Table of Contents
Overview
Monosaccharides are the simplest carbohydrates. They are the “basic sugar units” from which more complex carbohydrates (disaccharides and polysaccharides) are built. In living organisms they are central as:
- immediate energy sources,
- starting materials for many metabolic pathways,
- building blocks for larger biomolecules (e.g., starch, cellulose, glycogen, nucleotides, glycoproteins).
They cannot be hydrolyzed into smaller sugars; any further breakdown destroys the sugar structure.
Classification of Monosaccharides
Monosaccharides are classified according to:
- Number of carbon atoms
- 3 C: trioses
- 4 C: tetroses
- 5 C: pentoses
- 6 C: hexoses
(Heptoses with 7 C also exist but are less important in basic biology.) - Type of carbonyl group
- Aldoses: have an aldehyde group at the end of the chain (
-CHO) - Ketoses: have a ketone group in the chain (
C=Oin the middle)
Combining both criteria gives names such as:
- Aldotriose (3 C, aldehyde): e.g. glyceraldehyde
- Ketotriose (3 C, ketone): dihydroxyacetone
- Aldohexose (6 C, aldehyde): e.g. glucose, galactose
- Ketohexose (6 C, ketone): e.g. fructose
The general empirical formula is often $\mathrm{C_n H_{2n} O_n}$ for common monosaccharides (e.g. glucose: $\mathrm{C_6 H_{12} O_6}$), but there are exceptions.
Important Biologically Relevant Monosaccharides
Trioses
Trioses are especially important as intermediates in metabolism (e.g. glycolysis), rather than as stored nutrients.
- Glyceraldehyde (an aldotriose)
- Exists as two stereoisomers (D- and L-glyceraldehyde).
- Used as a reference molecule to define D- and L-forms of all other monosaccharides.
- Dihydroxyacetone (a ketotriose)
- The only common monosaccharide without a chiral center.
- Also a key intermediate in metabolic pathways.
Pentoses
Pentoses are crucial as components of nucleic acids and coenzymes.
- Ribose (aldopentose)
- Sugar component of RNA (ribonucleic acid).
- Present in important molecules such as ATP, NAD⁺, FAD.
- Deoxyribose (2-deoxyribose, aldopentose)
- Sugar component of DNA (deoxyribonucleic acid).
- Differs from ribose by having one less oxygen atom on carbon 2.
Hexoses
Hexoses are among the main energy-supplying sugars in organisms.
- Glucose (aldohexose)
- Central “blood sugar” in many animals, including humans.
- Primary energy source for many cells, especially nerve cells.
- Can be stored in polymers (e.g. glycogen, starch).
- Fructose (ketohexose)
- Common in fruits and honey.
- Often found together with glucose in household sugar (sucrose).
- Enters metabolic pathways after being converted to intermediates of glycolysis.
- Galactose (aldohexose)
- Component of lactose (milk sugar).
- Important for the structure of some glycolipids and glycoproteins.
Linear and Ring Forms
In aqueous solution, most monosaccharides with 5 or more carbon atoms exist mainly in ring form, not as an open chain.
Formation of the Ring
The ring is formed by an internal reaction between:
- the carbonyl group (aldehyde or ketone) and
- a hydroxyl (–OH) group on one of the other carbons.
This creates a hemiacetal (from aldehyde) or hemiketal (from ketone) and introduces a new special carbon atom, the anomeric carbon:
- In aldoses (e.g. glucose): the aldehyde carbon becomes the anomeric carbon (C1).
- In ketoses (e.g. fructose): usually C2 becomes the anomeric carbon.
The ring can be:
- Pyranose (6-membered ring: 5 carbons + 1 oxygen), e.g. glucose in its common form.
- Furanose (5-membered ring: 4 carbons + 1 oxygen), e.g. ribose in nucleotides, fructose frequently.
The linear form is in equilibrium with the ring form; the ring dominates under physiological conditions.
Anomers: α and β
When the ring closes, the orientation of the OH group on the anomeric carbon can differ, giving rise to two forms:
- α-anomer: OH on the anomeric carbon is oriented one way (for D-sugars typically “down” in the Haworth projection).
- β-anomer: OH is oriented the opposite way (“up” in the Haworth projection).
These forms are called anomers and can interconvert in solution via the open-chain form (mutarotation). The ratio of α to β is characteristic for each monosaccharide (for D-glucose in water, β is more abundant).
Chirality and D-/L-Notation
Most monosaccharides (except dihydroxyacetone) have one or more chiral centers (asymmetric carbon atoms with four different substituents). As a result, multiple stereoisomers exist.
- D-form vs L-form:
- Defined by the configuration at the highest-numbered chiral center compared to D-glyceraldehyde.
- In basic biology, naturally occurring sugars in organisms are almost exclusively D-monosaccharides.
Biological systems usually recognize only one specific stereoisomer. Enzymes involved in metabolism are highly stereospecific, so the “wrong” isomer is often not metabolized.
Reducing and Non-Reducing Properties
Monosaccharides with a free (or ring-opening) anomeric carbon that can form an aldehyde or α-hydroxy ketone in solution are called reducing sugars. They can reduce mild oxidizing agents.
Characteristics:
- All common monosaccharides (e.g. glucose, fructose, ribose) are chemically reducing sugars.
- In their ring form, the anomeric OH can open to regenerate the reactive carbonyl.
These reducing properties are exploited in certain biochemical tests and in metabolic reactions.
Chemical Reactivity and Derivatives
Monosaccharides participate in many types of reactions, generating derivatives with important biological functions.
Ester and Phosphate Derivatives
Hydroxyl groups can be esterified. In biology, phosphorylated monosaccharides are especially important:
- Example: glucose-6-phosphate, fructose-1,6-bisphosphate.
- Roles:
- Trap sugars inside cells (charged phosphate group prevents diffusion out).
- Activate monosaccharides for further reactions in metabolic pathways.
Amino Sugars
One hydroxyl group is replaced by an amino group (–NH₂), often further modified (e.g. acetylation):
- Examples:
- Glucosamine
- N-acetylglucosamine
- Functions:
- Building blocks of structural polysaccharides (e.g. chitin in arthropod exoskeletons, components of bacterial cell walls).
- Components of glycoproteins and glycolipids.
Deoxy Sugars
One hydroxyl group is replaced by a hydrogen:
- Example: 2-deoxyribose in DNA.
- This small change in sugar structure has large consequences for the properties and stability of nucleic acids.
Biological Roles of Monosaccharides
Although more complex carbohydrates and macromolecules are discussed elsewhere, monosaccharides themselves have distinct roles:
- Immediate energy supply
- Glucose is the main energy substrate for many cells.
- Rapidly metabolized through central energy pathways.
- Building blocks for larger molecules
- Formation of disaccharides and polysaccharides via glycosidic bonds.
- Components of nucleotides (ribose, deoxyribose), lipopolysaccharides, glycoproteins, and glycolipids.
- Recognition and signaling
- Specific monosaccharide residues on cell surfaces contribute to cell–cell recognition, immune responses, and receptor binding (usually as part of larger carbohydrate chains).
These functions make monosaccharides a central hub between energy metabolism, structural biology, and information-carrying molecules.