Biochemistry of Carbohydrates

Biochemistry of Carbohydrates

In this article I talk about Biochemistry of Carbohydrates.

Biochemistry of Carbohydrates

Carbohydrates are the most abundant biomolecules on earth, they are found everywhere. Carbohydrates are a major source of energy for all living organisms such as animals and plants, But they are not only important for energy but also serve as important structural components for example DNA contained the carbohydrate.

Ribose and the plant cell wall are made up of the carbohydrates cellulose.

Carbohydrates mainly contain carbon hydrogen and oxygen atoms in a molar ratio of (1:2:1) one carbon two hydrogen’s and one oxygen.

Carbohydrates can be divided into four types. these are monosaccharides, disaccharides, oligosaccharides and polysaccharides. The word ( saccharide ) is derived from the Greek word for sugar.

Now let us look at each of these types of carbohydrates and learn a bit more about their structure and how they are formed.

this is a biochemistry lesson let’s begin with monosaccharides.


Monosaccharides are also referred to as simple sugars and they are the smallest units that make up any carbohydrate. they are the building blocks the three main monosaccharides in the human diet include glucose, galactose and fructose.

Biochemistry of CarbohydratesBiochemistry of Carbohydrates

These structures may look intermidating but all you need to know is that they contain carbons hydrogen’s and oxygens.


Glucose molecule is the main source of energy for humans. you can see glucose in its cyclic chemical form what’s important to know about glucose is that it contains six carbon atoms. A carbon atom – we can label these carbon atoms of glucose with numbers one to six in this specific order – Β So glucose has six carbon atoms. This particular type of glucose is actually an alpha glucose because it has an alpha configuration ( alpha carbohydrates ) is where the hydroxyl group the OH group of carbon number ( one is pointing in the opposite direction to the carbon number six ) so these are opposite each other.

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There is another type of glucose known as a beta glucose. this is essentially where the hydroxyl group of carbon number one and carbon number six are pointing in the same direction.

Biochemistry of CarbohydratesBiochemistry of Carbohydrates

Two carbohydrates where the hydroxyl group of carbon number one is pointing in the same direction as carbon number six

These alpha and beta carbohydrates also apply to other types of carbohydrates such as galactose as well as fructose, So for example1- galactose molecule is actually a beta galactose because the hydroxyl group and the carbon number 6 is pointing in the same direction.

2- fructose molecule is actually in a beta configuration so it’s a bead of fructose because the hydroxyl group is pointing in the same direction as carbon number six.

so I hope you understood the structure of the three major monosaccharides in the human diet.


Disaccharides are made up of two monosaccharides for example a glucose molecule and another glucose molecule. They can form a bond with each other to form disaccharide is called maltose.


maltose is essentially two glucose molecules linked together. It is linked together by an alpha (1-4) glycosidic bond because carbon number 1 of glucose and carbon number 4 of other are involved in the linking process and it’s called alpha because both these glucose molecules are in an alpha configuration.

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another process of linking monosaccharides with one another is called condensation and here water is released therefore the reverse reaction is hydrolysis and this is where we add water.

Biochemistry of CarbohydratesBiochemistry of Carbohydrates

Adding water to an alpha glycosidic bond will break the bond, So maltose is only one example of a disaccharide.

Let’s look at some other common examples galactose molecule can link with a glucose molecule, So this particular glucose molecule is actually in a beta configuration because the hydroxyl group in the same direction as carbon number 6, So galactose molecule and the glucose molecule can form a link and through the condensation process which it will form lactose.


Lactose is made up of galactose and glucose which are linked together by a beta 1 to 4 glycosidic bond, Because the galactose and glucose are in a beta configuration and also carbon number 1 and carbon number 4 are involved

The reverse reaction to break lactose requires hydrolysis by adding water.

Lactose as you all probably know is found naturally in milk.


The third type of disaccharide I want to talk about is where we form a bond between one glucose molecule and one fructose. This glucose molecule is an alpha glucose because Β the hydroxyl and carbon number six are pointing the opposite direction, So glucose and fructose can form a link and through the condensation reaction by removing of water to form a disaccharide called sucrose.

Sucrose molecule is made up of one glucose molecule and one fructose molecule. The bond between the glucose and fructose is a little more complicated as it is our glucose alpha one and fructose beta two bond, So the glucose is in an alpha configuration and the fructose is in a bead of configuration and it’s carbon number one of glucose and carbon number two of fructose that are involved in the linking process.

Sucrose as you all know is table sugar and is formed by plants and not formed by animals so humans cannot form sucrose.

Sucrose is broken down through a hydrolysis reaction.

The disaccharides ( maltose, lactose and sucrose ) are all good examples that we encounter in our normal diet.


All of those saccharides basically consists of short chains of monosaccharides typically less than 20 monosaccharides linked together.

now let’s look at an example of an oligosaccharide so if we were to take maltose and add another glucose molecule to it through a condensation reaction. We can form an additional alpha 1 to 4 glycosidic bond. this oligosaccharide is called maltotriose.

Maltotriose is made up of 3 glucose units and they’re linked together by alpha 1 to 4 glycosidic bonds to break down these bonds requires hydrolysis ( the addition of water ).

The structure can keep growing with the addition of more glucose molecules but when the oligosaccharide eventually exceeds 20 monosaccharides with 20 bonds. the carbohydrate is then referred to as a polysaccharide.


Most carbohydrates found in nature occur as polysaccharides. polysaccharides are also known as glycans.

To simplify things, polysaccharides can be a homo polysaccharide or they can be a hetero polysaccharide A homo polysaccharide means the polysaccharide only contains a single type of monosaccharides. For example, It only contains glucose molecules linked together.

a hetero polysaccharide means that the polysaccharide contains two or more different monosaccharides. For example a long chain of fructose and glucose molecules to make things.

polysaccharide can also be unbranched like what you see here.

or it can be branched. This goes for both homo polysaccharides and hetero polysaccharides.

Hetero polysaccharides can also be unbranched or branched.

Polysaccharide contains thousands are made up of thousands of monosaccharides linked together.

Homo-polysaccharides serve as storage forms of monosaccharides in both humans and plants and even bacteria, So they’re very important.


It is an example of homo polysaccharides. starch is a storage form of monosaccharides in plants. starch is the main carbohydrate in the human diet and are found in our bread cereal and rice. Starch is only made up of glucose because it is a homo polysaccharide.

Starch has regular alpha 1 to 4 glycosidic bond between two glucose molecules and this is because carbon number 1 and carbon number 4 of these glucose molecules are involved in the linking process. however starch has alpha 1 to 6 glycosidic bond between two glucose molecules and that is because carbon number 1 of first glucose and carbon number 6 of other glucose are involved in the linking process so what you take out of this is that starch has two forms. It can be branched like what I just explained or starch can be unbranched.

If starch is unbranched so it is only a chain of glucose linked together by alpha 1 to 4 glycosidic bonds. It is referred to as amylose.

If starch is branched it contains both alpha 1 to 4 and alpha 1 to 6 glycosidic bonds between glucose and therefore it is referred to as amylopectin.

amylose and amylopectin are two forms of glucose polymers.


glycogen Β is the other good example of a polysaccharide.

glycogen is a homo polysaccharide because it is made up of glucose.

glycogen can also be branched or unbranched.

glycogen is a storage form of glucose in animals such as humans.

Starch and glycogen are actually very similar in structure which both are made up of glucose. they can either be branched or unbranched, So both starch and glycogen contain amylose and amylopectin.

The only difference is that glycogen has branch points occurring every eight to twelve glucose residues but in starch these branch points occur every 24 to 30 glucose residues so the branch points occur more frequently in glycogen and starch and this of course will influence the structure in some way.


Another type of polysaccharide is dextran strands. Dextran are structural components in bacteria and yeast.

These polysaccharides are made up of alpha 1 to 3 and alpha 1 to 6 glycosidic bonds, however the dextran can also contain alpha 1 to 2 and alpha 1 to 4 glycosidic bonds.


Finally the other polysaccharide worth mentioning is cellulose. cellulose are structural components in plants they make up the plant cell wall. They are unbranched homo polysaccharide consisting of thousands of glucose molecules, So you can see unbranched cellulose on top of each other in cellulose

The glucose molecules have a beta configuration and therefore the bonds between these glucose molecules are beta bonds.

The glucose molecules are linked together by beta 1 to 4 glycosidic bond.

Humans do not have enzymes that break down that hydrolyzed beta 1 to 4 glycosidic bonds of cellulose, So humans cannot digest cellulose.

Now even though cellulose are only chains of beta glucose so unbranched beta glucose these chains can form hydrogen bonds with each other forming a very strong structure.

47 thoughts on “Biochemistry of Carbohydrates”

  1. Really awesome. Lots of praise to you Professor. I personally tell you this type of teaching is required across the globe for a better understanding of the depth of the course. It will be a fandamental base for newcomers the researchers of next-generation research.

  2. Just a suggestion, when you were explaining why the molecules were alpha and beta…u should have gone into detail about the chiral carbons so that viewers will know why the hydroxide group needs to be directed in that configuration

  3. Also…in maltose the alpha 1, 4 glycosidic bond…the alpha only applies to the first residue because the anomer was not envolved in the second residue….therefore the alpha does not apply to the 4 part in the alpha 1,4 glycodisic bond…it only applies to the first residue since that is where the anomeric carbon is

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