Step 1: What a linkage means.
A linkage is simply the kind of bond that joins the small building blocks of a biomolecule. Different biomolecules are built using different linkages.
Step 2: Peptide linkage.
A peptide linkage is the amide bond \((-CO-NH-)\) that joins two amino acids. It forms when the \(-COOH\) group of one amino acid reacts with the \(-NH_2\) group of the next amino acid, with loss of a water molecule. It is found in proteins.
Step 3: Glycosidic linkage.
A glycosidic linkage is the oxygen bridge \((-O-)\) that joins two monosaccharide (sugar) units. It forms when the \(-OH\) groups of two sugar molecules react, again losing a water molecule. It is found in carbohydrates like sucrose, starch and cellulose.
Step 4: Essential amino acids.
These are amino acids that our body cannot make on its own, so they must come from our diet (food). Examples are valine, leucine and lysine.
Step 5: Non-essential amino acids.
These are amino acids that our body can make by itself, so they need not be taken from food. Examples are glycine and alanine.
Answer: A peptide linkage is an amide bond \((-CO-NH-)\) between amino acids in proteins, while a glycosidic linkage is an ether-type \(-O-\) bond between sugar units in carbohydrates. Essential amino acids must be supplied by the diet because the body cannot synthesise them, whereas non-essential amino acids are made by the body itself.