Biological Molecules- Carbohydrates

  • Created by: DonaJ2002
  • Created on: 18-12-18 20:34

Carbohydrates= Polymers made of long chains of monsaccharides which are monomers.

Monosaccharides and disaccharides are sugars that are monomers which join via condensation reactions and form disaccharides. The condensation between 2 monosaccharides releases H2O and forms a glycosidic bond.

Examples of Monosaccharides are:

1) Galactose

2) Fructose

3) Glucose

Glucose is a hexose sugar, which means a monosaccharide with 6 carbon atoms. There are 2 types of glucose: 

Alpha and Beta glucose which are isomers (molecule with same molecular formula but different structure). They have the same molecular formula but the H and OH at the end are arranged differently.

Image result for alpha and beta glucose

Disaccharides are formed by the condensation reaction od 2 monosaccharides that form a glycosidic bond:

Glucose+Glucose => Maltose

Fructose+Glucose => Sucrose

Galactose+Glucose => Lactose


Benedict's Test for Sugars (Carbohydrates):

Sugars are classified as reducing or non-reducing.

Testing for Reducing sugars- Monosaccharides and some Disaccharides

1) Prepare food sample with H2O, Mortar, Pestle and add 3cm(cubed) to a test tube.

2) Add 2-3 drops of Benedict's reagent to the test tubeand put in a water bath that's been brought to boil.

3) + result= form coloured precipitate(solid particles suspended into the solution) Green to Yellow to Orange to Brick Red. - result= stays blue and means there is Non- reducing present. 

Higher the concentration of reducing sugar, the further the colour change= compare the amount of reducing sugars in different solutions. 

Filter the solution and weigh precipitate/ remove precipitate and use colorimeter to measure absorbance of remaining Benedict's reagent, is an accurate way.

Non-reducing sugar test- break them down into their monosaccharides.

1) Do Reducing and if it's - result, then there is non-reducing.

1) Get new sample and add 3cm(cubed) dilute HCl and gently heat in water bath.

2) Neutralise with sodium hydrogencarbonate.

3) Do reducing sugar test with Benedict's reagent- + result= Green, Yellow, Orange, Brick Red. - result= stays Blue (No reducing/non-reducing)

Polysaccharides are formed when more than 2 monosaccharides join via condensation reaction.

Glycogen and starch are formed by the condensation of many alpha glucose units.

Cellulose is formed by the condensation of many beta glucose units.

Animals get energy from glucose. Excess glucose in animals is stored as glycogen. Glycogen is a polysaccharide of alpha glucose. It's long, many side branches that allow enzymes to break glycosidic bonds and release glucose, when it's needed. It has more side branches than amylopectin so, it can release the stored glucose quicker. It's compact= good for storage.

Cells get energy from glucose, plants store excess glucose as starch. Strach is insoluble in water so, it doesn't affect the water potential and doesn't let water enter the cells by osmosis and turn turgid. This makes it good for storage. It's a mixture of 2 alpha glucose's polysaccharides called amylose and amylopectin:

Amylose is long and unbranched chain of alpha glucose, glycosidic bonds' angles give it a coiled structure therefore, glucose can be stored into small space.

Amylopectin has similar structure to glycogen but has less number of side branches.


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