Carbohydrates: Structure, Function, and Diversity

Carbohydrates

Introduction to Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically in the ratio (CH2O)n. They play essential roles in living organisms, including energy storage, structural support, and cell recognition.

• General Formula: Most carbohydrates have the empirical formula (CH2O)n.

• Key Functions: Energy storage, structural components, and cell identification.

Main Roles of Carbohydrates

• Cell Structure: Carbohydrates are integral to the formation of cell walls in plants (cellulose) and exoskeletons in arthropods (chitin).

• Cell Identity: Carbohydrates on cell surfaces act as markers for cellular recognition and communication.

• Energy Storage: Carbohydrates such as starch and glycogen store energy for later use.

Monosaccharides

Structure of Monosaccharides

Monosaccharides are the simplest form of carbohydrates, consisting of a single sugar unit. They serve as the building blocks for more complex carbohydrates.

• Definition: Monosaccharides are simple sugars with the general formula CnH2nOn.

• Functional Groups: Contain a carbonyl group (either an aldehyde or ketone) and multiple hydroxyl groups.

• Examples: Glucose (an aldose), Fructose (a ketose).

Key Structural Features

• Number of Carbons: Monosaccharides can have 3-7 carbon atoms (trioses, tetroses, pentoses, hexoses, heptoses).

• Carbonyl Group Position: If the carbonyl group is at the end, the sugar is an aldose; if within the chain, it is a ketose.

• Hydroxyl Group Arrangement: The spatial arrangement of -OH groups around asymmetric carbons varies among sugars.

• Ring Formation: In aqueous solutions, monosaccharides often form ring structures.

Examples of Monosaccharides

Name	Type	Structure
Glucose	Aldose	6-carbon sugar with an aldehyde group
Fructose	Ketose	6-carbon sugar with a ketone group

Variation Among Monosaccharides

How Monosaccharides Differ

Monosaccharides can differ in several ways, leading to a diversity of structures and functions.

• Number of Carbons: Determines the length and type of sugar (e.g., pentose vs. hexose).

• Position of Carbonyl Group: Aldose vs. ketose.

• Spatial Arrangement of Hydroxyl Groups: Isomers such as glucose and galactose differ in the orientation of -OH groups.

• Alternative Ring Forms: Monosaccharides can cyclize to form different ring structures (e.g., alpha and beta forms of glucose).

Disaccharides and Polysaccharides

Formation of Disaccharides

Disaccharides are formed when two monosaccharides are joined by a glycosidic linkage, a type of covalent bond.

• Glycosidic Linkage: A bond formed between the hydroxyl groups of two monosaccharides, releasing a molecule of water (condensation reaction).

• Examples: Maltose (glucose + glucose), Lactose (glucose + galactose).

Polysaccharides

Polysaccharides are large polymers composed of many monosaccharide units linked together. They serve various structural and storage functions in cells.

• Examples: Starch (energy storage in plants), Glycogen (energy storage in animals), Cellulose (structural component in plants).

• Properties: The type of glycosidic linkage (alpha or beta) determines the structure and function of the polysaccharide.

Summary Table: Types of Carbohydrates

Type	Monomer	Example	Main Function
Monosaccharide	—	Glucose, Fructose	Immediate energy, building block
Disaccharide	2 monosaccharides	Maltose, Lactose	Transport, energy
Polysaccharide	Many monosaccharides	Starch, Glycogen, Cellulose	Energy storage, structure

Additional info:

• Carbohydrates are also precursors for the synthesis of nucleotides and amino acids.

• Cell-surface carbohydrates are critical for immune recognition and signaling.