Chapter 1: Carbohydrates

Features of monosaccharides (CH2O)n

• Linear forms have free carbonyl (C=O) group → reducing sugars

• Small in size, have multiple hydroxyl (OH) groups which can form H bonds with water → readily soluble in water

• Ring structures exhibit a- and b- isomerism

Examples of monosaccharides

Glucose, galactose, fructose

Formula of dissacharides

Cn(H2O)n-1

Features of disaccharides

• Made up of 2 monosaccharides joined by a glycosidic bond formed between 2 monosaccharides by a condensation rxn that involves the loss of a water molc

• can be split into component monosaccharides via hydrolysis rxn, where a glycosidic bond can be broken with the addition of a water molecule

• Have any OH groups which extend out of the ring → form H bond with water → readily soluble in water

• All are reducing sugars except sucrose

Examples of disaccharides and their constituent monomers

Sucrose (glucose + fructose), lactose (glucose + galactose), maltose (glucose + glucose)

Polysaccharides (C6H10O5)n are made up of many monosaccharides joined by ___ bonds formed between them by ___ reactions which involve the loss of ___ ___.

Glycosidic, condensation, water molecules

Starch is made up of 20% ___ and 80% ___.

Amylose, amylopectin

What is the function of starch?

Plant storage molecule

Starch is stored as granules in ___.

Chloroplasts

Starch is made up of ___ monomers

a-glucose

What are the bonds between the monomers in starch?

• Amylose: a(1-4) glycosidic bonds

• Amylopectin: a(1-4) glycosidic bonds within branch, a(1-6) glycosidic bonds at branch points

Structure of amylose and amylopectin

Amylose is a helical molecule while amylopectin is a helical and branched molecule

Orientation of starch molecules

All glucose monomers in the chain have the same orientation

Function of glycogen

Animal storage polysaccharide

Glycogen is stored in the ___ and ___ cells

Liver, muscle

Glycogen is made up of ___ monomers

a-glucose

Bonds between glycogen monomers

a(1-4) glycosidic bonds within branch, a(1-6) glycosidic bonds at branch points

All glucose monomers in glycogen in the chain have the ___ orientation.

Same

Structure of glycogen

Helical, more extensively branched than amylopectin

Function of cellulose

Plant structural polysaccharide

Cellulose is found in the ___ ___ of plants

Cell walls

Monomers of cellulose

B-glucose

Bonds between cellulose monomers

b(1-4) glycosidic bonds

Orientation of cellulose monomers

Alternate B glucose monomers are rotated 180 degrees with respect to each other

Structure of cellulose

Long, straight chain

Bonds between OH molecules in cellulose form…

OH groups projecting outwards in both directions allow interchain hydrogen bonding between cellulose molecules that are parallel to each other → form microfibrils

Why do the structures of starch and glycogen make them good energy storage molecules?

1. Helical molecules, arrangement allows many a-glucose monomers to be packed per unit volume → compact energy store

2. Most OH groups involved in intramolecular H bonding within the helix → few OH groups available for H bonding with water → insoluble in water, water potential unaffected by their presence

3. Branched → multiple branch ends which hydrolytic enzymes can work on → more glucose molecules can be released rapidly at the same time → more ATP can be generated by respiration per unit time

4. Large molecules → insoluble in water

Why the structure of cellulose makes it a good structural molecule

1. Alternate glucose monomers are rotated 180 deg wrt eo → long, straight molecule with OH groups projecting out in both directions → interchain H bonding between cellulose molecules parallel to eo → microfibrils → high tensile strength

2. Most OH groups involved in interchain H bonding → few OH groups available for H bonding with water → insoluble in water

3. Meshwork of microfibrils that form the cell wall

   1. have a porous structure → cell freely permeable to water and solutes → allow movement of substances across cell wall

   2. strong and rigid, distribute stress in all directions to prevent plant cells from bursting due to osmotic stress

4. Cellulases that hydrolyse cellulose are found in very few organisms → cellulose cannot be hydrolysed by most organisms and be used as respiratory substrate —> good structural molecule

Benedict’s test for reducing sugars

1. Place 2cm³ of test solution in a test tube

2. Add equal volume of Benedict’s reagent

3. Shake mixture

4. Heat by immersing tube in boiling water bath for 3-4 minutes

Brick-red ppt → reducing sugar is present

Test for non-reducing sugars

If a negative result for BT is obtained for test solution, then

1. boil equal volume of test solution with dilute HCl for ~1min → hydrolyse disaccharide to monosaccharides

2. Cool contents of tube

3. Neutralise the acidic content with sodium bicarbonate solution

4. Carry out Benedict’s test for reducing sugar

Presence of non-reducing sugar indicated by:

• Blue solution remains when BT is first carried out

• After acid hydrolysis, BT carried out again → colour of final suspension depends on amount of sugar present

Iodine test for starch

1. Add a few drops of iodine solution to 1cm³ of test solution

2. Observe any colour change (blue-black → starch present, orange → starch absent)

Cellulose has a structural function while starch has a storage function. Relate these functional differences to the differences in molecular structure of cellulose and starch. [4]

• Cellulose has B-glucose monomers that are linked by B(1-4) glycosidic bonds: enzymes that hydrolysis these bonds are rarely found in nature and therefore likely to remain intact → suitable as structural molc

• Cellulose alternate glucose residues inverted 180˚ wrt one another allowing straight chains to be formed with hydroxyl groups projecting out in either direction. Numerous H bonding b/w adjacent cellulose molcs form microfibrils: straight chains allow packing of cellulose chains into bundles of microfibrils with high tensile strength that make up cell wall

• Starch has A glucose molecules that are linked by a(1-4) glycosidic bonds. Enzyme that hydrolysis these bonds are commonly available → glucose units readily released for respiration to yield energy

• Starch A glucose monomers are linked by a(1-4) glycosidic bonds which give rise to helical molecules of amylose as each residue in bent in one direction wrt adjacent molc: helical arrangement allows more glucose residues per unit volume → compact storage molc

Suggest why amylase (hydrolyse starch) will not catalyse the hydrolysis of cellulose. [2]

1. Amylase has a specific active site with complementary charge and conformation to starch which it binds and catalyses the hydrolysis of a(1-4) glycosidic bonds

2. Cellulose has B(1-4) glycosidic bonds whose 3D conformation is not complementary to AS of amylase

Use an annotated diagram to show how the bond between P (main branch of glycogen) and Q (branch) is broken

Suggest why cellulose must be synthesised at the cell surface membrane and not inside the cell. [3]

1. Cellulose is a macromolecule found outside the cell as part of the cell wall therefore it is easier to deposit it there

2. Cellulose molecule may be too large to be transported through the cell membrane if it has to be transported to the exterior of the cell

3. Cellulose is insoluble in the hydrophobic core of the lipid bilayer, hence synthesised at the cell surface membrane and transported out of the cell

Identify one property of starch that could account for its stability over long periods of time. [1]

Large molecule and insoluble in water, hence will not participate in metabolic reactions in aq medium of the cell

Explain how cellulose differs from starch in function