Carbohydrates: Structure and Function

Introduction to Carbohydrates

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

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

• Oligosaccharide: Short chains of sugar molecules (few-sugars)

• Polysaccharide: Long chains of sugar molecules (many-sugars), such as starch, glycogen, and cellulose

Key Point: The function of carbohydrates in living organisms is determined by how their monomers are linked together.

Molecular Structure of Carbohydrates

Carbohydrates generally have the molecular formula , where "n" is the number of carbon-hydrate groups and can range from 3 to over a thousand. Their structure includes:

• Carbonyl group ()

• Hydroxyl groups ()

• Carbon-hydrogen bonds ()

Not all compounds with the formula are carbohydrates (e.g., formaldehyde is not a carbohydrate).

Sugars as Monomers

Monosaccharides, or simple sugars, serve as the building blocks for larger carbohydrates and provide chemical energy in cells. They also played a significant role in chemical evolution, such as ribose in nucleotide formation.

• Example: Ribose is required for the formation of nucleotides, the building blocks of RNA.

Structural Diversity of Monosaccharides

What Distinguishes One Monosaccharide from Another?

Monosaccharides vary in several structural aspects, which affect their function:

• Location of the carbonyl group:

  • At the end of the molecule: aldose

  • In the middle of the molecule: ketose

• Number of carbon atoms:

  • Three: triose

  • Five: pentose

  • Six: hexose

• Spatial arrangement of atoms: Different arrangement of hydroxyl groups can result in different sugars (e.g., glucose vs. galactose).

• Linear and ring forms: Sugars often form ring structures in aqueous solutions, and these forms can have different properties.

Key Point: Each monosaccharide has a unique structure and function due to these variations.

Polysaccharides: Structure and Linkages

Formation and Types of Polysaccharides

Polysaccharides are polymers made from monosaccharide monomers. Two sugars linked together form a disaccharide. The linkage occurs via a condensation reaction between two hydroxyl groups, forming a covalent bond called a glycosidic linkage. These linkages can be broken by hydrolysis reactions.

• Glycosidic linkages can form between any two hydroxyl groups.

• Common types:

  • α-1,4-glycosidic linkage

  • β-1,4-glycosidic linkage

• Both linkages are between the C-1 and C-4 carbons, but differ in geometry.

Major Polysaccharides and Their Functions

Additional info: In cellulose and chitin, every other monomer is flipped, allowing for linear strands and hydrogen bonding between adjacent strands, which increases structural strength.

Functions of Carbohydrates in Cells

Diverse Cellular Roles

Carbohydrates serve multiple functions in living organisms:

• Precursors for other molecules (e.g., nucleotides, amino acids)

• Provide fibrous structural materials (e.g., cellulose, chitin, peptidoglycan)

• Indicate cell identity (e.g., glycoproteins, glycolipids)

• Store chemical energy

Carbohydrates as Structural Support

Structural polysaccharides such as cellulose, chitin, and peptidoglycan form long strands with bonds between adjacent strands, organized into fibers or sheets. This provides strength and elasticity to cells and organisms.

• β-1,4-glycosidic linkages are resistant to hydrolysis, making these fibers difficult to break down.

• These fibers exclude water, further hindering hydrolysis.

• Dietary fiber from carbohydrates is important for digestive health.

Role in Cell Identity

Carbohydrates on the cell surface play a key role in cell recognition and signaling:

• Glycoproteins: Proteins with attached carbohydrates

• Glycolipids: Lipids with attached carbohydrates

• These molecules are crucial for distinguishing "self" cells and for communication between cells.

Carbohydrates and Energy Storage

Energy Storage and Release

Carbohydrates store and provide chemical energy. In photosynthesis, plants convert sunlight into chemical energy stored in carbohydrate bonds:

• Carbohydrates have more energy than because electrons in and bonds have higher potential energy than those in and bonds.

Enzymatic Hydrolysis of Polysaccharides

Energy-storage polysaccharides like starch and glycogen are easily hydrolyzed due to their α-glycosidic linkages.

• Glycogen is hydrolyzed by the enzyme phosphorylase in animal cells to release glucose.

• Starch is hydrolyzed by amylase enzymes, which are important in digestion.

ATP Production from Glucose

When a cell needs energy, it breaks down glucose, and the captured energy is used to make ATP:

• ATP is the universal energy currency in cells, driving processes such as polymerization and muscle movement.