Carbohydrates

Question 1

What is the general formula of monosaccharides?

Answer 1

(CH_{2O)n where n is either 3 (triose), 5 (pentose) or 6 (hexose)}

Question 2

What are some examples of pentose monosaccharides?

Answer 2

• Ribose found in RNA and ATP
• Deoxyribose in DNA
• Ribulose 1,5 bisphosphate (RuBP) in photosynthesis

Question 3

What are some examples of hexose monosaccharides?

Answer 3

• Glucose
• Fructose
• Galactose

Question 4

Draw alpha glucose

Answer 4

Answer on revision card

Question 5

Draw beta glucose

Answer 5

Answer on revision card

Question 6

All hexose are structural isomers. What is a structural isomer?

Answer 6

They all have the same chemical formula but a different molecular structure

Question 7

What are some examples of disaccharides?

Answer 7

• Maltose
• Lactose
• Sucrose

Question 8

What is the structure, bonding and function of maltose? And draw it

Answer 8

Structure: two alpha glucose
Function: present in germinating seeds
Bonding: alpha 1,4 glycosidc bond

Drawing on revision card

Question 9

What is the structure and function of sucrose?

Answer 9

Structure: composed of alpha glucose and fructose
Function: transport sugar in phloem plants

• It is a non reducing sugar
• It is also referred to as cane sugar or just sugar

Question 10

What makes sucrose a non-reducing sugar?

Answer 10

• It cannot reduce Cu²⁺ to Cu⁺
• This is because the chemical group required for this reduction reaction is involved in the glycosidic bonds between the monosaccharides

Question 11

What is the function, structure and bonding of lactose?

Answer 11

Function: found in milk/milk products
Structure: composed of glucose (alpha or beta) and beta galactose
Bonding: beta 1,4 glycosidic bond

Question 12

What the reagent is used to test for sugars?

Answer 12

Benedict’s reagant, an alkaline solution of copper(II) sulfate

Question 13

What is the procedure for the qualitative test for reducing sugars?

Answer 13

• Place the sample to be tested in a boiling tube
• If it is not in liquid form, grind it up or blend it in water
• Add an equal amount of Benedict’s reagant
• Heat the mixture gently in a boiling water bath for five minutes

Question 14

What is the procedure for the qualitativ test for non-reducing sugars?

Answer 14

• Place the sample to be tested in a boiling tube and add dilute hydrochloric acid
• Heat the mixture gently in a boiling water bath for a few minutes
• Neutralise the pH by adding sodium bicarbonate (NaHCO₃₎
• Add an equal amount of Benedict’s reagant
• Heat the mixture gently in a boiling water bath for five minutes

Question 15

What is the procedure for the quantitative test for reducing sugars?

Answer 15

• Set the colorimeter to the red filter
• Place a cuvette containing distilled water into the colorimeter and set the colorimeter to zero
• Place 2cm3 of each of the unknown glucose concentration samples in separate boiling tubes
• Add 2cm3 of Benedict’s solution to all boiling tubes
• Place boiling tubes in a boiling water bath for 5 minutes
• Use tongs to take the boiling tubes out of the water bath, leave to cool and allow the supernatant to settle
• Use a pipette to take 2cm3 of the solution left at the top from each boiling tube into seperate cuvettes
• Place each cuvette in the colrimeter and measure the % transmission

Question 16

What is the general formula for the polymerisation of glucose?

Answer 16

Answer on revision card

Question 17

What is the function of starch?

Answer 17

It is the storage carbohydrate in plants and is a source of glucose for respiration

Question 18

Where is starch found?

Answer 18

• Found in chloroplasts and amyloplasts=> it is insoluble so does not affect the water potential
• Found in grain => seed germination
• Found in tubers (e.g. potato) => for asexual reproduction and can carry out respiration at night

Question 19

What are the two types of starch?

Answer 19

Amylose and amylopectin

Question 20

What is the bonding, structure, mass and composition of amylose?

Answer 20

• 20% starch
• The monomer is α glucose
• The bond is an α 1,4 glycosidc bond
• It is a helical molecule, it is a helix held together by intramolecular hydrogen bonds between -OH groups
• Relative molecular mass ≈ 50,000 => there are about 300 glucose residues in a molecule of amylose (can’t go past that as it would be unstable)

Question 21

What is the bonding, structure, mass and composition of amylopectin?

Answer 21

• 80% starch
• The monomer is α glucose
• The bond is an α 1,4 glycosidc bond between adjacent molecules AND α 1,6 glycosidc bonds between branching points
• It has a branched structure
• Relative molecular mass ≈ 500,000 => there are about 3000 glucose residues in a molecule of amylopectin

Question 22

What is the bonding, structure, mass and composition of glycogen?

Answer 22

• The monomer is α glucose
• The bond is an α 1,4 glycosidc bond between adjacent molecules AND α 1,6 glycosidc bonds between branching points
• It has a highly branched structure
• There are about up to 60,000 glucose residues in a molecule of glycogen

Question 23

What is the bonding, structure, mass and composition of cellulose?

Answer 23

• The monomer is β glucose
• The bond is an β 1,4 glycosidc bond between adjacent molecules
• It has linear structure
• There are about up to 10,000 glucose residues in a molecule of cellulose

Question 24

What occurs when iodine is mixed with amylose?

Answer 24

• Iodine ions enter the helix (absorbs light of a different wavelength)
• Turns starch/iodine complex blue-black
• Heat breaks the hydrogen bonds and releases the iodine
• This cause the blue-black colour to disappear

Question 25

What are the properties of glycogen and amylopectin that make them a good energy store?

Answer 25

**Branched** - Meaning that => many glucose monomers are available at branch endings and glycogen is more highly branched than amylopectin - Advantage => rapid release of glucose for respiration to provide energy for the cell => more glucose monomers are hydrolysed i.e. more rapid release of glucose for animals which are more metabollicaly active than plants **Compact** - Meaning that => many glucose monomers exist in a small volume - Advantage => large quantities of energy can be store in the cell **Insoluble** - Meaning that => they do not affect the water potential - Advantage => does not cause osmotic swell

Question 26

What is the function of cellulose?

Answer 26

- Structural carbohydrate - Component of plant and algae cell walls - Homopolysaccharide (made of only one type of monomer)

Question 27

Why doesn't a cellulose chain coil?

Answer 27

- The chain does not coil because each beta glucose monomer is 180°/opposite orienation with respect to its neigbour - Each chain lies parallel to the next and can form intramolecular hydrogen bonds with it

Question 28

What do chains of cellulose molecules form?

Answer 28

- Chains of cellulose molecules form microfibrils - A bundle of microfibrils makes macrofibrils - A bundle of macrofibrils makes cellulose fibres

Question 29

What are the properties of cellulose and what are their significance?

Answer 29

- Linear => ß glucose monomers flip 180 degrees alternately giving rise to strands which are parallel to each other - Unbranched => Only ß 1,4 glycosidic bonds between the ß glucose monomers present, no 1,6 glycosidic bonds - Unreactive => Glucose monomers held in a chain (plus many H bonds between cellulose molecules) which are not easily hydrolysed to be available for respiration - High tensile strength => Many intermolecular H bonds between the individual cellulose molecules within the microfibrils hold the chains firmly together - Insoluble => Very large polymer with many H bonds keeping strands together so no access for water molecules to bond with polar OH groups