Introduction

Macromolecules are naturally occurring compounds that have a large molecular weight. There are four classes of macromolecules – carbohydrates, lipids, nucleic acids and proteins. However, for the VCE Biology course, we will only be exploring nucleic acids and proteins.

Polymers

Polymers are molecules which are comprised of many repeating identical subunits, which are referred to as monomers. A condensation reaction is energy-requiring and occurs when monomers join to form a polymer. The name originates from the fact that a water molecule is released during this reaction.

Nucleic Acids

DNA, or deoxyribonucleic acid, is a type of nucleic acid that is the genetic material of all cellular organisms. It is a polymer that consists of monomers called nucleotides. A nucleotide is composed of a five-carbon deoxyribose sugar, a phosphate group, and a nitrogenous base. There are four types of nucleotides in DNA: adenine (A), guanine (G), thymine (T) and cytosine (C).

The carbons labelled 3 and 5 are important features in determining the orientation of the nucleotide (i.e. which way the genetic code is ‘read’), referred to as 3’ and 5’ carbons. The side of the nucleotide with the 3’ carbon is called the 3’ end, and the side of the nucleotide with the 5’ carbon is called the 5’ end. A DNA molecule is composed of two strands that are antiparallel (running in opposite directions) and held together by hydrogen bonds.

RNA, or ribonucleic acid, is a type of nucleic acid that is the genetic material of some viruses and is involved in protein synthesis in cellular organisms. The structure of RNA is very similar to that of DNA (i.e. composed of nucleotides), but there are three key differences between RNA and DNA:

• The sugar in RNA nucleotides is ribose, while the sugar in DNA nucleotides is deoxyribose (the exact meaning of the terms ribose and deoxyribose is not required knowledge).
• RNA is an unpaired chain of nucleotides, while DNA is composed of two chains of nucleotides.
• RNA has nucleotides Adenine, Guanine, Cytosine and Uracil, while DNA has Adenine, Guanine, Cytosine and Thymine.

DNA

Proteins

Proteins are polymers built from monomers called amino acids, which are made up of hydrogen, nitrogen, carbon, oxygen and sometimes sulphur atoms. There are twenty amino acids that can be coded for in organisms.

Proteome

The proteome refers to the complete set of proteins produced by a single cell or organism in an environment at a particular time. Scientists generally study this as proteins usually interact with each other and rarely in isolation.

Amino Acids

Proteins are polymers that are composed of monomers called amino acids. Amino acids are made up of a carboxyl group (COOH), an amino group (NH2) and a R variable group. The carboxyl and amino groups remain constant for all amino acids, while the R variable groups are specific for each amino acid. When two amino acids join, a peptide bond is formed.

Chemical Structure of an Amino Acid

Protein Structure

The structure of a protein essentially dictates its function, therefore, a change in just one amino acid can mean the difference between a function and non-function protein. There are four levels of protein structure, which are:

• The primary structure is the sequence of amino acids that make up the polypeptide chain. In this structure, adjacent amino acids are joined together by peptide bonds.
• The secondary structure is the way the polypeptide chain is twisted and folded into alpha helices and beta pleated sheets through hydrogen bonding. Note, these structure names cannot be abbreviated, therefore, you must write the full name in your VCE exam.
• The tertiary structure is the way the secondary structure is folded into a large, complex and functional 3D structure. The chemical nature of the R variable groups determines how the amino acids interact with one another and hence how the protein folds. This is held together by ionic, hydrogen, covalent bonding and disulphide bridges.
• The quaternary structure is when two or more polypeptide chains are grouped together to form a protein. For example, haemoglobin, which is made up of four polypeptide chains. These would be held together by hydrogen, ionic or covalent bonding.

Levels of Protein Structure