2.1 Biological macromolecules

Nucleic acids

You will remember that we mentioned earlier that each protein is different because of its unique sequence of amino acids. But what controls how the amino acids arrange themselves to form the specific proteins that are needed by an organism? This task is for the gene . A gene contains DNA (deoxyribonucleic acid) which is a polymer that belongs to a class of compounds called the nucleic acids . DNA is the genetic material that organisms inherit from their parents. It is DNA that provides the genetic coding that is needed to form the specific proteins that an organism needs. Another nucleic acid is RNA (ribonucleic acid). The diagram in [link] shows an RNA molecule.

The DNA polymer is made up of monomers called nucleotides . Each nucleotide has three parts: a sugar, a phosphate and a nitrogenous base. DNA is a double-stranded helix (a helix is basically a coil). Or you can think of it as two RNA molecules bonded together.

There are five different nitrogenous bases: adenine (A), guanine (G), cytosine (C), thymine (T) and uracil (U). It is the sequence of the nitrogenous bases in a DNA polymer that will determine the genetic code for that organism. Three consecutive nitrogenous bases provide the coding for one amino acid. So, for example, if the nitrogenous bases on three nucleotides are uracil , cytosine and uracil (in that order), one serine amino acid will become part of the polypeptide chain. The polypeptide chain is built up in this way until it is long enough (and with the right amino acid sequence) to be a protein. Since proteins control much of what happens in living organisms, it is easy to see how important nucleic acids are as the starting point of this process.

[link] shows some other examples of genetic coding for different amino acids.

Nucleic acids

1. For each of the following, say whether the statement is true or false . If the statement is false , give a reason for your answer.
   1. Deoxyribonucleic acid (DNA) is an example of a polymer and a nucleotide is an example of a monomer .
   2. Thymine and uracil are examples of nucleotides.
   3. A person's DNA will determine what proteins their body will produce, and therefore what characteristics they will have.
   4. An amino acid is a protein monomer.
   5. A polypeptide that consists of five amino acids, will also contain five nucleotides.
2. For each of the following sequences of nitrogenous bases, write down the amino acid/s that will be part of the polypeptide chain.
   1. UUU
   2. UCUUUU
   3. GGUUAUGUUGGU
3. A polypeptide chain consists of three amino acids. The sequence of nitrogenous bases in the nucleotides of the DNA is GCUGGUGCU. Give the structural formula of the polypeptide.

Summary

• A polymer is a macromolecule that is made up of many repeating structural units called monomers which are joined by covalent bonds.
• Polymers that contain carbon atoms in the main chain are called organic polymers .
• Organic polymers can be divided into natural organic polymers (e.g. natural rubber) or synthetic organic polymers (e.g. polystyrene).
• The polymer polyethene for example, is made up of many ethene monomers that have been joined into a polymer chain.
• Polymers form through a process called polymerisation .
• Two examples of polymerisation reactions are addition and condensation reactions.
• An addition reaction occurs when unsaturated monomers (e.g. alkenes) are added to each other one by one. The breaking of a double bond between carbon atoms in the monomer, means that a bond can form with the next monomer. The polymer polyethene is formed through an addition reaction.
• In a condensation reaction , a molecule of water is released as a product of the reaction. The water molecule is made up of atoms that have been lost from each of the monomers. Polyesters and nylon are polymers that are produced through a condensation reaction.
• The chemical properties of polymers (e.g. tensile strength and melting point) are determined by the types of atoms in the polymer, and by the strength of the bonds between adjacent polymer chains. The stronger the bonds, the greater the strength of the polymer, and the higher its melting point.
• One group of synthetic organic polymers, are the plastics .
• Polystyrene is a plastic that is made up of styrene monomers. Polystyrene is used a lot in packaging.
• Polyvinyl chloride (PVC) consists of vinyl chloride monomers. PVC is used to make pipes and flooring.
• Polyethene , or polyethylene , is made from ethene monomers. Polyethene is used to make film wrapping, plastic bags, electrical insulation and bottles.
• Polytetrafluoroethylene is used in non-stick frying pans and electrical insulation.
• A thermoplastic can be heated and melted to a liquid. It freezes to a brittle, glassy state when cooled very quickly. Examples of thermoplastics are polyethene and PVC.
• A thermoset plastic cannot be melted or re-shaped once formed. Examples of thermoset plastics are vulcanised rubber and melanine.
• It is not easy to recycle all plastics, and so they create environmental problems.
• Some of these environmental problems include issues of waste disposal, air pollution and recycling.
• A biological macromolecule is a polymer that occurs naturally in living organisms.
• Examples of biological macromolecules include carbohydrates and proteins , both of which are essential for life to survive.
• Carbohydrates include the sugars and their polymers, and are an important source of energy in living organisms.
• Glucose is a carbohydrate monomer. Glucose is the molecule that is needed for cellular respiration .
• The glucose monomer is also a building block for carbohydrate polymers such as starch , glycogen and cellulose .
• Proteins have a number of important functions. These include their roles in structures, transport, storage, hormonal proteins and enzymes.
• A protein consists of monomers called amino acids , which are joined by peptide bonds .
• A protein has a primary , secondary and tertiary structure.
• An amino acid is an organic molecule, made up of a carboxyl and an amino group, as well as a carbon side chain of varying lengths.
• It is the sequence of amino acids that determines the nature of the protein.
• It is the DNA of an organism that determines the order in which amino acids combine to make a protein.
• DNA is a nucleic acid . DNA is a polymer, and is made up of monomers called nucleotides .
• Each nucleotide consists of a sugar , a phosphate and a nitrogenous base . It is the sequence of the nitrogenous bases that provides the 'code' for the arrangement of the amino acids in a protein.

Summary exercise

1. Give one word for each of the following descriptions:
   1. A chain of monomers joined by covalent bonds.
   2. A polymerisation reaction that produces a molecule of water for every two monomers that bond.
   3. The bond that forms between an alcohol and a carboxylic acid monomer during a polymerisation reaction.
   4. The name given to a protein monomer.
   5. A six-carbon sugar monomer.
   6. The monomer of DNA, which determines the sequence of amino acids that will make up a protein.
2. For each of the following questions, choose the one correct answer from the list provided.
   1. A polymer is made up of monomers, each of which has the formula CH ${}_2$ =CHCN. The formula of the polymer is:
      1. -(CH ${}_2$ =CHCN) ${}_n$ -
      2. -(CH ${}_2$ -CHCN) ${}_n$ -
      3. -(CH-CHCN) ${}_n$ -
      4. -(CH ${}_3$ -CHCN) ${}_n$ -
   2. A polymer has the formula -[CO(CH ${}_2$ ) ${}_4$ CO-NH(CH ${}_2$ )6NH] ${}_n$ -. Which of the following statements is true ?
      1. The polymer is the product of an addition reaction.
      2. The polymer is a polyester.
      3. The polymer contains an amide linkage.
      4. The polymer contains an ester linkage.
   3. Glucose...
      1. is a monomer that is produced during cellular respiration
      2. is a sugar polymer
      3. is the monomer of starch
      4. is a polymer produced during photosynthesis
3. The following monomers are involved in a polymerisation reaction:

   

   1. Give the structural formula of the polymer that is produced.
   2. Is the reaction an addition or condensation reaction?
   3. To what group of organic compounds do the two monomers belong?
   4. What is the name of the monomers?
   5. What type of bond forms between the monomers in the final polymer?
4. The table below shows the melting point for three plastics. Suggest a reason why the melting point of PVC is higher than the melting point for polyethene, but lower than that for polyester.

   Plastic	Melting point ( ${}^0$ C)
   Polyethene	105 - 115
   PVC	212
   Polyester	260

5. An amino acid has the formula H ${}_2$ NCH(CH ${}_2$ CH ${}_2$ SCH ${}_3$ )COOH.
   1. Give the structural formula of this amino acid.
   2. What is the chemical formula of the carbon side chain in this molecule?
   3. Are there any peptide bonds in this molecule? Give a reason for your answer.