(A) (a) (i) Invert sugar
Definition : The equimolar mixture of D-(+)-glucose and D-(−)-fructose obtained by the hydrolysis of sucrose is called invert sugar.
$$\underset{\text{sucrose}}{C_{12}H_{22}O_{11}} + H_2O \xrightarrow{H^+ \text{ or invertase}} \underset{\text{glucose}}{C_6H_{12}O_6} + \underset{\text{fructose}}{C_6H_{12}O_6}$$
Why "invert" : the sign of optical rotation reverses during the hydrolysis.
| Species | Specific rotation |
|---|
| Sucrose | $+66 \cdot 5^\circ$ (dextrorotatory) |
| Product mixture | $-19 \cdot 9^\circ$ (laevorotatory) |
Fructose $(-92 \cdot 4^\circ)$ rotates light to the left far more strongly than glucose $(+52 \cdot 5^\circ)$ rotates it to the right, so the mixture is overall laevorotatory.
(A) (a) (ii) Polysaccharides
Definition : Carbohydrates that on hydrolysis give a large number of monosaccharide units are called polysaccharides.
Key points :
- They are not sweet and are therefore also called non-sugars.
- They are generally insoluble in water.
- Most are non-reducing, since almost no free anomeric carbon remains.
Examples : starch (plant storage), glycogen (animal storage), cellulose (plant structure).
(A) (b) Fibrous versus globular proteins — structural difference
| Fibrous protein | Globular protein |
|---|
| Polypeptide chains lie parallel to one another, held together by hydrogen and disulphide bonds, giving a thread-like (fibre) shape | Chains coil around to give a compact spherical shape |
| Generally insoluble in water | Generally soluble in water |
| Serve a structural role | Serve a functional role |
| Examples : keratin, myosin, collagen | Examples : insulin, haemoglobin, albumin |
OR
(B) (a) Why glycogen is called animal starch
Glycogen is the polysaccharide in which animals store their carbohydrate, held mainly in the liver and muscle.
Its role in animals is exactly the role starch plays in plants — a reserve of glucose that can be broken down by enzymes whenever the body needs energy.
Structurally too it resembles starch, being a polymer of $\alpha$-D-glucose units, though glycogen is more highly branched than amylopectin.
For these reasons — the same storage function and a similar structure — it is called animal starch.
(B) (b) Essential versus non-essential amino acids
| Essential amino acids | Non-essential amino acids |
|---|
| Cannot be synthesised by the human body | Can be synthesised within the body |
| Must be supplied through the diet | Need not be supplied in the diet |
| Ten of the twenty amino acids | The remaining ten |
| Examples : valine, leucine, lysine, phenylalanine, tryptophan | Examples : glycine, alanine, glutamic acid, serine |
(B) (c) Effect of denaturation on protein structure
What is destroyed : the secondary and tertiary structures.
The hydrogen bonds and other weak forces (disulphide and hydrophobic interactions) that hold the folded shape are broken, so the globular protein uncoils into a random, fibrous form and loses its biological activity.
What survives : the primary structure remains completely intact, because the strong covalent peptide bonds are not broken. The sequence of amino acids is unchanged.
Everyday examples : the coagulation of egg white on boiling; the curdling of milk.
