CARBOHYDRATES

LEARNING OUTCOME

3.1.2 The digestive system provides an interface with the environment. Digestion involves enzymic hydrolysis producing smaller molecules that can be absorbed and assimilated.

YOU SHOULD KNOW :

That the basic carbohydrate units are monosaccharides

The monosaccharides act as monomers and link to form disaccharides and polysaccharides

The structure of a-glucose. About linking of a-glucose by glycosidic bonds

formed by condensation reactions to form maltose and starch.

Disaccharides, sucrose and lactose. About lactose intolerance. Biochemical tests for reducing sugars, non

reducing sugars and starch

CARBOHYDRATES - SUGARS AND STARCHES C H 2O Hydrates of carbon

Functions Fuel for cell activity food reserve (starch) structural part of DNA and RNA (deoxyribose and ribose)

BUILDING LARGER CARBOHYDRATES

Carbohydrates are made up of chains of individual monomer components joined together to form a polymer.

Single monomer = monosaccharide.

2 monosaccharides joined = disaccharide.

3 or more = polysaccharide.

MONOSACCHARIDES - SIMPLE SUGARS

Contain from 3 to 7 carbons Trioses 3 carbons Tetroses 4 carbons Pentoses 5 carbons Hexoses 6 carbons Heptoses 7 carbons

Beta-glucose.

The hydroxyl group on carbon number 1 is UP.

The hydroxyls alternate “up and down” around the molecule.

β-glucose

DISACCHARIDES - 2 MONOSACCHARIDES Glucose + Fructose = Sucrose

Glucose + Galactose = Lactose

Glucose + Glucose = Maltose

The bond between sugars is a

glycosidic bond

POLYSACCHARIDES - A CHAIN OF MONOSACCHARIDES 1. Glycogen- a storage compound in animals.

We store glycogen in liver and muscle cells. The monomer unit is α gluclose.

2. Starch-a storage compound in plants. We eat this in foods such as bread and potatoes. Also chains of α gluclose.

3. Cellulose – makes plant cell walls. When we eat fruit / vegetables we eat this. It forms fibre/roughage in our diet. It consists of chains of β glucose

amylose

Starch - storage form of glucose in plants-amylopectin

GLYCOGEN

3. Cellulose - main structural component of plant cell walls

BENEDICT’S TEST FOR A REDUCING SUGAR.TheoryNeed to know this for practical exam and written exams. All monosaccharides and some disaccharides, (like maltose)

are reducing sugars.Receiving an electron is reduction.A reducing sugar is a sugar that can donate an electron to (or

reduce), another chemical, in this case Benedict’s reagent.Benedict’s contains Copper II sulphate and is alkaline.Cu2+ ions from the copper sulphate are reduced by the –CHO

Aldose or C=O Ketone groups in reducing sugars to form Cu+ ions.

When heated with a reducing sugar pale blue Benedict’s solution forms an insoluble red precipitate of Copper I oxide.

 

TEST FOR NON-REDUCING SUGARS

Some disaccharides such as sucrose are non-reducing sugars. This means they do not change the colour of Benedict’s when heated. To test a non-reducing sugar it must be broken down into its monosaccharide components by hydrolysis. These monosaccharides can then be tested with Benedicts as a reducing sugar.

METHOD

The non-reducing sugar is first hydrolysed by boiling with hydrochloric acid so that it will be broken down into its monosaccharides.

These can then reduce Benedict’s reagent in the normal way.

So a non-reducing sugar is identified by a negative reaction to Benedict’s before hydrolysis and a positive result after hydrolysis.

METHOD

 Carry out the reducing sugar test. If negative then in a boiling tube, add 2cm3 of dilute

hydrochloric acid, to a sample to be tested, mix the solution and heat in a hot water bath for 2-3 mins

 Then add sodium hydroxide, to the boiling tube until

the solution is neutral, Add until the fizzing stops. (This is important because Benedict’s is not effective in acid conditions).

 Carry out the reducing sugar test again. This time it

will be positive