2.7 Monosaccharides

Introduction

Monosaccharides are the simplest carbohydrates. They are the building blocks (monomers) of all larger carbohydrates like disaccharides and polysaccharides. They are sweet in taste, highly soluble in water, and cannot be broken down further by hydrolysis.

Definition

Monosaccharides: Monosaccharides are true carbohydrates which are either polyhydroxy aldehydes or polyhydroxy ketones.

In simple words: Monosaccharides are single sugar units that cannot be hydrolyzed into smaller carbohydrates.

Core Concept

• Monosaccharides are true carbohydrates.
• They are either:
• Polyhydroxy aldehydes (aldoses)
• Polyhydroxy ketones (ketoses)
• The general molecular formula is:
• The number of carbon atoms in monosaccharides ranges from 3 to 7.
• All carbon atoms in a monosaccharide except one have a hydroxyl group (-OH).
• The remaining carbon atom is either part of an aldehyde or ketone group.

Detailed Explanation

General Structure of Monosaccharides

Key Features:

• Aldehyde group (-CHO) → Aldose
• Ketone group (C=O) → Ketose

General Formula: (CH₂O)_n

Where n = number of carbon atoms.

Examples:

Classification of Monosaccharides

Monosaccharides are classified based on:

• Trioses (3C)
• Tetroses (4C)
• Pentoses (5C)
• Hexoses (6C)
• Heptoses (7C)

• Aldoses – Contain aldehyde group (-CHO)
• Ketoses – Contain ketone group (C=O)

Types of Monosaccharides

Important Monosaccharides

1. Glucose

• Also called: Dextrose, Grape sugar
• Formula: C₆H₁₂O₆
• Type: Aldohexose (contains aldehyde group)
• Structure: Polyhydroxy aldehyde
• Source: Fruits, honey, blood
• Function: Primary energy source for cells

2. Fructose

• Also called: Levulose, Fruit sugar
• Formula: C₆H₁₂O₆
• Type: Ketohexose (contains ketone group)
• Structure: Polyhydroxy ketone
• Source: Fruits, honey
• Function: Sweetest natural sugar; energy source

3. Ribose

• Formula: C₅H₁₀O₅
• Type: Aldopentose
• Source: RNA, ATP, NAD
• Function: Component of RNA and ATP

4. Deoxyribose

• Formula: C₅H₁₀O₄
• Type: Aldopentose
• Source: DNA
• Function: Component of DNA

5. Galactose

• Formula: C₆H₁₂O₆
• Type: Aldohexose
• Source: Milk (as part of lactose)
• Function: Energy source; component of lactose

Chemical Structures of Monosaccharides

Monosaccharides can exist in two structural forms:

• Found in crystalline form
• Contains aldehyde or ketone group

• Found when dissolved in water
• Most pentoses and hexoses convert to ring structure

Ring Structures

Two types of ring structures are found in monosaccharides:

• Five-membered ring
• Contains one oxygen and four carbon atoms
• C1 and C4 are attached to oxygen to form ring
• All pentoses and ketohexoses form furanose ring structure
• Example: Ribose → Ribofuranose

• Six-membered ring
• Contains one oxygen and five carbon atoms
• C1 and C5 are attached to oxygen to form ring
• Only aldohexoses are converted into pyranose ring structure
• Example: Glucose → Glucopyranose

Conversion of Open-Chain to Ring Structure

• In dried form: Open-chain structure
• In aqueous medium: Ring structure
• When dissolved in water:
• Oxygen atom from aldehyde group reacts with second last carbon (C5)
• Oxygen forms a link between C1 and C5
• OH group of second last carbon is shifted to C1
• Ring structure called glucopyranose is formed

• In dried form: Open-chain structure
• In aqueous medium: Ring structure
• When dissolved in water:
• Oxygen atom from aldehyde group reacts with second last carbon (C4)
• Oxygen forms a link between C1 and C4
• OH group of second last carbon is shifted to C1
• Ring structure called ribofuranose is formed

Isomers and Stereoisomers of Glucose

• Compounds with the same molecular formula but different structural arrangement.
• Example: Glucose and Fructose are functional group isomers (both C₆H₁₂O₆ but glucose is aldo sugar and fructose is keto sugar).

Stereoisomerism: - Molecules with the same molecular formula but differ only in the relative arrangement of -H and -OH groups in 3D space at the anomeric and asymmetric carbon atoms.

• The carbon in the ring structure derived from the carbonyl carbon of the open-chain form.
• Example: C-1 in glucose and ribose.

• A carbon that makes bonds with four different atoms or groups.
• Example: C-2, C-3, C-4, C-5 in glucose.

• The last asymmetric carbon or second last in the carbon chain.
• Example: C-5 in glucose.

• Formula: 2ⁿ
• Where n = number of asymmetric carbons
• Glucose has 4 asymmetric carbons → 2⁴ = 16 stereoisomers

Types of Stereoisomers

Anomers

• Stereoisomers that have different position of -H and -OH group on the C-1 (anomeric carbon) of pentose (e.g., ribose) and hexose (e.g., glucose).
• Called α or β isomers.
• If -OH group is found downward on C-1 → α sugar
• If -OH is present upward on C-1 → β sugar

Enantiomers

• Those stereoisomers which are non-superimposable mirror images of one another.
• Example: D (Dextro) and L (Levo) isomers of glucose.
• D isomers → right-handed form
• L isomers → left-handed form
• In D-glucose: Penultimate carbon (C-5) has -OH group on right side.
• In L-glucose: Penultimate carbon has -OH group on left side.
• Out of 16 stereoisomers of glucose, 8 are enantiomers of the other 8.

Diastereoisomers

• Those stereoisomers which have different position of -H and -OH groups at more than one asymmetrical carbon atoms.
• Unlike enantiomers, diastereoisomers are not mirror images of each other.
• Example: D-Glucose and D-Altrose.

Epimers

• Those stereoisomers which have different arrangement of -H and -OH groups at only one asymmetrical carbon atom.
• Example: D-Glucose and D-Mannose.

Science Tidbits (from textbook)

Laboratory manufactured (artificial) sugars such as tagatose, sucralose, etc., are left-handed forms (L-sugars), and are used as sweeteners. Whereas living organisms can exclusively use naturally occurring right-handed sugars (D-sugars).

Cellular proteins, receptors, and enzymes are designed to react only with particular enantiomers (D-sugars). For example, the enzymes in the human stomach are right-handed and can digest only D-sugars. Hence, right-handed human enzymes cannot metabolize left-handed sugars because their active sites cannot fit onto a left-handed substrate. So, for the left-handed substrate, the enzyme must be left-handed.

Intermediate Level

Classification of Monosaccharides

Important Monosaccharides in Detail

Ring Structures in Detail

• 5-membered ring
• 1 oxygen + 4 carbons
• C1 and C4 attached to oxygen
• Formed by pentoses and ketohexoses
• Example: Ribose → Ribofuranose

• 6-membered ring
• 1 oxygen + 5 carbons
• C1 and C5 attached to oxygen
• Formed by aldohexoses
• Example: Glucose → Glucopyranose

Stereoisomers in Detail

Why Are D-Sugars Important?

• Living organisms use D-sugars exclusively.
• Enzymes in the human body are right-handed and can only digest D-sugars.
• L-sugars (left-handed) cannot be metabolized by human enzymes.
• Artificial sweeteners like tagatose and sucralose are L-sugars.
• They taste sweet but are not metabolized, so they provide fewer calories.

Important Points

• 1. Monosaccharides are true carbohydrates which are either polyhydroxy aldehydes or polyhydroxy ketones.
• 2. The general molecular formula for the representation of monosaccharides is (CH₂O)_n where 'n' is the number.
• 3. Number of carbon atoms in monosaccharides ranges from 3 to 7.
• 4. All the carbon atoms in a monosaccharide except one have a hydroxyl group (-OH), while the remaining carbon atom is either part of an aldehyde or ketone.
• 5. Monosaccharides with five or more carbon atoms may exist in two structural forms:
• - Open-chain (acyclic) structure
• - Ring (cyclic) structure
• 6. Monosaccharides are usually found in open-chain structure in crystalline form, but when they are dissolved in water, most of them (pentoses and hexoses) are converted into ring chain structure.
• 7. Two types of ring structure are found in monosaccharides:
• - Furanose – Five-cornered ring structure with one oxygen and four carbon atoms. All pentoses and ketohexoses form furanose ring structure.
• - Pyranose – Six-cornered ring structure with one oxygen and five carbon atoms. Only aldohexoses are converted into pyranose ring structure.
• 8. Glucose and Ribose:
• - In dried form: Open-chain structure
• - In aqueous medium: Ring structure
• - Glucose → Glucopyranose
• - Ribose → Ribofuranose
• 9. Isomerism: Compounds with the same molecular formula but different structural arrangement or formula.
• 10. Stereoisomers: Molecules with the same molecular formula but differ only in the relative arrangement of -H and -OH groups in 3D space at the anomeric and asymmetric carbon atoms.
• 11. Anomeric carbon: The carbon in the ring structure derived from the carbonyl carbon of the open-chain form.
• 12. Asymmetric carbon: A carbon that makes bonds with four different atoms or groups.
• 13. Penultimate carbon: The last asymmetric carbon or second last in the carbon chain.
• 14. Number of stereoisomers: 2ⁿ where n = number of asymmetric carbons. Glucose has 16 stereoisomers.
• 15. Types of stereoisomers:
• - Anomers: Differ in position of -H and -OH on C-1 (anomeric carbon).
• - Enantiomers: Non-superimposable mirror images (e.g., D-Glucose and L-Glucose).
• - Diastereoisomers: Differ at more than one asymmetric carbon (e.g., D-Glucose and D-Altrose).
• - Epimers: Differ at only one asymmetric carbon (e.g., D-Glucose and D-Mannose).
• 16. α and β sugars: If -OH group is found downward on C-1 → α sugar; if -OH is present upward on C-1 → β sugar.
• 17. D and L sugars: In D-glucose, penultimate carbon (C-5) has -OH group on right side; in L-glucose, penultimate carbon has -OH group on left side.
• 18. Science Tidbits: Laboratory manufactured sugars such as tagatose, sucralose are left-handed forms (L-sugars) and are used as sweeteners. Living organisms can exclusively use naturally occurring right-handed sugars (D-sugars). Cellular proteins, receptors, and enzymes are designed to react only with particular enantiomers (D-sugars).

Remember

• - Monosaccharides = true carbohydrates; polyhydroxy aldehydes or ketones
• - General formula = (CH₂O)_n
• - **Carbon atoms** = 3 to 7
• - Types by carbon number: Triose, Tetrose, Pentose, Hexose, Heptose
• - Types by carbonyl group: Aldose, Ketose
• - Ring structures: Furanose (5-membered), Pyranose (6-membered)
• - Glucose → Glucopyranose
• - Ribose → Ribofuranose
• - **Anomeric carbon** = C-1
• - Asymmetric carbon = bonded to 4 different groups
• - Penultimate carbon = second last carbon
• - Stereoisomers = 2ⁿ
• - Types: Anomers, Enantiomers, Diastereoisomers, Epimers
• - **α sugar** = -OH down on C-1
• - **β sugar** = -OH up on C-1
• - D sugar = -OH on right on penultimate carbon
• - L sugar = -OH on left on penultimate carbon
• - D-sugars = natural, used by organisms
• - L-sugars = synthetic, used as sweeteners

Common Mistakes

• 1. Thinking monosaccharides can be hydrolyzed – Monosaccharides cannot be further hydrolyzed into smaller carbohydrates.
• 2. Confusing aldoses and ketoses – Aldoses contain an aldehyde group; Ketoses contain a ketone group.
• 3. Confusing furanose and pyranose – Furanose = 5-membered ring; Pyranose = 6-membered ring.
• 4. Thinking all monosaccharides form pyranose rings – Only aldohexoses form pyranose rings. Pentoses and ketohexoses form furanose rings.
• 5. Confusing anomeric and asymmetric carbon – Anomeric carbon = C-1 (derived from carbonyl carbon); Asymmetric carbon = bonded to 4 different groups.
• 6. Confusing penultimate carbon – The penultimate carbon is the second-to-last carbon in the chain.
• 7. Forgetting the formula for stereoisomers – 2ⁿ where n = number of asymmetric carbons.
• 8. Confusing anomers and enantiomers – Anomers differ at C-1; Enantiomers are mirror images.
• 9. Confusing diastereoisomers and epimers – Diastereoisomers differ at more than one asymmetric carbon; Epimers differ at only one.
• 10. Thinking all sugars are D-sugars – L-sugars exist but are synthetic and used as sweeteners.
• 11. Forgetting that humans can only metabolize D-sugars – Human enzymes are right-handed and can only digest D-sugars.
• 12. Thinking L-sugars are harmful – L-sugars are not harmful; they are used as low-calorie sweeteners.
• 13. Confusing glucose and fructose – Glucose is an aldohexose; Fructose is a ketohexose.
• 14. Forgetting the examples of pentoses – Ribose and deoxyribose are pentoses.
• 15. Thinking all monosaccharides are hexoses – Monosaccharides can have 3, 4, 5, 6, or 7 carbon atoms.

Quick Revision

• Monosaccharides are true carbohydrates which are either polyhydroxy aldehydes or polyhydroxy ketones.
• The general molecular formula for the representation of monosaccharides is (CH₂O)_n where 'n' is the number.
• Number of carbon atoms in monosaccharides ranges from 3 to 7.
• All the carbon atoms in a monosaccharide except one have a hydroxyl group (-OH), while the remaining carbon atom is either part of an aldehyde or ketone.
• Monosaccharides with five or more carbon atoms may exist in two structural forms, i.e., open-chain or acyclic structure and a ring or cyclic structure.
• Monosaccharides are usually found in open-chain structure in crystalline form but when they are dissolved in water, most of them (pentoses and hexoses) are converted into ring chain structure.
• Two types of ring structure are found in monosaccharides, i.e., furanose and pyranose.
• Furanose is a five-cornered ring structure with one oxygen and four carbon atoms. In furanose, C1 and C4 are attached to oxygen to form ring structure. All pentoses and ketohexoses form furanose ring structure.
• Pyranose is a six-cornered ring structure with one oxygen and five carbon atoms. In pyranose, C1 and C5 are attached to oxygen to form ring structure. Only aldohexoses are converted into pyranose ring structure.
• Let us understand it by taking examples of aldohexose glucose C₆H₁₂O₆ and aldopentose ribose C₅H₁₀O₅.
• Glucose and ribose can exist in open-chain structure in dried form but it exists in ring structure in aqueous medium.
• When they dissolve in water, the oxygen atom from aldehyde group reacts with second last carbon, i.e., C5 in case of glucose and C4 in case of ribose.
• In this way oxygen atom forms a link between C1 and C5 in case of glucose and between C1 and C4 in case of ribose.
• While the OH group of second last carbons is shifted to C1.
• After this modification ring structure of glucose called glucopyranose whereas, ring structure of ribose is called ribofuranose.
• 2.8 Isomers and Stereoisomers of Glucose:
• Phenomenon in which compounds have the same molecular formula but they differ in structural arrangement or formula is called isomerism.
• For example, glucose and fructose both are hexoses with same molecular formula, i.e., C₆H₁₂O₆ but both have different structure of their functional groups.
• Glucose is an aldo sugar and fructose is a keto sugar so they are functional group isomers of each other.
• Stereoisomerism in glucose:
• Stereoisomers are molecules that have the same molecular formula but differ only in the relative arrangement of -H and -OH group in 3D space at the anomeric and asymmetric carbon atoms.
• Anomeric carbon is the carbon in the ring structure of monosaccharides that is derived from the carbonyl carbon (-6) of ketone or aldehyde functional group of their open-chain, e.g., C-1 in the glucose and ribose.
• Asymmetric carbon is that carbon which makes bonds with four different atoms or groups around it. For example, in glucose, four carbon atoms (C-2, C-3, C-4, and C-5) are asymmetric carbon atoms.
• Last asymmetric carbon atom or second last in carbon chain (C-5 in case of glucose) is called penultimate carbon.
• In monosaccharide the number of stereoisomers actually depends upon the number of asymmetric carbons in its structure and can be calculated by the formula 2ⁿ where n is the number of asymmetric carbon atoms so glucose has 16 stereoisomers.
• Stereoisomers can be classified into following groups:

60-Second Revision

• Monosaccharides = true carbohydrates; polyhydroxy aldehydes or ketones
• General formula = (CH₂O)_n
• **Carbon atoms** = 3 to 7
• Types by carbon number: Triose, Tetrose, Pentose, Hexose, Heptose
• Types by carbonyl group: Aldose, Ketose
• Ring structures: Furanose (5-membered), Pyranose (6-membered)
• Glucose → Glucopyranose
• Ribose → Ribofuranose
• **Anomeric carbon** = C-1
• Asymmetric carbon = bonded to 4 different groups
• Penultimate carbon = second last carbon
• Stereoisomers = 2ⁿ
• Types: Anomers, Enantiomers, Diastereoisomers, Epimers
• **α sugar** = -OH down on C-1
• **β sugar** = -OH up on C-1
• D sugar = -OH on right on penultimate carbon
• L sugar = -OH on left on penultimate carbon
• D-sugars = natural, used by organisms
• L-sugars = synthetic, used as sweeteners
• Warning: Monosaccharides cannot be hydrolyzed further. Not all monosaccharides form pyranose rings — only aldohexoses do. Pentoses and ketohexoses form furanose rings. Anomers differ at C-1; enantiomers are mirror images; epimers differ at only one asymmetric carbon. D-sugars are natural; L-sugars are synthetic. Human enzymes can only digest D-sugars.