Polysaccharides are polymers containing many monosaccharides linked by glycosidic bonds. Like disaccharides, polysaccharides are formed by condensation reactions.
As polysaccharides are very large molecules, they are insoluble and so are suitable for storage.
When hydrolysed, they break down into disaccharides or monosaccharides.
The main polysaccharides are starch and cellulose in plants and glycogen in animals.
Cellulose in plants is used for structural support for the cells rather than storage.
Starch is made up of two compounds: Amylose and Amylopectin.
- glucose joined with 1-4 linkages
- chains curve to form spiral structures held in place by hydrogen bonds.
- glucose with 1-4 linkages and 1-6 branches
- shorter chains
It is an effecient energy storage molecule:
-amylose and amylopectin are compact
-insoluble and remains in one place
-easily broken down into glucose
In carbohydrates, the basic monomer unit is a sugar, otherwise known as a saccharide. A single monomer is therefore called a monosaccharide.
Monosaccharides are sweet-tasting, soluble substances that have the general formula (CH2O)n , where n can be any number from 3-7.
Perhaps the best known monosaccharide is glucose. This molecule is a hexose(6-carbon) sugar and has the formula C6H12O6 . However, the atoms or cabon, hydrogen and oxygen can be arranged in many different ways.
When combined in pairs, monosaccharides form a disaccharide. For example:
- glucose linked to glucose forms maltose.
- glucose linked to fructose forms sucrose.
- glucose linked to galactose forms lactose.
When the monosaccharides join, a molecule of water is removed and the reaction is therefore called a condensation reaction. This reaction forms a glycosidic bond.
When water is added to a disaccharide under suitable conditions, it breaks the glycosidic bond releasing the constituent monosaccharides. This is called hydrolysis.
The enzyme amylase is produced in the mouth and the pancreas. It hydroylyses the alternate glycosidic bonds of the starch molecules to produce the disaccharide maltose. The maltose is then hyrdrolysed by maltase (formed in the lining of the intestine) to make glucose.
- Food is chewed by teeth in the mouth, breaks into pieces = larger surface area.
- saliva containing salivary amylase enters the mouth and mixes with the food to start hydrolysing any starch to form maltose.
- food is swalloed and enters the stomach, the acidic conditions denature the amylase and prevents further hydrolysis.
- food passed into small intestine and mixed with pancreatic juice containing pancreatic amylase. this continues the hydrolysis os starch into maltose
- muscles in the intestine push the food along as its epithelial lining produces maltase to hydrolyse the maltose from the starch breakdown into glucose.
In addition to the digestion of maltose, our bodies must also break down two other common disaccharides.
- sucrose is usually contained within the cells and must be physically broken down by the teeth in order to release it
- passes through the stomach and the small intestine where sucrase is prodcued (in the epithelial lining)
- sucrase hydrolyses the single glycosidic bond to produce glucose and fructose.
- the sugar found in milk
- digested in the small intestine, whose epithelial lining produces the enzyme lactase
- lactase hydrolyses the glycosidic bond to form glucose and galactose.
Test for reducing sugars
- All monosaccharides and some disaccharides are reducing sugars. When a reducing sugar is heated with Benedict's reagent it forms and insoluble red precipitate of copper oxide. Add equal quantities of food and Benedict's and heat in boiling water for five minutes.
Test for non-reducing sugars
- First test as if it's a reducing sugar. When you confirm the negative results, add equal amounts of food and hydrochloric acid and heat (this will hydroylse any disaccharides). Then add sodium hydrogencarbonate to neutralise acid and re-test the resulting solution with Benedict's.
- Results vary from strenth from a low of reducing sugar present to a high concentration. (blue to green to yellow to orange to red)
Test for starch
- Add iodine to food sample, blue-black colour=positive reaction.