Monomers & Polymers (OCR A Level Biology)

• There is a massive variety of life within and between organisms however the biochemical basis of life is similar for all living things
• The key molecules that are required to build structures that enable organisms to function are:
  • Carbohydrates
  • Proteins
  • Lipids
  • Nucleic Acids
  • Water

• Monomers are the smaller units from which larger molecules are made
• Polymers are molecules made from a large number of monomers joined together in a chain
• Carbon compounds can form small single subunits (monomers) that bond with many repeating subunits to form large molecules (polymers) by a process called polymerisation
• Macromolecules are very large molecules
  • They contain 1000 or more atoms and so have a high molecular mass
  • Polymers can be macromolecules, however, not all macromolecules are polymers as the subunits of polymers have to be the same repeating units

Covalent bonding

• A covalent bond is the sharing of two or more electrons between two atoms
  • The electrons can be shared equally forming a nonpolar covalent bond or unequally (where an atom can be more electronegative δ^-) to form a polar covalent bond

• Generally, each atom will form a certain number of covalent bonds due to the number of free electrons in the outer orbital e.g. H = 1 bond, C = 4 bonds
• Covalent bonds are very stable as high energies are required to break the bonds
• Multiple pairs of electrons can be shared forming double bonds (e.g. unsaturated fats C=C) or triple bonds

Different types of covalent bonds

• When two monomers are close enough that their outer orbitals overlap this results in their electrons being shared and a covalent bond forming. If more monomers are added then polymerisation occurs (and / or a macromolecule forms)

Condensation

• Also known as dehydration synthesis (‘to put together while losing water’)
• A condensation reaction occurs when monomers combine together by covalent bonds to form polymers (polymerisation) or macromolecules (lipids) and water is removed

Written and symbolic illustrations of the removal of water to form a covalent bond between two or more monomers during a condensation reaction

Hydrolysis

• Hydrolysis means ‘lyse’ (to break) and ‘hydro’ (with water)
• In the hydrolysis of polymers, covalent bonds are broken when water is added

Written and symbolic illustrations of the addition of water to break down covalent bond/s during a hydrolysis reaction

Covalent Bonds in Organic Molecules Table

• Carbohydrates, lipids, proteins and nucleic acids contain the chemical elements carbon (C) and hydrogen (H) making them organic compounds
• Carbon atoms are key to organic compounds because:
  • Each carbon atom can form four covalent bonds – this makes the compounds very stable (as covalent bonds are so strong they require a large input of energy to break them)
  • Carbon atoms can form covalent bonds with oxygen, nitrogen and sulfur
  • Carbon atoms can form straight chains, branched chains or rings

Carbohydrates

• All carbohydrates contain the chemical elements C, H and O
• As H and O atoms are always present in the ratio of 2:1 (eg. water H₂O, which is where ‘hydrate’ comes from in 'carbohydrate') they can be represented by the formula C_x (H₂O)_y
• The three types of carbohydrates are monosaccharides, disaccharides and polysaccharides
• Carbohydrates have many different functions:
  • Source of energy e.g. glucose is used for energy-release during cellular respiration
  • Store of energy e.g. glycogen is stored in the muscles and liver of animals
  • Structurally important e.g. cellulose in the cell walls of plants

Types of Carbohydrates Table

Lipids

• All lipids contain the chemical elements C, H and O
  • However, the proportion of O in lipids is low compared to carbohydrates

• There are many types of lipids, including triglycerides (fats and oils), phospholipids, waxes, and steroids (such as cholesterol)
• Lipids have many different functions:
  • Source of energy that can be respired (lipids have a high energy yield)
  • Store of energy e.g. lipids are stored in animals as fats in adipose tissue and in plants as lipid droplets
  • Insulating layer e.g. thermal insulation under the skin of mammals and electrical insulation around nerve cells
  • An essential component of biological membranes

Proteins

• Like carbohydrates and lipids, all proteins contain the chemical elements C, H and O
  • However, all proteins also contain N (nitrogen) and some proteins contain S (sulphur)

• Proteins have many different functions:
  • Required for cell growth, cell repair and the replacement of biological materials
  • Structurally important e.g. in muscles, collagen and elastin in the skin, collagen in bone and keratin in hair
  • Proteins can also act as carrier molecules in cell membranes, antibodies, enzymes or hormones

Nucleic Acids

• Like carbohydrates, lipids and proteins, all nucleic acids contain the chemical elements C, H and O
  • However, all nucleic acids also contain N (nitrogen) in their bases and P (phosphorous) in the form of phosphate groups

• Nucleic acids (DNA and RNA) have one function:
  • Carrying the genetic code in all living organisms
  • Nucleic acids are essential in the control of all cellular processes including protein synthesis

The key biological molecules for living organisms