(a) Non-essential amino acids
Amino acids that can be synthesised in the human body and therefore need not be supplied through the diet are called non-essential amino acids.
Ten of the twenty amino acids found in proteins are non-essential.
Examples : glycine, alanine, aspartic acid, glutamic acid, serine.
For contrast : the other ten, such as valine, leucine and lysine, are essential — the body cannot make them, so they must come from food.
(b) Monosaccharides
Carbohydrates that cannot be hydrolysed into simpler carbohydrate units are called monosaccharides. They are the simplest sugars and the building blocks of all larger carbohydrates.
Examples : glucose, fructose, galactose, ribose.
Key points :
- About twenty monosaccharides occur in nature.
- They are classified by the functional group present — aldose (has $-CHO$) or ketose (has $>C=O$) — and by the number of carbons, e.g. glucose is an aldohexose.
- All monosaccharides are reducing sugars, giving positive Tollens' and Fehling's tests.
(c) Anomers
A pair of cyclic sugar molecules that differ only in the configuration of the $-OH$ group at C-1, the anomeric carbon, are called anomers.
Example : $\alpha$-D-glucopyranose and $\beta$-D-glucopyranose.
How they arise : when the open-chain form of glucose cyclises, the $-CHO$ carbon at C-1 becomes a new chiral centre. The $-OH$ formed there can point either downward ($\alpha$-form) or upward ($\beta$-form), giving two different cyclic hemiacetals.
They differ in physical properties — for instance $\alpha$-D-glucose melts at 419 K with $[\alpha]_D = +111^\circ$, while $\beta$-D-glucose melts at 423 K with $[\alpha]_D = +19 \cdot 2^\circ$.
