Water
- Water molecules consist of 2 hydrogen molecules covalently to an oxygen molecule.
- The molecules are slightly polar because the oxygen nucleus pulls the shared electrons away from the hydrogen nuclei. Giving the oxygen nuclei a δ- charge, and the hydrogen nuclei a δ+ charge.
- The polarity of water causes attraction between water molecules. This force of attraction is called a hydrogen bond.
Property of Water | Why is it useful |
Liquid medium | Provides habitats for aquatic organisms, medium for chemical reactions & used for transport |
Important metabolite | Used in hydrolysis & condensation reactions |
High specific heat capacity | Keeps aquatic & cellular environments stable |
High latent heat of vaporisation | Evaporation has a cooling effect on organisms |
Cohesion of molecules | Water is drawn up the xylem |
Surface tension | Allows pond-skaters to walk on the surface |
Good solvent and transport medium | Dissolves ionic and polar molecules, allowing them to easily be transported |
Good reaction medium | The cytoplasm in cells is an aqueous solution where many chemical reactions happen |
Incompressible | Can prevent plants from wilting & act as a hydrostatic skeleton for invertebrates |
Monomers & Polymers
- Monomers are individual molecules that make up a polymer.
- Polymers are long chains that are composed of many individual monomers that have been bonded together in a repeating pattern.
- Condensation Reactions occurs when two molecules combine to form a more complex molecule with the removal of water.
- Hydrolysis Reactions occurs when larger molecules are broken down into smaller molecules with the addition of water.
Carbohydrates
- Monosaccharides are the simplest carbohydrates, consisting of only one sugar molecule (e.g. Glucose, Fructose & Galactose)
- Ribose sugars (pentose) are found in many important biological molecules such as ribonucleic acid (RNA), ATP, NAD
- Glucose is a hexose sugar with 2 isomers
- Disaccharides are sugars that are composed of two monosaccharides joined together in a condensation reaction, forming a glycosidic bond.
Disaccharide | Constituent monosaccharides |
Maltose | 2 × a-glucose |
Sucrose | a-glucose and fructose |
Lactose | b-glucose and galactose |
- Polysaccharides are formed by many monosaccharides joined together.
- Amylose, amylopectin (starch) is the main polysaccharide energy store in plants, is composed of α-glucose.
- In animals, the polysaccharide energy store is called glycogen, composed of α-glucose.
- Cellulose is a structural component of plant cell walls, composed of long unbranched chains of b-glucose.
Lipids
- Fatty acids can be:
- Saturated – there are no C=C bonds and the molecule has as many hydrogen atoms as possible.
- Unsaturated – there is at least one C=C bond, therefore the molecule contains fewer hydrogen atoms than is maximally possible.
- A triglyceride molecule is formed by joining one molecule of glycerol to three fatty acids through three condensation reactions, forming ester bonds.
- Triglycerides have key roles in respiration and energy storage due to its insolubility and high carbon to hydrogen ratio.
- Phospholipids replace one of the fatty acid chains in triglycerides with a phosphate molecule.
- The hydrophobic tails and hydrophilic heads of phospholipids allow them to form phospholipid bilayers
Proteins
- Amino acids are the monomer units used to make proteins.
- The 20 naturally occurring amino acids only differ in their R groups
- Dipeptides are formed when two amino acids are joined together by a condensation reaction, forming a peptide bond.
- A polypeptide is a polymer made of many amino acids joined together by peptide bonds.
- A protein may contain one or more polypeptide chains.
- There are four structural levels:
Level | Definition | Bond Type |
Primary | The specific sequence of amino acids in a polypeptide chain | Peptide Bonds |
Secondary | The curling or folding of the polypeptide chain into α-helices and β-pleated sheets due to the formation of hydrogen bonds | Hydrogen Bonds |
Tertiary | The overall specific 3-D shape of a protein, which is determined by interactions between R groups and the properties of R groups | Hydrogen bonds Ionic bonds Disulphide bridges |
Quaternary | The specific 3-D shape of a protein that is determined by the multiple polypeptide chains and/or prosthetic groups bonded together | Hydrogen bonds Ionic bonds Disulphide bridges |