The little-chain version
A is a short chain of amino acids connected by bonds. Think of a string of beads: each bead stands for one , and each connection stands for a bond.
The order of the amino acids is the sequence. That sequence helps shape the molecule and what it can do.
A familiar example
Your body makes many peptides. Some act as chemical messages between cells; signaling is one role, not the definition of every . is a that helps regulate blood .
One word, many different molecules
Peptides and proteins share amino-acid building blocks. The terms overlap in scientific use; a single bead-count rule is not the whole story.
A naturally occurring molecule, an experimental compound, and an approved medicine describe different things. Knowing the structure or natural origin of a does not establish that a product is safe or effective.