Biomolecule

Carbohydrates

What are the three basic necessities of any human being? Food, shelter and clothing? And did you know that carbohydrates are the source of all three? It will be safe to say that without carbohydrates life as we known it would not exist on planet earth. Now let us learn a bit more about the chemical properties of carbohydrates.

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Carbohydrates

The term carbohydrate is itself a combination of the “hydrates of carbon”. They are also known as “Saccharides” which is a derivation of the Greek word “Sakcharon” meaning sugar. The definition of carbohydrates in chemistry is as follows:

“Optically active polyhydroxy aldehydes or polyhydroxy ketones or substances which give these on hydrolysis are termed as carbohydrates”.

Some of the most common carbohydrates that we come across in our daily lives are in form of sugars. These sugars can be in form of Glucose, Sucrose, Fructose, Cellulose, Maltose etc.

General Formula of Carbohydrate

Carbohydrates

The general formula for carbohydrate is Cx(H2O)y. Although, it must be remembered that this is just a general formula. There are various exceptions to this that we will see. Let us take a look at Acetic Acid which is CH3COOH. Now although this will fit in the general formula of carbohydrate i.e. Cx(H2O)y, we know that acetic acid is not a carbohydrate.

Formaldehyde (HCHO) also falls under this category of this general formula but is also not a carbohydrate. And on the other hand, Rhamnose (C6H12O6) which is very much a carbohydrate but does not follow the general formula.

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Classification of Carbohydrates

The main classification of carbohydrate is done on the basis of hydrolysis. This classification is as follow:

  1. Monosaccharides: These are the simplest form of carbohydrate that cannot be hydrolyzed any further. They have the general formula of (CH2O)n. Some common examples are glucose, Ribose etc.
  2. Oligosaccharides: Carbohydrates that on hydrolysis yield two to ten smaller units or monosaccharides are oligosaccharides.  They are a large category and further divides into various subcategories.
  3. Disaccharides: A further classification of oligosaccharides, these give two units of the same or different monosaccharides on hydrolysis. For example, sucrose on hydrolysis gives one molecule of glucose and fructose each. Whereas maltose on hydrolysis gives two molecules of only glucose,
  4. Trisaccharides: Carbohydrates that on hydrolysis gives three molecules of monosaccharides, whether same or different. An example is Raffinose.
  5. Tetrasaccharides: And as the name suggests this carbohydrate on hydrolysis give four molecules of monosaccharides. Stachyose is an example.
  6. Polysaccharides: The final category of carbohydrates. These give a large number of monosaccharides when they undergo hydrolysis, These carbohydrates are not sweet in taste and are also known as non-sugars. Some common examples are starch, glycogen etc.

Importance of Carbohydrates

Now as we previously discussed, carbohydrates are absolutely essential for life on the planet. Let us take a more detailed look at the importance of carbohydrates.

  • Carbohydrates are responsible for storing chemical energy in living organisms. You must hear all the time when athletes carbo-load before a game. This is so they can provide themselves with extra energy. They are also an important constituent for supporting tissues in plants and even in some animals.
  • As I am sure you are already aware of photosynthesis. It is the process by which plants utilize solar energy to generate energy for themselves and food for us. Through this process, plants fix CO2 and synthesize carbohydrate. Let us take a look at the chemical reaction occurring during photosynthesis.

x(CO2) + y(H2O) + Solar energy ⇒ Cx (H2O)y + O2

  • So carbohydrates due to photosynthesis are the repository of solar energy in plants, Then when plants or animals metabolize the said carbohydrate this energy releases. The metabolizing equation is just the reverse of the photosynthesis equation

Cx (H2O)y + O2 ⇒ x(CO2) + y(H2O) + Energy

Solved Example for You

Q: Which one of the following is an example of non-reducing sugar?

  1. Sucrose
  2. Lactose
  3. Maltose
  4. Cellobiose

Ans: The correct answer is “A”. Sucrose is not a reducing sugar because it lacks the ability to form either aldehyde or a ketone in a basic solution. Sucrose’s anomeric carbon is not free since this carbon is used to link fructose and glucose together. Therefore it cannot open up the ring structure and react with the reagent.

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