2.9 Properties and Roles of Disaccharides

Introduction

Disaccharides are a type of oligosaccharide made of two monosaccharide units joined together by a glycosidic bond. They are an important energy source for living organisms and are commonly found in nature. Examples include maltose, lactose, and sucrose.

Definition

Disaccharides: Two monosaccharides combine to form a disaccharide. It is a kind of oligosaccharide.

In simple words: A disaccharide is a sugar made of two monosaccharide units joined together.

Core Concept

• Disaccharides are formed by the condensation of two monosaccharides.
• They are less sweet in taste and comparatively less soluble in water than monosaccharides.
• They can be hydrolyzed to give monosaccharides.
• Disaccharides are a familiar energy source for living organisms.
• The general formula of a disaccharide is:

Examples: Maltose, lactose, and sucrose.

Detailed Explanation

Properties of Disaccharides

Common Disaccharides

2.9.1 Sucrose

• Common name: Cane sugar
• Composition: α-Glucose + β-Fructose
• Glycosidic bond: α-1,2-glycosidic bond (or α-1, β-2-glycosidic bond)
• Source: Sugar cane, sugar beet
• Uses: Widely used as a sweetener at homes for making sweet dishes
• Role in plants: Also called transport disaccharide as prepared food in plants is transported in the form of sucrose
• Properties:
• Very soluble
• Can be moved efficiently in high concentration in plants
• Relatively unreactive chemically
• Hydrolysis: Upon hydrolysis, sucrose yields one molecule of α-glucose and one molecule of β-fructose

2.9.2 Maltose

• Common name: Malt sugar
• Composition: α-Glucose + α-Glucose
• Glycosidic bond: α-1,4-glycosidic bond
• Source: Germinating seeds, sprouting potatoes
• Production: Produced in the brewing industry by the malting process in which barley starch is catabolized using the enzyme amylase
• Role: Intermediate disaccharide produced during the breakdown of starch and glycogen; acts as an energy-producing molecule
• Function: Acts as an energy source for growth and development of young seedlings

2.9.3 Lactose

• Common name: Milk sugar
• Composition: β-Galactose + β-Glucose
• Glycosidic bond: β-1,4-glycosidic bond
• Source: Milk of mammals
• Ratio in different mammals:
• 3–4% in goat milk
• 4–6% in cow milk
• 5–8% in human milk
• Production: Also produced as a by-product during the cheese production process
• Role:
• Used as a direct energy source by infants
• Acts as a substrate for the synthesis of various macro-biomolecules which are involved in vital nervous and immunological processes

Glycosidic Bonds in Disaccharides

Definition: A glycosidic bond is a covalent chemical bond between two glycosides (sugar molecules) that holds them together.

• Formed due to a condensation reaction between two sugar molecules (mostly hexoses).
• The H-group from one ringed sugar interacts with the -OH group of another sugar.
• One water molecule is released.

• Named on the basis of carbon number of interacting carbons of both linked sugars.
• Named alpha or beta on the basis of the alpha or beta type of the first sugar involved in the linkage.
• Example: α-1,4-glycosidic linkage is formed when the first sugar is alpha, donates -OH group from its C-1, and the other sugar donates H-group from its fourth carbon.

Glycosidic Linkages in Common Disaccharides:

Critical Thinking (from textbook)

• Why do potatoes taste sweet when stored for long?
• How do the initially tasteless or sour-tasting young fruits become sweet when they ripen?

• When potatoes are stored for a long time, the starch in them is broken down into maltose and glucose by enzymes. These sugars are sweet in taste.
• When fruits ripen, starch is converted into sugars (fructose, glucose, sucrose) by enzymes. This makes the fruits sweet.

Intermediate Level

Properties of Disaccharides

Comparison of Common Disaccharides

Glycosidic Bond Formation

• Two monosaccharides come close together.
• The H-group from one sugar interacts with the -OH group of another sugar.
• A water molecule is released.
• A glycosidic bond is formed.

• First sugar: α or β
• Carbon number of first sugar: 1
• Second sugar: α or β
• Carbon number of second sugar: 2, 4, etc.

• Sucrose: α-1,2-glycosidic bond
• Maltose: α-1,4-glycosidic bond
• Lactose: β-1,4-glycosidic bond

Roles of Disaccharides

Why Are Disaccharides Less Sweet Than Monosaccharides?

• Sweetness depends on the structure of the sugar molecule.
• Monosaccharides have a free aldehyde or ketone group.
• In disaccharides, these groups are involved in the glycosidic bond.
• Therefore, disaccharides are less sweet.

Why Are Disaccharides Less Soluble Than Monosaccharides?

• Disaccharides are larger molecules.
• They have more hydrogen bonding between molecules.
• This makes it harder for water to dissolve them.
• Therefore, disaccharides are less soluble.

Important Points

• 1. Disaccharides: Two monosaccharides combine to form a disaccharide.
• 2. Disaccharides are a kind of oligosaccharide.
• 3. Disaccharides are less sweet in taste and comparatively less soluble in water.
• 4. They can be hydrolyzed to give monosaccharides.
• 5. Disaccharides are a familiar energy source for living organisms.
• 6. The general formula of a disaccharide is C₁₂H₂₂O₁₁.
• 7. Common disaccharides:
• - Sucrose: Cane sugar; glucose + fructose; α-1,2-glycosidic bond
• - Maltose: Malt sugar; glucose + glucose; α-1,4-glycosidic bond
• - Lactose: Milk sugar; galactose + glucose; β-1,4-glycosidic bond
• 8. Sucrose:
• - Commonly known as cane sugar.
• - Widely used as a sweetener at homes.
• - In plants, also called transport disaccharide.
• - Very soluble and can be moved efficiently in high concentration.
• - Relatively unreactive chemically.
• - Upon hydrolysis, yields one molecule of α-glucose and one molecule of β-fructose.
• 9. Maltose:
• - Commonly known as malt sugar.
• - Intermediate disaccharide produced during the breakdown of starch and glycogen.
• - Acts as an energy-producing molecule.
• - Produced in the brewing industry by the malting process.
• - Found in germinating seeds and sprouting potatoes.
• - Acts as an energy source for growth and development of young seedlings.
• 10. Lactose:
• - Commonly known as milk sugar.
• - Found in milk of mammals.
• - Ratio varies: 3–4% in goat milk, 4–6% in cow milk, 5–8% in human milk.
• - Produced as a by-product during cheese production.
• - Used as a direct energy source by infants.
• - Acts as a substrate for synthesis of various macro-biomolecules involved in vital nervous and immunological processes.
• 11. Glycosidic bond: A covalent chemical bond between two glycosides that holds them together.
• 12. Glycosidic bond is formed due to a condensation reaction between two sugar molecules.
• 13. Glycosidic bonds are named on the basis of carbon number of interacting carbons of both linked sugars.
• 14. Glycosidic linkage is also named alpha or beta on the basis of alpha or beta type of the first sugar involved in linkage.
• 15. Glycosidic linkages in disaccharides:
• - Sucrose: α-1,2-glycosidic linkage (or α-1, β-2-glycosidic linkage)
• - Maltose: α-1,4-glycosidic linkage (or α-1, α-4-glycosidic linkage)
• - Lactose: β-1,4-glycosidic linkage (or β-1, β-4-glycosidic linkage)

Remember

• - Disaccharides = two monosaccharides joined together
• - General formula = C₁₂H₂₂O₁₁
• - Less sweet and less soluble than monosaccharides
• - Can be hydrolyzed to monosaccharides
• - Examples:
• - Sucrose = Glucose + Fructose (α-1,2)
• - Maltose = Glucose + Glucose (α-1,4)
• - Lactose = Galactose + Glucose (β-1,4)
• - Sucrose = transport disaccharide in plants
• - Maltose = intermediate in starch breakdown
• - Lactose = energy source for infants
• - Glycosidic bond = covalent bond between two sugars
• - Formation = condensation reaction; water released

Common Mistakes

• 1. Thinking disaccharides are as sweet as monosaccharides – Disaccharides are less sweet than monosaccharides.
• 2. Thinking disaccharides are more soluble than monosaccharides – Disaccharides are less soluble than monosaccharides.
• 3. Confusing the composition of disaccharides – Sucrose = glucose + fructose; Maltose = glucose + glucose; Lactose = galactose + glucose.
• 4. Confusing the glycosidic bonds – Sucrose = α-1,2; Maltose = α-1,4; Lactose = β-1,4.
• 5. Thinking sucrose is a reducing sugar – Sucrose is a non-reducing sugar because both anomeric carbons are involved in the glycosidic bond.
• 6. Thinking maltose is found in milk – Maltose is found in germinating seeds and sprouting potatoes; lactose is found in milk.
• 7. Confusing lactose and maltose – Lactose = milk sugar; Maltose = malt sugar.
• 8. Forgetting that sucrose is the transport sugar in plants – Sucrose is the main transport sugar in plants.
• 9. Thinking all disaccharides are equally sweet – Sucrose is sweeter than maltose and lactose.
• 10. Forgetting the roles of disaccharides – Sucrose = transport; Maltose = intermediate in starch breakdown; Lactose = energy for infants.
• 11. Confusing the enzymes that hydrolyze disaccharides – Sucrase → sucrose; Maltase → maltose; Lactase → lactose.
• 12. Thinking disaccharides are only found in plants – Lactose is found in milk (animals); maltose is found in germinating seeds (plants); sucrose is found in plants.

Quick Revision

• Disaccharides: Two monosaccharides combine to form a disaccharide. It is a kind of oligosaccharide.
• Disaccharides are less sweet in taste and comparatively less soluble in water.
• These can be hydrolyzed to give monosaccharides.
• Disaccharides are familiar energy source for living organisms.
• Examples include maltose, lactose, and sucrose.
• The general formula of disaccharide is C₁₂H₂₂O₁₁.
• Sucrose:
• Commonly known as cane sugar.
• Widely used as sweetener at homes for making sweet dishes.
• In plants sucrose is also called transport disaccharide as prepared food in plants is transported in the form of sucrose.
• It is very soluble and can therefore be moved efficiently in high concentration in plants.
• It is also relatively unreactive chemically.
• Upon hydrolysis sucrose yields one molecule of α-glucose and one molecule of β-fructose.
• Maltose:
• Commonly known as malt sugar.
• It is an intermediate disaccharide produced during the breakdown of starch and glycogen and act as energy producing molecule.
• Maltose is produced in brewing industry by malting process in which barley starch is catabolized using enzyme amylase.
• Maltose is generally found in germinating seeds and sprouting potatoes where it act as energy source for growth and development of young seedlings.
• Lactose:
• Commonly known as milk sugar as it is found in milk of mammals.
• Ratio of lactose in different mammals is variable e.g., 3–4% in goat milk, 4–6% in cow milk, 5–8% in human milk.
• It is also produced as by-product during cheese production process.
• Lactose is used as direct energy source by infants.
• Lactose act as substrate for synthesis of various macro-biomolecules which are involved in vital nervous and immunological processes.
• Glycosidic Bonds in Disaccharides:
• A glycoside is basically a ring chain hexose (6C) or pentose (5C) sugar molecule that is attached usually to another sugar molecule.
• Covalent chemical bonds between two glycosides that hold them together is called glycosidic bond.
• Glycosidic bond is formed due to condensation reaction between two sugar molecules mostly hexoses.
• For the formation of glycosidic bond, the H-group from one ringed sugar interacts with the -OH group of another sugar and one water molecule is released.
• Glycosidic linkage is present in disaccharides, oligosaccharides, and polysaccharides.
• Glycosidic bonds are named on the basis of carbon number of interacting carbons of both linked sugars.
• Glycosidic linkage is also named alpha or beta on the basis of alpha or beta type of first sugar involved in linkage.
• e.g., α-1,4 glycosidic linkage is formed when first sugar is alpha, donates -OH group from its C-1 and other sugar donates H-group from its fourth carbon.
• Glycosidic linkage in sucrose:
• The sucrose is formed by the condensation of α-glucose and β-fructose.
• In this reaction, the -OH group at C-1 of α-glucose interacts with the H of -OH group at C-2 of β-fructose, releasing a water molecule forming α-1,2-glycosidic linkage or α-1, β-2-glycosidic linkage.
• Glycosidic linkage in maltose:
• Maltose is formed by the condensation of two molecules of α-glucose.
• In this reaction, the -OH group at C-1 of first α-glucose interacts with the H of -OH group at C-4 of other α-glucose, releasing a water molecule forming α-1,4-glycosidic linkage or α-1, α-4-glycosidic linkage.
• Glycosidic linkage in lactose:
• The lactose is formed by the condensation of β-galactose and β-glucose.
• In this reaction, the -OH group at C-1 of β-galactose interacts with the H of -OH group at C-4 of β-glucose, releasing a water molecule forming β-1,4-glycosidic linkage or β-1, β-4-glycosidic linkage.

60-Second Revision

• Disaccharides = two monosaccharides joined together
• General formula = C₁₂H₂₂O₁₁
• Less sweet and less soluble than monosaccharides
• Can be hydrolyzed to monosaccharides
• Examples:
• Sucrose = Glucose + Fructose (α-1,2)
• Maltose = Glucose + Glucose (α-1,4)
• Lactose = Galactose + Glucose (β-1,4)
• Sucrose = transport disaccharide in plants
• Maltose = intermediate in starch breakdown
• Lactose = energy source for infants
• Glycosidic bond = covalent bond between two sugars
• Formation = condensation reaction; water released
• Warning: Disaccharides are less sweet and less soluble than monosaccharides. Sucrose, maltose, and lactose have different compositions and glycosidic bonds. Sucrose is a non-reducing sugar. Lactose is found in milk, maltose in germinating seeds, and sucrose in plants.