Carbohydrates Flashcards
What is a carbohydrate?
Carbon molecules (carbo-) combined with water (-hydrate).
Why are living organisms based on carbon?
Carbon atoms very readily form bonds with other carbon atoms, this allows a sequence of carbon atoms of various lengths to be built up. These forma ‘backbone’ along which other atoms can be attached.
In carbohydrates what is the basic unit/monomer?
The basic monomer unit is sugar, otherwise known as saccharide.
What are monosaccharides?
A single monomer, they are sweet-tasting, soluble substances that have the general formula (CH’2’O)n. For example, glucose.
What are organic molecules?
Molecules that contain carbon.
What is reduction?
Reduction is a chemical reaction involving the gain of electrons or hydrogen.
What is a reducing sugar?
A sugar that can donate electrons to another chemical, in order to reduce them.
What is the test for reducing sugar called?
Benedict’s test, as Benedict’s reagent is used in the test.
Which chemicals are reducing sugars?
All monosaccharides and some disaccharides.
What is Benedict’s reagent?
An alkaline solution of copper (II) sulfate.
Why is Benedict’s reagent used to test for reducing sugars?
When a reducing sugar is heated with Benedict’s reagent it forms an insoluble red precipitate of copper (I) oxide.
Describe the method for the test for reducing sugars.
1) Add 2cm3 of the food sample to be tested to a test tube. If the sample is not already in liquid form, first grind it up in water.
2) Add an equal volume of Benedict’s reagent.
3) Heat the mixture in a gently boiling water bath for five minutes.
When combined in pairs, monosaccharides create…
Disaccharides. For example, glucose joined to glucose to form maltose.
What happens when monosaccharides join?
A molecule of water is removed and the reaction is therefore called a condensation reaction. The bond that is formed is called a glycosidic bond.
What happens when water is added to a disaccharide?
It breaks down the glycosidic bond releasing the constituent monosaccharides (hydrolysis)
How do we detect a non-reducing sugar?
First it is hydrolysed into its monosaccharide components.
1) If not in liquid form, ground up into water.
2) Add 2cm3 of the food sample being tested to 2cm3 of Benedict’s reagent in a test tube and filter.
3) Place the test tube in a gently boiling water bath for five minutes, if it changes blue there is no reducing sugar present.
4) Add the same quantity of food sample again and also an additional 2cm3 of hydrochloric acid and place in the water bath for 5 minutes to hydrolyse.
5) Make the solution neutral so the reagent works, by adding some sodium hydrogencarbonate solution.
6) Re-test the resulting solution by heating it with 2cm3 of reagent in gently boiling water for five minutes.
7) If a non-reducing sugar was present in the original sample the Benedict’s reagent will turn orange-brown.
Why will the Benedict’s reagent turn orange-brown if a non-reducing sugar was present originally?
This is due to the reducing sugars that were produced from the hydrolysis of the non-reducing sugar.
What are polysaccharides?
Polymers formed by combining together many monosaccharide molecules. They are joined together by glycosidic bonds that were formed by condensation reactions.
Why are polysaccharides insoluble?
They are very large molecules, this feature makes them suitable for storage.
What happens when polysaccharides are hydrolysed?
They break down into disaccharides and monosaccharides.
What is the test for starch?
Starch changes potassium iodide solution from yellow to blue-black.
When are carbohydrates formed?
Carbohydrates are molecules created when carbon is combined with water.
The general formula of a monosaccharide is…
Monosaccharides have the general formula , where n can be 3-7.
State the formula of glucose.
C₆H₁₂O₆.
