Nucleosides and nucleotides are both important components of nucleic acids, such as DNA and RNA. While they are similar in structure, there is a key difference between them – the presence or absence of a phosphate group.

Nucleosides:
A nucleoside is composed of two main components:
1. Nitrogenous Base: This is a heterocyclic compound that contains nitrogen and forms the basis of the nucleoside. Examples of nitrogenous bases include adenine, guanine, cytosine, thymine, and uracil.
2. Pentose Sugar: This is a five-carbon sugar molecule that is attached to the nitrogenous base. The most common pentose sugars found in nucleosides are ribose and deoxyribose.

Nucleotides:
A nucleotide, on the other hand, is composed of three main components:
1. Nitrogenous Base: Like nucleosides, nucleotides also contain a nitrogenous base. This can be adenine, guanine, cytosine, thymine, or uracil.
2. Pentose Sugar: Similar to nucleosides, nucleotides also contain a pentose sugar. It can be either ribose or deoxyribose.
3. Phosphoric Acid: This is the key difference between nucleosides and nucleotides. Nucleotides have a phosphate group attached to the sugar molecule. The phosphate group can be esterified at one, two, or three positions of the sugar molecule, resulting in mono-, di-, or triphosphate nucleotides, respectively.

The absence of phosphoric acid in nucleosides is what distinguishes them from nucleotides. Nucleosides do not have a phosphate group attached to the sugar molecule, whereas nucleotides do. This phosphate group is crucial for the formation of phosphodiester bonds, which are responsible for linking nucleotides together to form the backbone of DNA and RNA molecules.

In summary, the absence of phosphoric acid is the key difference between nucleosides and nucleotides. Nucleosides do not have a phosphate group, while nucleotides do. This difference in structure has important implications for their roles and functions in cellular processes, particularly in the synthesis and replication of genetic material.