so an enzyme will only bind to one substrate. They are SPECIFIC. Lock and key theory.
B) Cell wall
The cell wall is a structure important cell protection and shape. It is necessary for the survival of unicellular organisms