Table of Contents
Overview
Disaccharides are carbohydrates made of two monosaccharide units linked together. They are still “small” sugars, but already show how combining simple building blocks can change sweetness, digestibility, and biological role. Here we focus on their structure, common types, how they are formed and broken down, and why they matter for organisms and human nutrition.
Formation of Disaccharides: Glycosidic Bonds
When two monosaccharides join, they do so via a glycosidic bond.
- The bond forms in a condensation reaction:
two hydroxyl groups (–OH) from the monosaccharides react, a molecule of water is released, and a covalent bond remains. - General scheme:
$$
\text{Monosaccharide}_1\text{–OH} + \text{HO–Monosaccharide}_2
\rightarrow \text{Monosaccharide}_1\text{–O–Monosaccharide}_2 + H_2O
$$ - The bond can be:
- α-glycosidic or β-glycosidic, depending on the configuration of the anomeric carbon in the first sugar.
- Between different carbon atoms, e.g. 1→4 or 1→2.
The notation α-1,4-glycosidic bond means:
- The anomeric carbon (C1) of the first monosaccharide is in the α configuration.
- It is linked to carbon 4 of the second monosaccharide.
Reducing vs. Non‑Reducing Disaccharides
Monosaccharides with a free anomeric carbon (hemiacetal group) can act as reducing sugars. For disaccharides:
- If one anomeric carbon remains free, the disaccharide is a reducing sugar.
- If both anomeric carbons are tied up in the glycosidic bond, the disaccharide is non‑reducing.
This difference influences:
- Their chemical reactivity (e.g. ability to reduce certain test reagents).
- Their behavior in food chemistry (browning reactions, stability).
Important Disaccharides
We will look at three biologically and nutritionally important disaccharides: sucrose, lactose, and maltose. Many others exist (e.g. trehalose, cellobiose), but these three are most relevant in basic biology and everyday life.
Sucrose
Composition and Bond Type
- Monosaccharide units: glucose + fructose
- Bond: α-1,2-glycosidic bond
- The anomeric carbon C1 of α-D-glucose binds to the anomeric carbon C2 of β-D-fructose.
- Because both anomeric carbons are involved, sucrose is a non‑reducing disaccharide.
Occurrence and Function
- Main “table sugar” from sugar cane and sugar beet.
- Widespread in plants:
- A primary transport form of sugar in many plants (moves from leaves to storage organs).
- For humans and many animals:
- Major dietary energy source.
Digestion
- Enzyme: sucrase (also called invertase in some contexts).
- Location: small intestine (in humans).
- Hydrolysis reaction:
$$
\text{sucrose} + H_2O \xrightarrow{\text{sucrase}} \text{glucose} + \text{fructose}
$$ - The released glucose and fructose are then absorbed and metabolized.
Technological and Nutritional Aspects (Brief)
- High sweetness; highly soluble.
- Involved in caries formation by supporting bacterial growth in the mouth.
- Excessive intake is linked to metabolic health issues (e.g. obesity, type 2 diabetes risk), mainly through overall energy surplus.
Lactose
Composition and Bond Type
- Monosaccharide units: galactose + glucose
- Bond: β-1,4-glycosidic bond
- The anomeric carbon C1 of β-D-galactose is linked to C4 of D-glucose.
- One anomeric carbon (of glucose) remains free:
- Lactose is a reducing disaccharide.
Occurrence and Function
- Found almost exclusively in milk of mammals:
- Main carbohydrate in milk.
- Provides energy for suckling young.
- Contributes to the osmotic properties of milk, influencing its water content.
Digestion and Lactose Intolerance
- Enzyme: lactase (β-galactosidase).
- Location: small intestine.
- Hydrolysis:
$$
\text{lactose} + H_2O \xrightarrow{\text{lactase}} \text{glucose} + \text{galactose}
$$ - Lactose intolerance:
- Caused by reduced lactase activity in adults.
- Undigested lactose reaches the large intestine, where bacteria ferment it:
- Leads to gas formation, abdominal pain, diarrhea.
- Prevalence varies among human populations; linked to genetic variants affecting lactase persistence.
Maltose
Composition and Bond Type
- Monosaccharide units: glucose + glucose
- Bond: α-1,4-glycosidic bond
- C1 of α-D-glucose linked to C4 of another D-glucose.
- One anomeric carbon remains free:
- Maltose is a reducing disaccharide.
Occurrence and Function
- Forms during starch breakdown:
- Partial digestion of starch in plants, animals, and in food processing (e.g. malting of barley).
- Present in:
- Germinating seeds.
- Malt products (e.g. malt extract, some beers).
- Acts more as an intermediate in carbohydrate digestion and metabolism than as a major storage or transport sugar.
Digestion
- Enzyme: maltase (α-glucosidase).
- Hydrolysis:
$$
\text{maltose} + H_2O \xrightarrow{\text{maltase}} \text{glucose} + \text{glucose}
$$ - The resulting glucose is readily absorbed and used in cellular respiration or stored as glycogen (in animals) or starch (in plants).
Other Biologically Relevant Disaccharides (Overview)
Without going into the detail reserved for polysaccharides or specialized chapters, it is useful to mention two additional disaccharides that connect to larger carbohydrate structures:
- Cellobiose
- Units: glucose + glucose.
- Bond: β-1,4-glycosidic.
- Derived from partial hydrolysis of cellulose (a structural polysaccharide in plant cell walls).
- Not easily digested by humans due to the β-linkage.
- Trehalose
- Units: glucose + glucose.
- Bond: α,α-1,1-glycosidic (both anomeric carbons are linked).
- A non‑reducing disaccharide.
- Functions as a stress-protective sugar in some organisms (e.g. yeast, insects), helping them tolerate drying or freezing.
These examples show that simply changing how two glucose molecules are connected drastically alters their properties and biological roles.
Hydrolysis of Disaccharides
Disaccharides are broken down by hydrolysis reactions, the reverse of the condensation that formed them:
- General hydrolysis:
$$
\text{Disaccharide} + H_2O \xrightarrow{\text{specific enzyme}}
\text{Monosaccharide}_1 + \text{Monosaccharide}_2
$$ - Each common disaccharide has a specific enzyme:
- Sucrose → sucrase
- Lactose → lactase
- Maltose → maltase
These reactions:
- Occur under relatively mild conditions in living organisms.
- Are highly specific (each enzyme recognizes a particular bond type and arrangement).
Biological and Nutritional Significance
Disaccharides occupy a middle position between monosaccharides and polysaccharides:
- Energy supply
- Provide readily available energy once hydrolyzed to monosaccharides.
- Sucrose and lactose are especially important in human diets.
- Transport and storage
- Sucrose is a major transport sugar in plants.
- Others act as intermediates during the mobilization of storage polysaccharides (e.g. maltose from starch).
- Regulation and adaptation
- The ability (or inability) to digest certain disaccharides shapes:
- Dietary specialization in animals.
- Nutritional tolerance and intolerance in humans (e.g. lactose).
Disaccharides thus illustrate how combining the same small building blocks (like glucose) in different ways creates molecules with distinct properties, functions, and biological implications.