Carbohydrates

Question 1

What are carbohydrates made out of and what is their general formula?

Answer 1

• Saccharides or sugars, repeating monosaccharides
• General formula: Cx(H2O)y
• Aldehyde group (-CHO) or ketone group (C=O) with hydroxyl (-OH) groups
• Ratio 1:2:2, C:H:O

Question 2

What are reducing sugars?

Answer 2

• Monosaccharides are reducing sugars since ketone or aldehyde functional groups can act as a reducing agent in a chemical reaction (undergo oxidation)

Question 3

What are the two different monosaccharide sugars called, depending on the type of functional group present?

Answer 3

• Monosaccharides with aldehyde functional group called aldose sugar
• Glucose (hexose) –> aldohexose sugar
• Monosaccharide containing ketone functional group called ketose sugars
• Fructose (hexose) –> ketohexose sugar

Question 4

What is the cyclisation of sugar?

Answer 4

• When a straight-chain form of sugars cyclise in solution to form ring structures w/ ether linkage
• Carbonyl group of the aldehyde or ketone reacts with -OH group further down the sugar
• Ether linkage forms between carbonyl-hyroxyl (C-O-C)
• Ketose form hemiketals, aldose form hemiacetals
• This process is only a rearrangement in bonds (molecular formula does NOT change), isomers
• Amount of cyclisation under normal conditions is high, those with 5/6 carbons exist in straight-chain form
• When sugars cyclise, form stable 5/6 membered rings
• Sugars with 3/4 carbons don’t cyclise (too much strain on ring structure, unstable)

Question 5

What are Haworth projections?

Answer 5

• Fisher projections show the straight-chain forms of monosaccharides and Haworth projections show cyclic form

Question 6

How do you show the conversion from straight-chain to cyclic?

Answer 6

1. Number carbons in straight chain, carbonyl carbon highest priority (lowest number)
2. Identify location of carbonyl group (ketone) and -OH group involved in the ether linkage
3. Chosen -OH group is second from last carbon
4. Oxygen from -OH group bonds to carbonyl carbon, forms ether linkage
5. Hydrogen from hydroxyl joins oxygen from carbonyl, forms new hydroxyl group

Question 7

What are the properties and uses of monosaccharides?

Answer 7

• Strong intermolecular forces between molecules due to hydroxyl functional groups
• At r.t.p solid crystals or powders with high melting points
• Functional groups are polar, very soluble in water in cyclic and straight-chain forms
• Hydrogen bonds between water molecules, dissolve sugar molecule
• Easily dissolved in bloodstream and carried to cells as source of energy
• Polar molecules can cross cell membrane easily and used as immediate fuel sources (cellular respiration)
• Used in pharmaceutical industry as a binding agent for tablets (often in powdered form)
• Used to sweeten foods and beverages

Question 8

Describe the structures of dissacharides.

Answer 8

• When two monosaccharides are bonded in a condensation reaction
• Functional groups (-OH) form ether linkage also called glycosidic bond
• General formula is still Cx(H2O)y (y is 1 less than x)

Question 9

Give three examples and their monomers of dissacharides.

Answer 9

• Matlose: glucose + glucose (beer)
• Lactose: glucose + galactose (milk)
• Sucrose: glucose + fructose (sugar cane)

Question 10

Describe the properties of dissacharides.

Answer 10

• Many polar functional groups, allow for stronger intermolecular forces, making them solid crystals or powders at r.t.p, highly soluble
• Two cyclic monomers bonded together by glycosidic bond, bond can contain a carbonyl group

Question 11

Why are only aldose sugars reducing agents?

Answer 11

• Aldose sugars don’t have carbonyl carbon part of the glycosidic bond, these sugars convert to straight-chain from, act as reducing agent, sugar
• Ketose sugars have carbonyl carbon as part of glycosidic bond, unable to convert to straight-chain, makes them non-reducing sugars
• Dissacharides made from aldose sugars are reducing sugars, sucrose (ketone sugar is not a reducing sugar)

Question 12

How do you test for reducing sugars?

Answer 12

• Using Benedict’s solution, contain copper (II) ions that react with carbonyl group by redox reaction
• Copper (II) ions undergo reduction to copper (I), carbonyl group undergoes oxidation to form carboxylic acid
• Copper (I) insoluble, form precipitate, colour change from blue to yellow-orange

Question 13

Explain the glycosidic bond.

Answer 13

• Link between two sugars between two carbons
• In sucrose link between C-1 (glucose) and C-2 (fructose), link is 1,2 glycosidic bond

Question 14

What are polysaccharides?

Answer 14

• When more than two monosaccharides polymerise by condensation reaction
• Result oligosaccharide (3-10 monomers) or polysaccharide (more than 10 monomers)

Question 15

Explain the structure of starch.

Answer 15

• Starch is hydrolysed by enzymes into glucose monomers and use for energy
• Starch polymers vary, depending on the bonding between monomers and whether or not there is branching
• Amylose: water soluble, straight chain
• Amylopectin: water insoluble, branched structure
• Uses D-glucose monomers
• Long starch chains are soluble in water only when heated, forms helical structures
• Found in high levels in food (potatoes, rice)

Question 16

How do you test for the presence of starch?

Answer 16

• Using solution of iodine called Lugol’s solution
• Starch is NOT a reducing sugar, will not show a positive result with Benedict’s
• If starch is present, turns dark blue

Question 17

Explain the function and structure of cellulose.

Answer 17

• Important structural component of the cell wall that surrounds plant cells
• Hydrolysing cellulose would require enzymes that are not present in animals
• Passes through digestive system, ‘dietary fibre’
• Long linear chains held together by strong intermolecular forces, very dense and insoluble in water

Question 18

Explain the function and structure of glycogen.

Answer 18

• Human body’s main source of energy, animal starch
• Polymerised glucose used as short-term energy reserve, stored in liver and muscle tissue
• Excess glucose stored for later use (night)
• More branched than starch produced by plants, still water-soluble
• Similar structure to amylopectin

Question 19

What are the major functions of polysaccharides in the body?

Answer 19

• Provides energy: potatoes, rice, grains
• Stores energy: starch, glycogen
• Precursors: components of nucleic acids, role in biosynthesis of proteins
• Dietary fibre: secreted by human digestive tract, cellulose, helps prevent obesity, irritable bowel syndrome