HO - 02.2 Carbohydrates

Carbohydrates Overview

Carbohydrates, also known as saccharides or colloquially as ‘sugars’, are fundamental biomolecules classified into four categories based on their structure:
- Monosaccharides: The simplest form of carbohydrates, also referred to as 'simple sugars'.
- Disaccharides: Composed of two monosaccharides joined together.
- Oligosaccharides: Short chains of monosaccharides typically ranging from three to ten units.
- Polysaccharides: Long chains that consist of many monosaccharides, sometimes referred to as polymers of saccharides.
In shorthand notation, carbohydrates are often represented as ‘CHO’ indicating their composition of carbon, hydrogen, and oxygen.

Monosaccharides are classified as polyhydroxy aldehydes or polyhydroxy ketones, characterized by the presence of multiple hydroxyl (–OH) groups and a carbonyl (C=O) group.
- Their names typically end with the suffix ‘-ose’ (e.g., glucose, fructose).
An important example includes glycerol, which is a sugar alcohol, with names often ending in ‘-ol’.

Glucose is a critical monosaccharide that serves as a primary source of carbon and energy in living organisms.
For example, Escherichia coli can survive on glucose alone, utilizing it effectively for metabolic processes.
It is imperative to note the carbon numbering (C-1 to C-6) in glucose, especially when discussing its structural forms.
Both 5- and 6-carbon monosaccharides typically exist as ring structures.

Pyranose: A 6-membered ring structure that consists of 5 carbon atoms and 1 oxygen atom. Glucose is a notable example with the molecular formula C6H12O6.
Furanose: A 5-membered ring structure made of 4 carbon atoms and 1 oxygen atom, such as fructose, which also has the formula C6H12O6.
The ring structures can exhibit variations, commonly identified as α- and β-glucose, which impact the properties of polymers derived from them.

Disaccharides are formed from the linkage of two monosaccharides and serve essential functions such as energy and carbon transport, as well as acting as a source for energy and carbon.
Common disaccharides include:
- Sucrose (table sugar) – composed of glucose and fructose.
- Maltose (malt sugar).
- Lactose (milk sugar).

Oligosaccharides, like fructans (fructooligosaccharides), are typically found in plants such as onions and artichokes, providing important dietary components like inulin, which is a soluble fiber with various physiological roles in plant cells.
Polysaccharides are extensive chains of monosaccharides that can contain hundreds to thousands of units.

Glycogen: An animal storage carbohydrate that serves as a polymer of α-glucose. It features specific linkages, comprising both α(1→6) for branch points and α(1→4) throughout the chains. Glycogen is notably stored in the liver and skeletal muscle, providing energy reserves.
Starch: The primary storage carbohydrate in plants, which exists in two forms: amylopectin and amylose, structured with α(1→4) and α(1→6) linkages.

Cellulose: A polymer of β-glucose that acts as a structural component in plant cell walls. It exists as long linear bundles termed microfibrils, making it one of the most abundant natural polymers on Earth, found in materials such as cotton.
- Cellulose features β(1→4) linkages between glucose units.
Chitin: A polymer of N-acetylglucosamine (NAG) characterized by β(1→4) linkages. It serves as a structural element in arthropod exoskeletons and fungal cell walls.
- NAG is crucial for the structural integrity of chitin forms in nature.