Summary

• The three types of carbohydrates are monosaccharides, disaccharides and polysaccharides
• Monosaccharides are the simplest sugars e.g. glucose. fructose and galactose
• Disaccharides are formed by condensation where there is linking of two monosaccharides together
• Disaccharides can be broken down to monosaccharides via hydrolysis
• Polysaccharides, also known as polymers contain three or more monosaccharides
• Polysaccharides; starch, glycogen and cellulose are important for storing energy and for providing support and protection for cells and whole organisms

General

Carbohydrates are within the four major classes of biomolecules along with proteins, lipids and nucleic acids. They are biological, organic macromolecules that include sugars, starches and fibres. They are made up of smaller molecules called monomers and are symbolized by the formula (CH₂O)_n.

From the formula it shows carbohydrates contain three elements; carbon, nitrogen and oxygen, a number of hydroxyl groups (-OH) and a carbonyl group (C=O).

• First carbohydrates serve useful functions within a cell for example are a source and storage of energy to carry out various processes. Carbohydrates are considered to be the fastest and most basic way to obtain energy. Sugars is what we eat to get our energy from and these carbohydrates can be found in pasta, rice and bread for example and ribose.
• Second, deoxyribose and ribose sugars form the main structural framework of DNA and RNA.
• Third, carbohydrate polysaccharides play an important structural role within a cell. For example a plant cell has an important feature known as the cell wall which provides strength and rigidity to the plant cell.

Classification of Carbohydrates

Monosaccharides

The simplest form of carbohydrates are called monosaccharides which are “soluble, sweet-tasting” sugars. They are the monomer building blocks that join together for more complex carbohydrates. “Mono” refers to one and “saccharide” refers to sugars. They all have the formula of (CH₂O)_n, where n can be a number between 3 and 7. The molecular formulae for each type of sugar can there be worked out using the general formula (CH₂O)_n.

Within biological molecules individual units are known as monomers and monomers joined together in chains are known as polymers.

The most common monosaccharide is known as glucose, a six-carbon sugar that has a formula of n=6; C₆H₁₂O₆. In eukaryotic cells glucose plays an important role in the transport of sugars within the blood and is the main energy source in respiration. Glucose is in a ring form and has 2 isomers called α-glucose and beta-glucose and differ by the position of the hydroxyl (-OH) group (Figure 2). α-glucose is when the hydroxyl group is present below carbon number 1 in the sugar molecule and beta-glucose is where the hydroxyl group is present below the carbon atom.

An isomer are molecules that have the same chemical formula but a different arrangement of atoms in space

Disaccharides

When two monosaccharides are combined in pairs, a disaccharide if formed. Table 1 shows the combinations of the two monosaccharides that can either be the same or different. It is important to remember these simple condensation reactions summarised in the Table 1:

Disaccharides	Component Monosaccharides	Role
Maltose	α-glucose and α-glucose	Energy source in germinating seeds
Sucrose	α-glucose and fructose	Transport in the phloem
Lactose	α-glucose and galactose	Energy source in milk

Table 1: shows the different types of disaccharides, the component monosaccharides that are present and the role each disaccharide plays

Glyosidic bonding – condensation (polymerisation)

When a polymer is formed from a monomer a condensation reaction occurs that forms a glyosidic bond. A condensation reaction involves the formation of water (H₂O), as one oxygen atom and two hydrogen atoms are removed from the monosaccharides. This covalent bond joins the monosaccharide together to form a disaccharide. The nomenclature of glyosidic bonds depends on which carbons atoms the bond being formed is between. An important rule of thumb is to name the carbons on each  sugar in an 3’ o clock orientation. The glyosidic bond is connects two carbons together  formed; number 1 carbon from α-glucose is connected to number 4 carbon. Using disaccharide, maltose as an example the glyosidic bond is formed between carbon 1 and carbon 4 and is called 1,4 glyosidic bond.

Glyosidic bonding – hydrolysis (breakdown)

The breakdown of disaccharides are broken down through a hydrolysis reaction to form two monosaccharides. This is the reverse of the condensation reaction and a hydrolysis reaction requires water (H2O). The chemistry will change again by the addition of water to the disaccharide and breaks the glyosidic bond to form two monosaccharides.

Roles of monosaccharides and disaccharides

Both the monosaccharides and disaccharides can function as substrates for respiration that are broken down to produce ATP energy. What makes them useful is the large number of C-H backbone groups that can be easily oxidised, which is able to yield lots of energy.

Tests for reducing and non-reducing sugars

Monosaccharides and disaccharides are both reducing sugars. The test to test for reducing sugars is known as the Benedict’s test. An example of a reducing sugar is maltose and a non-reducing sugar example is the disaccharide sucrose. The overall process of the test is as follows:

A sample is heated with Benedict’s reagent; an alkaline solution of copper sulfate, if the solution remains blue there is no reducing sugar present
If the alkaline solution forms an insoluble precipitate of copper oxide and the solution changes from green to yellow, orange and brown to deep red. The difference in the colour intensity depends on the quantity of the amount of reducing sugar is present in the sample being tested.

A reducing sugar is a sugar within its structure and has a free aldehyde or ketone body that has the capability of acting as a reducing agent.

A non-reducing agent has no aldehyde or ketone and cannot act as a reducing agent

Polysaccharides

Read more about the Properties of Polysaccharides

Polysaccharides also known as polymers, are long complex chains of many monosaccharides that are joined together by glyosidic bonds. They are formed by a series of condensation reaction and yield more than 10 molecules monosaccharides on hydrolysis. Their properties are summarized in Table 2. Their general formula is (C₆H₁₀O₅)_n. Unlike both monosaccharides and disaccharides,  polysaccharides are insoluble and not sugars. They are very large molecules (macromolecules) and the feature of them being insoluble makes them suited for storage.

Starch, glycogen and cellulose are examples of polysaccharides (Table 3).

• Starch is the polysaccharide found in many parts of the plant cell and form granulates and is mixture of two substances, amylose and amylopectin.
• Glycogen is the major carbohydrate storage product found in humans and are stored in smaller granules mainly found in the liver and muscles.
• Cellulose is a polysaccharide is known as the most abundant organic molecule on earth, it makes up around 50% of all organic carbon. The structure differs from starch and glycogen and is made up of beta-glucose rather than α-glucose.

Properties of Polysaccharides	Importance for Energy Storage
Large molecule	Cannot diffuse out of the cell
Insoluble molecule	Does not affect the osmotic balance of the cell
Compact	Lots of energy stored in little space
Easily broken down	Allows for each accessible energy

Table 2: Summary of Polysaccharides

Characteristic	Amylose	Amylopectin	Glycogen	Cellulose
Found in	Plants	Plants	Animals and Fungi	Plants
Found as	Grains	Grains	Tiny granules	Fibres
Function	Energy store	Energy store	Energy store	Structural support
Basic monomer unit	α-glucose	α-glucose	α-glucose	-glucose
Type of bond between monomer unit	1,4 glyosidic	1,4 and 1,6 glyosidic	1,4 and 1,6 glyosidic	1,4 6 glyosidic
Type of chain	Unbranched Helical chains	Branched Less branched than glycogen	Short and highly branched	Long, unbranched straight chains No coils

Table 3: Comparison of cellulose with other polysaccharides amylose, amylopectin and glycogen

Frequently Asked Questions

What are monosaccharides?

Monosaccharides are the simplest carbohydrates. They are hydrated carbon compounds having a simple structure. They are sweet in taste and soluble in water. Examples include glucose, fructose, ribose, etc. 

What are oligosaccharides?

These are the carbohydrates that consist of two to nine monosaccharide molecules. Monosaccharides are linked together via glycosidic bonds to form oligosaccharides. These include disaccharides, trisaccharides, etc. 

How do you define disaccharides?

These are oligosaccharides that are made up of two monosaccharide molecules joined together via a glycosidic bond. The most important disaccharides are lactose, sucrose, and maltose. 

What do you know about polysaccharides?

These are polymeric carbohydrates made up of hundreds or thousands of monosaccharide residues linked together via glycosidic bonds. They have a complex structure and are insoluble in water. Examples include starch, glycogen and cellulose. 

References

[1]. https://openstax.org/books/biology-ap-courses/pages/3-2-carbohydrates