What is a peptide bond or linkage? Distinguish between a peptide, oligopeptide polypeptide and protein.

Explanation

### Step-by-Step Solution:

1. **Definition of Peptide Bond:**
A peptide bond, also known as peptide linkage, is a covalent bond that forms between two amino acids. This bond is created when the carboxyl group (-COOH) of one amino acid reacts with the amino group (-NH2) of another amino acid, resulting in the release of a water molecule (H2O). The resulting bond between the carbon atom of the carboxyl group and the nitrogen atom of the amino group is the peptide bond.

2. **Formation of Peptide Bond:**
For example, consider two amino acids: glycine (NH2-CH2-COOH) and alanine (NH2-CH(CH3)-COOH). When these two amino acids link together, the hydroxyl group (-OH) from the carboxyl group of glycine and a hydrogen atom (H) from the amino group of alanine are removed, forming water (H2O). The remaining parts of the amino acids then bond together, forming the structure: NH2-CH2-CO-NH-CH(CH3)-COOH. The bond between the carbon (C) of the carboxyl group and the nitrogen (N) of the amino group is the peptide bond.

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