Monosaccharides and Disaccharides

Chemistry for Biologists

What are organic compounds?

Compounds that contain carbon atoms.

Which elements are commonly found in organic compounds?

Carbon and hydrogen, as well as oxygen and less frequently nitrogen, sulfur and phosphorus.

Why is carbon important in biological molecules?

Each carbon atom can form four bonds, allowing carbon atoms to join together into long chains, branched chains and rings.

What is a monomer?

A small molecule that is a single unit of a larger molecule called a polymer.

What is a polymer?

A long-chain molecule made up of many smaller, repeating monomer units joined together by chemical bonds.

What is a macromolecule?

A very large molecule often formed by polymerisation.

Why can carbon form a wide variety of biological molecules?

Carbon atoms can form four strong bonds and can join into chains, branched chains and rings.

What are carbohydrates made from?

Carbon, hydrogen and oxygen.

What are the three main groups of carbohydrates?

Monosaccharides, disaccharides and polysaccharides.

What are the main functions of carbohydrates?

They provide a usable energy source, store energy and form important structural components.

What is starch?

A polysaccharide formed from glucose monomers that is used for energy storage in plants.

What is a monosaccharide?

A single sugar monomer.

What is the general formula of a monosaccharide?

(CH₂O)n, where n is usually a small number.

What are the three types of monosaccharide based on their number of carbon atoms?

Triose sugars, pentose sugars and hexose sugars.

What is a triose sugar?

A monosaccharide containing three carbon atoms.

What is the general formula of a triose sugar?

C₃H₆O₃.

Why are triose sugars important in respiration?

Respiration breaks glucose down into triose sugars as intermediate molecules.

What is a pentose sugar?

A monosaccharide containing five carbon atoms.

What is the general formula of a pentose sugar?

C₅H₁₀O₅.

Which important biological molecules contain pentose sugars?

DNA and RNA.

Which pentose sugar is found in RNA?

Ribose.

Which pentose sugar is found in DNA?

Deoxyribose.

What is a hexose sugar?

A monosaccharide containing six carbon atoms.

What is the general formula of a hexose sugar?

C₆H₁₂O₆.

What are three common hexose sugars?

Glucose, galactose and fructose.

What are the two isomers of glucose?

α-glucose and β-glucose.

What is the difference between α-glucose and β-glucose?

They have different arrangements of atoms around carbon 1.

What is the arrangement around carbon 1 in α-glucose?

The OH group on carbon 1 is below the ring.

What is the arrangement around carbon 1 in β-glucose?

The OH group on carbon 1 is above the ring.

What is an isomer?

A molecule that has the same chemical formula as another molecule but a different molecular structure.

Why is the difference between α-glucose and β-glucose biologically important?

The different arrangements affect the bonds formed between glucose molecules and therefore the polymers produced.

What is a disaccharide?

A sugar made of two monosaccharide units joined by a glycosidic bond.

How is a disaccharide formed?

Two monosaccharides join in a condensation reaction, forming a glycosidic bond and releasing one molecule of water.

What is a condensation reaction?

A reaction in which two molecules join and a molecule of water is removed.

What is a glycosidic bond?

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

What is a 1,4-glycosidic bond?

A glycosidic bond formed between carbon 1 of one monosaccharide and carbon 4 of another.

What is a 1,6-glycosidic bond?

A glycosidic bond formed between carbon 1 of one monosaccharide and carbon 6 of another.

Which monosaccharides combine to form maltose?

Glucose + glucose.

How is maltose formed?

Two α-glucose molecules join in a condensation reaction, forming a 1,4-glycosidic bond and releasing water.

Where is maltose commonly found?

In germinating seeds such as barley.

Which monosaccharides combine to form sucrose?

Glucose + fructose.

Where is sucrose commonly found?

It is stored in plants such as sugar cane.

Which monosaccharides combine to form lactose?

Glucose + galactose.

Where is lactose commonly found?

In milk, where it is the main carbohydrate.

How can a glycosidic bond be broken?

By hydrolysis.

What is hydrolysis?

A reaction in which water is added to break a chemical bond.

What happens when a disaccharide undergoes hydrolysis?

A molecule of water is added, breaking the glycosidic bond and producing two monosaccharides.

What is Benedict's solution used to test for?

Reducing sugars.

What colour is Benedict's solution before a positive reaction?

Bright blue.

What does Benedict's solution contain?

Copper(II) ions.

How do you carry out Benedict's test for reducing sugars?

Add Benedict's solution to the sample and heat it gently.

What happens during a positive Benedict's test?

The blue solution changes towards orange and an orange-red precipitate forms as copper(II) ions are reduced to copper(I) ions.

What colour change indicates a positive Benedict's test?

Blue to orange or brick-red, with a precipitate forming.

What are reducing sugars?

Sugars that react with Benedict's solution and reduce copper(II) ions to copper(I) ions, producing an orange-red precipitate.

Which monosaccharides are reducing sugars?

All monosaccharides.

Are all disaccharides reducing sugars?

No. Some are reducing sugars, but sucrose is non-reducing.

What is a non-reducing sugar?

A sugar that does not react directly with Benedict's solution.

Is sucrose a reducing or non-reducing sugar?

Sucrose is a non-reducing sugar.

How do you test for a non-reducing sugar such as sucrose?

First test with Benedict's solution to confirm no reducing sugar is present. Then add dilute hydrochloric acid and heat to hydrolyse the glycosidic bonds. Cool and neutralise with sodium hydrogencarbonate, then repeat Benedict's test.

Why is hydrochloric acid used when testing for a non-reducing sugar?

It hydrolyses the glycosidic bond, breaking the non-reducing disaccharide into monosaccharides.

Why must the solution be neutralised before repeating Benedict's test?

Benedict's reagent requires alkaline conditions to work correctly.

What indicates the presence of a non-reducing sugar after hydrolysis?

A positive Benedict's test after hydrolysis, shown by an orange-red precipitate.

Define a monomer.

A small molecule that is a single unit of a larger molecule called a polymer.

Define a polymer.

A long-chain molecule made up of many smaller, repeating monomer units joined together by chemical bonds.

Define a macromolecule.

A very large molecule often formed by polymerisation.

Define starch.

A long-chain polymer formed of glucose monomers.

Define sucrose.

A sweet-tasting disaccharide formed by joining glucose and fructose.

Define glucose.

A hexose sugar.

Define a monosaccharide.

A single sugar monomer.

Define a disaccharide.

A sugar made up of two monosaccharide units joined by a glycosidic bond, formed in a condensation reaction.

Define a polysaccharide.

A polymer consisting of long chains of monosaccharide units joined by glycosidic bonds.

Define a triose sugar.

A sugar with three carbon atoms.

Define a pentose sugar.

A sugar with five carbon atoms.

Define ribose.

A pentose sugar that forms part of the structure of RNA.

Define deoxyribose.

A pentose sugar that forms part of the structure of DNA.

Define deoxyribonucleic acid (DNA).

A nucleic acid that is the genetic material in many organisms.

Define ribonucleic acid (RNA).

A nucleic acid that is the genetic material in some organisms and is involved in protein synthesis.

Define a hexose sugar.

A sugar with six carbon atoms.

Define an isomer.

A molecule with the same chemical formula as another molecule but a different molecular structure.

Define a condensation reaction.

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

Define a glycosidic bond.

A covalent bond formed between two monosaccharides in a condensation reaction that can be broken by hydrolysis.

Define a reducing sugar.

A sugar that reacts with blue Benedict's solution and reduces copper(II) ions to copper(I) ions, producing an orange-red precipitate.

Define a non-reducing sugar.

A sugar that does not react with Benedict's solution.