Define and give examples of polysaccharides, triacylglycerols, nucleotides, and nucleosides. Explain how to name nucleosides.

Polysaccharides are complex carbohydrates formed by many monosaccharide units linked together.

• Homopolysaccharides are composed of only one type of monosaccharide unit. Examples include starch, glycogen, and cellulose (all made of glucose units).
• Heteropolysaccharides are composed of two or more different types of monosaccharide units. Examples include hyaluronic acid and heparin.

TAG stands for Triacylglycerol (also known as triglyceride). It is a lipid molecule composed of a glycerol backbone esterified to three fatty acid molecules. TAGs serve as the primary form of energy storage in animals and plants.

A nucleotide is the basic building block of nucleic acids (DNA and RNA). It consists of three components: a nitrogenous base (adenine, guanine, cytosine, thymine, or uracil), a pentose sugar (deoxyribose in DNA or ribose in RNA), and one or more phosphate groups.

A nucleoside is a part of a nucleotide, consisting of a nitrogenous base covalently linked to a pentose sugar (deoxyribose or ribose), but without any phosphate groups.

Naming Nucleosides: The nitrogenous base's name is modified.

• For purine bases (Adenine, Guanine), the suffix "-osine" is added: Adenine becomes Adenosine, Guanine becomes Guanosine.
• For pyrimidine bases (Cytosine, Thymine, Uracil), the suffix "-idine" is added: Cytosine becomes Cytidine, Thymine becomes Thymidine, Uracil becomes Uridine.

Naming Nucleotides: Nucleotides are named by adding "monophosphate," "diphosphate," or "triphosphate" to the corresponding nucleoside name, indicating the number of phosphate groups attached.

• Example: Adenine $\rightarrow$ Adenosine (nucleoside) $\rightarrow$ Adenosine monophosphate (AMP), Adenosine diphosphate (ADP), Adenosine triphosphate (ATP).
• Similarly: Guanine $\rightarrow$ Guanosine $\rightarrow$ Guanosine monophosphate (GMP).
• Cytosine $\rightarrow$ Cytidine $\rightarrow$ Cytidine monophosphate (CMP).
• Thymine $\rightarrow$ Thymidine $\rightarrow$ Thymidine monophosphate (TMP).
• Uracil $\rightarrow$ Uridine $\rightarrow$ Uridine monophosphate (UMP).

A peptide bond is formed between two amino acids through a dehydration synthesis (or condensation) reaction. The carboxyl group $(-\text{COOH})$ of one amino acid reacts with the amino group $(-\text{NH}_2)$ of another amino acid. A molecule of water is removed, and a covalent bond forms between the carbon atom of the carboxyl group and the nitrogen atom of the amino group, resulting in a $-\text{CO}-\text{NH}-$ linkage.