Chapter 5: Carbohydrates – Structure, Function, and Biological Roles

Carbohydrates: Structure and Function

Introduction to Carbohydrates

Carbohydrates are essential biomolecules that play critical roles in cell structure, cellular identity, and energy storage. They are classified based on the number of sugar units they contain.

• Monosaccharide: Single sugar monomers (e.g., glucose, fructose).

• Oligosaccharide: Short chains of monosaccharide units ("few-sugars"), typically 2–10 units.

• Polysaccharide: Long chains of monosaccharide units ("many-sugars"), such as starch, glycogen, and cellulose.

Key Point: The structure and linkage of carbohydrate monomers determine their biological function.

Structure of Monosaccharides

Monosaccharides are the simplest carbohydrates and serve as building blocks for larger carbohydrate molecules. They generally follow the formula (CH2O)n, where n is typically 3 or more.

• Basic Structure: At least 3 carbon atoms, a carbonyl group (C=O), and two or more hydroxyl groups (-OH).

• Classification: Monosaccharides can be classified by the position of the carbonyl group and the number of carbon atoms.

Naming Monosaccharides by Carbonyl Position

The location of the carbonyl group determines whether a monosaccharide is an aldose or a ketose:

• Aldose: Carbonyl group at the end of the carbon chain (e.g., glucose).

• Ketose: Carbonyl group within the carbon chain (e.g., fructose).

Naming Monosaccharides by Number of Carbon Atoms

Monosaccharides are also named according to the number of carbon atoms:

• 3C = Triose

• 4C = Tetrose

• 5C = Pentose

• 6C = Hexose

Isomers are molecules with the same molecular formula but different structural arrangements, leading to different properties.

Monosaccharide	Number of Carbons	Type	Example
Glyceraldehyde	3	Aldose	Triose
Dihydroxyacetone	3	Ketose	Triose
Ribose	5	Aldose	Pentose
Glucose	6	Aldose	Hexose
Fructose	6	Ketose	Hexose

Sugars Exist in Both Linear and Ring Forms

Monosaccharides can exist in linear (open-chain) or ring (cyclic) forms. In aqueous solutions, the ring form is usually predominant.

• Ring Formation: The carbonyl group reacts with a hydroxyl group on the same molecule, forming a ring structure.

• Isomeric Forms: For glucose, the ring can form as either α-glucose or β-glucose, depending on the orientation of the hydroxyl group attached to the anomeric carbon.

Example: Linear glucose can cyclize to form α-glucose or β-glucose.

Summary Table: Monosaccharide Classification

Classification	Criteria	Examples
By Carbonyl Position	Aldose (end), Ketose (internal)	Glucose (aldose), Fructose (ketose)
By Carbon Number	Triose (3C), Tetrose (4C), Pentose (5C), Hexose (6C)	Glyceraldehyde (triose), Ribose (pentose), Glucose (hexose)

Key Terms and Definitions

• Monosaccharide: The simplest carbohydrate, consisting of a single sugar unit.

• Aldose: A monosaccharide with a carbonyl group at the end of the carbon chain.

• Ketose: A monosaccharide with a carbonyl group within the carbon chain.

• Isomer: Molecules with the same chemical formula but different structures.

• Ring Structure: A cyclic form of a monosaccharide, common in aqueous solutions.

Example: Glucose

• Linear Formula: $C_6H_{12}O_6$

• Ring Forms: α-glucose and β-glucose

• Biological Role: Primary energy source for cells

Additional info: These notes cover the first part of Chapter 5, focusing on the structure and classification of carbohydrates, especially monosaccharides. Later sections (not shown in these slides) typically address polysaccharide formation, carbohydrate function in cell identity, and energy storage.