1.2.1 - Carbohydrates 1 – monosaccharides and disaccharides

What are biological molecules important for?

Biological molecules are important for the structure and function of living organisms.

What class are biological molecules usually?

Organic compounds?

What do organic compounds contain?

• Organic compounds contain carbon atoms.

• They also contain atoms such as hydrogen and oxygen, and less frequently nitrogen, sulphur and phosphorus.

What are some examples of organic biological molecules?

• carbohydrates

• lipids

• proteins

• nucleic acids

How many covalent bonds can carbon form?

Carbon can form four covalent bonds. This allows carbon atoms to bond with many other atoms and with other carbon atoms.

What can carbon atoms form?

• long chains

• branched chains

• rings

What are Carbohydrates important for?

Carbohydrates are important in cells as a usable energy source and for storing energy.

What are the best known Carbohydrates?

The best-known carbohydrates are sugars and starch.

What are the three main groups of carbohydrates?

• Monosaccharides

• Disaccharides

• Polysaccharides

What is a monosaccharide?

A monosaccharide is a single sugar unit and is the monomer from which larger carbohydrates can be formed.

What is the formula for monosaccharides?

(CH2O)n

What is a three carbon sugar called?

A triose is a monosaccharide containing 3 carbon atoms. Its general formula is C₃H₆O₃.

What is a five carbon sugar called?

A pentose is a monosaccharide containing 5 carbon atoms. Its general formula is C₅H₁₀O₅.

Then:

• ribose → RNA

• deoxyribose → DNA

What is a 6 carbon sugar called?

A hexose sugar has 6 carbon atoms with a general formula of C6H12O6

What are the three important hexose sugars?

• glucose

• fructose

• galactose

What is an isomer?

Isomers are molecules that have the same chemical formula but different molecular structures.

What are the two isomers of glucose?

• Alpha glucose

• Beta glucose

Why does glucose form a ring?

Glucose can form a ring when atoms within the molecule react to form a new covalent bond. The resulting ring structure is important because the arrangement of groups determines whether glucose is α or β, and glucose's ring structure is involved when forming glycosidic bonds.

What is a Disaccharide?

A disaccharide is a sugar made up of two monosaccharide units joined together by a glycosidic bond.

What are the three important Disaccharides?

• Sucrose

• Lactose

• Maltose

How is sucrose formed?

Glucose + Fructose

How is lactose formed?

Glucose + Galactose

How is maltose formed?

Glucose + Glucose

What is a condensation reaction?

A condensation reaction is a reaction in which a molecule of water is removed as a bond is formed between molecules.

What is a Glycosidic bond?

A glycosidic bond is a covalent bond formed between two monosaccharides in a condensation reaction.

What is a Hydrolysis reaction?

Hydrolysis is essentially the opposite of condensation.

A hydrolysis reaction uses water to break a glycosidic bond and release the monosaccharides.

How does benedict’s test for testing reducing sugars work?

Benedict's solution

• Benedict's solution is a chemical test for reducing sugars.

• It is a bright blue solution containing copper(II) ions.

• When heated with a reducing sugar, the copper(II) ions are reduced.

• A precipitate forms.

• The colour changes from blue towards orange/red depending on the amount of reducing sugar.

Reducing sugars

Monosaccharides are reducing sugars.

Some disaccharides are also reducing sugars.

What are non reducing sugars and how would you test for them?

Some sugars do not react with Benedict's solution.

The textbook gives sucrose as an example.

A non-reducing sugar such as sucrose can be:

1. heated with hydrochloric acid

2. the glycosidic bonds are hydrolysed

3. the solution is cooled

4. sodium hydrogen carbonate is added to neutralise the acid

5. the resulting monosaccharides can then give a positive Benedict's test.