(a) Hydrolysis of sucrose
$$\underset{\text{sucrose}}{C_{12}H_{22}O_{11}} + H_2O \xrightarrow{H^+ \text{ or invertase}} \underset{\text{glucose}}{C_6H_{12}O_6} + \underset{\text{fructose}}{C_6H_{12}O_6}$$
Products : D-(+)-glucose and D-(−)-fructose, in equal amounts.
The two units in sucrose are joined by an $\alpha$-1,2-glycosidic linkage between C-1 of glucose and C-2 of fructose.
A point worth adding — invert sugar
The equimolar mixture obtained is called invert sugar, because the sign of optical rotation reverses during the hydrolysis :
| Species | Specific rotation |
|---|
| Sucrose | $+66 \cdot 5^\circ$ (dextrorotatory) |
| Glucose | $+52 \cdot 5^\circ$ |
| Fructose | $-92 \cdot 4^\circ$ |
| Product mixture | $-19 \cdot 9^\circ$ (laevorotatory) |
Fructose rotates light to the left far more strongly than glucose rotates it to the right, so the mixture is overall laevorotatory.
Note also : sucrose itself is non-reducing (both anomeric carbons are locked in the linkage), yet both its hydrolysis products are reducing sugars.
(b) Essential amino acids
Definition : Amino acids that cannot be synthesised by the human body and must therefore be obtained from the diet are called essential amino acids.
Key points :
- Ten of the twenty amino acids found in proteins are essential.
- A deficiency in the diet leads to poor growth and protein deficiency disorders such as kwashiorkor.
Examples : valine, leucine, isoleucine, lysine, phenylalanine, threonine, tryptophan, methionine, histidine, arginine.
The remaining ten, which the body can make for itself, are the non-essential amino acids — glycine, alanine, aspartic acid, glutamic acid, serine and others.
