25 WJEC Biology AS Level - Marianne Izen - 2nd Edition

Question 1

Peptide bond

Answer 1

The chemical bond formed by a condensation reaction between the amino group of one amino acid and the carboxyl group of another.

Question 2

What is the bond in a dipeptide called?

Answer 2

It is called a ‘peptide’ bond.

Remember it is not called a ‘dipeptide bond.

Question 3

Describe the basic structure of an amino acid

Answer 3

All amino acids have the same basic structure.

Attached to a central carbon atom are:

1. An amino group, -NH2, at one end of the molecule, called the N-terminal.
2. A carboxyl, group-COOH, at the other end of the molecule, called the C-terminal.
3. A hydrogen atom.
4. The R group, which is different in each amino acid.

Question 4

Describe the ‘zwitterion’ nature of an amino acid

Answer 4

The amino group is basic.

At pH 7, the pH of the cell, it gains an H and becomes positively charged.

The carboxyl group is acidic and at pH 7, it loses an H, becoming negatively charged.

So at pH 7, an amino acid has both a positive and a negative charge.

Such an ion is a ‘zwitterion’.

Question 5

Describe the formation of a dipeptide

Answer 5

Proteins are linear sequences of amino acids.

The amino group of one amino acid reacts with the carboxyl group of another, with the elimination of water.

The bond that is formed from this condensation reaction is a peptide bond, and the resulting compound is a dipeptide.

The dipeptide can be written as NH2.CHR1.COO-NH.CHR2.COOH.

But if the amino acids bonded the other way, they would form the dipeptide NH.CHR2.COO-NH.CHR1.COOH.

This is a different dipeptide with different properties.

Question 6

Describe the features of proteins

Answer 6

Proteins differ from carbohydrates and lipids in that, in addition to carbon, hydrogen and oxygen, they always contain nitrogen.

Many proteins also contain sulphur and some contain phosphorus.

Proteins are polymers made of monomers called amino acids.

The chains of amino acids are called polypeptides.

About 20 different amino acids are used to make up proteins.

There are thousands of different proteins and their shape is determined by the specific sequence of amino acids in the chain.

Question 7

What are the elements in the following macromolecules?

Carbohydrates
Lipids
Proteins

Answer 7

Make sure you remember the elements in macromolecules:

Carbohydrates-C,H,O
Lipids - C, H, a little O
Proteins-C, H, O, N

Question 8

What gives amino acids their name?

Answer 8

The amino group and the carboxyl group give amino acids their name.