A disaccharide (also called a double sugar or ''biose'') is the
sugar
Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or do ...
formed when two
monosaccharide
Monosaccharides (from Greek '' monos'': single, '' sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built.
They are usually colorless, water- so ...
s are joined by
glycosidic linkage. Like monosaccharides, disaccharides are simple sugars
soluble in water. Three common examples are
sucrose
Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula .
For human consumption, sucrose is extracted and refine ...
,
lactose
Lactose is a disaccharide sugar synthesized by galactose and glucose subunits and has the molecular formula C12H22O11. Lactose makes up around 2–8% of milk (by mass). The name comes from ' (gen. '), the Latin word for milk, plus the suffix ' ...
, and
maltose
}
Maltose ( or ), also known as maltobiose or malt sugar, is a disaccharide formed from two units of glucose joined with an α(1→4) bond. In the isomer isomaltose, the two glucose molecules are joined with an α(1→6) bond. Maltose is the tw ...
.
Disaccharides are one of the four chemical groupings of
carbohydrate
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may o ...
s (
monosaccharide
Monosaccharides (from Greek '' monos'': single, '' sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built.
They are usually colorless, water- so ...
s, disaccharides,
oligosaccharide
An oligosaccharide (/ˌɑlɪgoʊˈsækəˌɹaɪd/; from the Greek ὀλίγος ''olígos'', "a few", and σάκχαρ ''sácchar'', "sugar") is a saccharide polymer containing a small number (typically two to ten) of monosaccharides (simple sug ...
s, and
polysaccharide
Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with w ...
s). The most common types of disaccharides—sucrose, lactose, and maltose—have 12
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...
atoms, with the general formula C
12H
22O
11. The differences in these disaccharides are due to atomic arrangements within the molecule.
The joining of monosaccharides into a double sugar happens by a
condensation reaction, which involves the elimination of a water molecule from the
functional group
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the r ...
s only. Breaking apart a double sugar into its two monosaccharides is accomplished by
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolysi ...
with the help of a type of
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
called a
disaccharidase. As building the larger sugar ejects a water molecule, breaking it down consumes a water molecule. These reactions are vital in
metabolism
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run ...
. Each disaccharide is broken down with the help of a corresponding disaccharidase (
sucrase,
lactase, and
maltase).
Classification
There are two functionally different classes of disaccharides:
*Reducing disaccharides, in which one monosaccharide, the
reducing sugar of the pair, still has a free
hemiacetal
A hemiacetal or a hemiketal has the general formula R1R2C(OH)OR, where R1 or R2 is hydrogen or an organic substituent. They generally result from the addition of an alcohol to an aldehyde or a ketone, although the latter are sometimes called hemi ...
unit that can perform as a reducing
aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl gro ...
group;
lactose
Lactose is a disaccharide sugar synthesized by galactose and glucose subunits and has the molecular formula C12H22O11. Lactose makes up around 2–8% of milk (by mass). The name comes from ' (gen. '), the Latin word for milk, plus the suffix ' ...
,
maltose
}
Maltose ( or ), also known as maltobiose or malt sugar, is a disaccharide formed from two units of glucose joined with an α(1→4) bond. In the isomer isomaltose, the two glucose molecules are joined with an α(1→6) bond. Maltose is the tw ...
and
cellobiose
Cellobiose is a disaccharide with the formula (C6H7(OH)4O)2O. It is classified as a reducing sugar. In terms of its chemical structure, it is derived from the condensation of a pair of β-glucose molecules forming a β(1→4) bond. It can be h ...
are examples of reducing disaccharides, each with one hemiacetal unit, the other occupied by the
glycosidic bond, which prevents it from acting as a
reducing agent
In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ).
Examples of substances that are commonly reducing agents include the Earth met ...
. They can easily be detected by the Woehlk test or Fearon's test on methylamine.
*Non-reducing disaccharides, in which the component monosaccharides bond through an
acetal
In organic chemistry, an acetal is a functional group with the connectivity . Here, the R groups can be organic fragments (a carbon atom, with arbitrary other atoms attached to that) or hydrogen, while the R' groups must be organic fragments n ...
linkage between their
anomer
In carbohydrate chemistry, a pair of anomers () is a pair of near-identical stereoisomers that differ at only the anomeric carbon, the carbon that bears the aldehyde or ketone functional group in the sugar's open-chain form. However, in order f ...
ic centers. This results in neither monosaccharide being left with a hemiacetal unit that is free to act as a reducing agent.
Sucrose
Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula .
For human consumption, sucrose is extracted and refine ...
and
trehalose are examples of non-reducing disaccharides because their glycosidic bond is between their respective hemiacetal carbon atoms. The reduced chemical reactivity of the non-reducing sugars in comparison to reducing sugars, may be an advantage where stability in storage is important.
Formation
The formation of a disaccharide molecule from two
monosaccharide
Monosaccharides (from Greek '' monos'': single, '' sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built.
They are usually colorless, water- so ...
molecules proceeds by displacing a
hydroxy group from one molecule and a
hydrogen nucleus (a
proton
A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
) from the other, so that the now vacant bonds on the monosaccharides join the two
monomer
In chemistry, a monomer ( ; '' mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization.
Classification
...
s together. Because of the removal of the water molecule from the product, the term of convenience for such a process is "
dehydration reaction" (also "
condensation reaction" or "
dehydration synthesis"). For example, milk sugar (lactose) is a disaccharide made by condensation of one molecule of each of the monosaccharides
glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...
and
galactose, whereas the disaccharide sucrose in sugar cane and sugar beet, is a condensation product of glucose and
fructose
Fructose, or fruit sugar, is a ketonic simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorb ...
.
Maltose
}
Maltose ( or ), also known as maltobiose or malt sugar, is a disaccharide formed from two units of glucose joined with an α(1→4) bond. In the isomer isomaltose, the two glucose molecules are joined with an α(1→6) bond. Maltose is the tw ...
, another common disaccharide, is condensed from two glucose molecules.
The dehydration reaction that bonds monosaccharides into disaccharides (and also bonds monosaccharides into more complex
polysaccharide
Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with w ...
s) forms what are called glycosidic bonds.
Properties
The glycosidic bond can be formed between any hydroxy group on the component monosaccharide. So, even if both component sugars are the same (e.g., glucose), different bond combinations (regiochemistry) and
stereochemistry
Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms that form the structure of molecules and their manipulation. The study of stereochemistry focuses on the relationships between stereoi ...
(''alpha-'' or ''beta-'') result in disaccharides that are
diastereoisomers with different chemical and physical properties. Depending on the monosaccharide constituents, disaccharides are sometimes crystalline, sometimes water-soluble, and sometimes sweet-tasting and sticky-feeling. Disaccharides can serve as
functional group
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the r ...
s by forming glycosidic bonds with other organic compounds, forming
glycoside
In chemistry, a glycoside is a molecule in which a sugar is bound to another functional group via a glycosidic bond. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. ...
s.
Assimilation
Digestion of disaccharides involves breakdown into monosaccharides.
Common disaccharides
:
Maltose, cellobiose, and chitobiose are
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolysi ...
products of the polysaccharides
starch
Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human die ...
,
cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...
, and
chitin
Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...
, respectively.
Less common disaccharides include:
:
References
External links
*
{{Authority control
Carbohydrate chemistry