The backbone is made of many small units of bone or cartilage. what is each unit called?

Osteocytes

Explanation:

The backbone is a major component of the vertebral skeleton. It consists of living bone cells (osteocytes), protein fibres (collagen) and minerals, mainly calcium phosphate and calcium carbonate.

Thus, osteocytes are the small units of backbone

The ossicles

Explanation:

The smaller parts of spinal cord bones are called the ossicles. They joined to make up the long spinal cord.

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products