Carbohydrates Flashcards
1
Q
What are carbohydrates made out of and what is their general formula?
A
- 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
2
Q
What are reducing sugars?
A
- Monosaccharides are reducing sugars since ketone or aldehyde functional groups can act as a reducing agent in a chemical reaction (undergo oxidation)
3
Q
What are the two different monosaccharide sugars called, depending on the type of functional group present?
A
- Monosaccharides with aldehyde functional group called aldose sugar
- Glucose (hexose) –> aldohexose sugar
- Monosaccharide containing ketone functional group called ketose sugars
- Fructose (hexose) –> ketohexose sugar
4
Q
What is the cyclisation of sugar?
A
- 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)
5
Q
What are Haworth projections?
A
- Fisher projections show the straight-chain forms of monosaccharides and Haworth projections show cyclic form
6
Q
How do you show the conversion from straight-chain to cyclic?
A
- Number carbons in straight chain, carbonyl carbon highest priority (lowest number)
- Identify location of carbonyl group (ketone) and -OH group involved in the ether linkage
- Chosen -OH group is second from last carbon
- Oxygen from -OH group bonds to carbonyl carbon, forms ether linkage
- Hydrogen from hydroxyl joins oxygen from carbonyl, forms new hydroxyl group
7
Q
What are the properties and uses of monosaccharides?
A
- 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
8
Q
Describe the structures of dissacharides.
A
- 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)
9
Q
Give three examples and their monomers of dissacharides.
A
- Matlose: glucose + glucose (beer)
- Lactose: glucose + galactose (milk)
- Sucrose: glucose + fructose (sugar cane)
10
Q
Describe the properties of dissacharides.
A
- 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
11
Q
Why are only aldose sugars reducing agents?
A
- 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)
12
Q
How do you test for reducing sugars?
A
- 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
13
Q
Explain the glycosidic bond.
A
- Link between two sugars between two carbons
- In sucrose link between C-1 (glucose) and C-2 (fructose), link is 1,2 glycosidic bond
14
Q
What are polysaccharides?
A
- When more than two monosaccharides polymerise by condensation reaction
- Result oligosaccharide (3-10 monomers) or polysaccharide (more than 10 monomers)
15
Q
Explain the structure of starch.
A
- 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)
16
Q
How do you test for the presence of starch?
A
- 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
17
Q
Explain the function and structure of cellulose.
A
- 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
18
Q
Explain the function and structure of glycogen.
A
- 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
19
Q
What are the major functions of polysaccharides in the body?
A
- 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
