Monomers & Polymers (OCR A Level Biology): Revision Note
Monomers & Polymers
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
Examiner Tips and Tricks
When discussing monomers and polymers, give the definition but also name specific examples eg. a nucleic acid is a polymer, made of nucleotide monomers.Remember, lipid molecules are not made from monomers or polymers as each fatty acid joins to a glycerol molecule, rather than to each other. Separate lipid molecules, such as triglycerides, are not held together by covalent bonds and therefore lipids cannot be classed as polymers.
Chemical Elements in Biological Molecules
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 Cx (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
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