Carbs, you either love them or hate them, but they are pretty vital for body function. Primarily they provide energy in the form of glucose.
Carbohydrates come in a few different forms, many of which all have slightly different functions. The main structures to be aware of are monosaccharides, disaccharides and polysaccharides.
All of these carbohydrates, for all their differences, have one key thing or element in common: they are all formed from the elements carbon, hydrogen and oxygen.
They’re all made up of sugar units and simply; monosaccharides consist of one single unit, disaccharides two and polysaccharides are made of many units!
Let’s first tackle the monosaccharide grouping and how they are used and made.
Carbohydrates are polymers made of monosaccharides. Monosaccharides or monomers are joined together by a condensation reaction. In the instance of a condensation reaction, water is removed. When they are joined they form a disaccharide, because there is now more than one unit of sugar.
The bond that is made from joining two monosaccharides together is a glycosidic bond. A commonly known monosaccharide is glucose.
Starch is also made from monosaccharides. Starch and glycogen are made of chains of alpha glucose molecules, all joined together by glycosidic bonds.
So far, so good!
In the case of disaccharides, just remember that via a condensation reaction, disaccharides can be made. For instance maltose is formed from two alpha glucose molecules, making it a disaccharide, and as it was before, a glycosidic bond joins two glucoses together.
Like disaccharides, polysaccharides consist of monomer units linked by a glycosidic bond. However, remembering the big difference: Instead of just two monomer units joining together (making a disaccharide), polysaccharides are made up of many different units.
Chains of these units are known as polymers and these larger molecules have important structural and storage roles.
For example, starch is stored in organisms as a future energy source. This is seen in potatoes which have a high starch content to supply the buds with energy to grow.
Amylose is the unbranched form of starch. Both glycogen and amylopectin, although formed from alpha glucose, are branched molecules. This is unlike cellulose which is formed from beta glucose!
Glycogen is also stored in the liver, which releases glucose for energy in times of low blood sugar. This makes it the main energy source of animals.
It is made from many alpha glucose molecules joined together by a condensation reaction, forming a glycosidic bond between each monomer – but you could have probably guessed that much by now.
In order to obtain glucose from glycogen, a hydrolysis reaction must take place. Hydrolysis is a reaction where water is added resulting in the breaking of a bond between two molecules.
Both starch and glycogen are insoluble in water which allows them to remain inside cells. Cellulose, which is made of beta glucose molecules and has a long unbranched structure, is found in the cell wall of a plant.
Chains of cellulose are held together by many hydrogen bonds to form strong fibres called microfibrils.
That’s all the hard stuff over, just one last thing to get your head around – testing for substances!
There are two main tests that are worth knowing, thankfully they’re both pretty straight forward:
When testing for a reducing sugar such as glucose, you would perform the benedict’s test. This involves adding benedict’s reagent and heating. If the test is positive, a red precipitate will form. If negative the solution will remain blue.
When testing to see if starch is present you would perform an iodine test. If starch is in the solution the sample will change colour from orange/brown to blue/black in colour. If there is no starch present, the sample will remain orange/brown.
And that’s it, carbs complete!