Molecules
Molecules and living organisms
The elements present in the largest proportions in living organisms are carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur. The first three are the most abundant. Atoms of the element found in living organisms combine to form biologically important molecules linked by strong covalent bonds.
Water
In a water molecule each hydrogen atom shares a pair of electrons with the oxygen atom forming a covalent bond.
The hydrogen has a slight positive charge while the oxygen has a slight negative charge
The molecule is described as being polar which is a term used to describe a molecule with an uneven distribution of charge
As the water molecules stick together it allows them to be cohesive
Structure
The polar nature of the water results in the formation of hydrogen bonds between adjacent water molecules
The hydrogen bonds extend between the oxygen on one water molecule and a hydrogen atom on another
Hydrogen bonds are important in many biological molecules but although common they are not strong and relatively easily broken
Water is an excellent solvent capable of dissolving a wide range of organic and inorganic substances
Molecules with charged groups such as glucose and amino acids and ions dissolve in water with the polar water molecules readily forming clusters around them.
Water forms between 50% to over 90% of the cells in a living organism. Its value as a solvent includes:
Water is needed for chemical reactions. Most of a cells reactions take place in an aqueous solution
Its ability to act as transport medium in living organisms. Water allows substances to dissolve to be transported.
The different effects it has on hydrophobic and hydrophilic molecules
Cohesion - Water molecules stick to each other. This is due to the hydrogen bonds among the molecules. Water molecules at the surface have a much greater attraction for each other than for molecules in the air. This cohesiveness creates a high surface tension at the surface of the water. Water molecules at the surface hold closely together, forming an invisible film. Aquatic insects such as water strider or pond skater rely on surface tension to walk on the water.
The hydrogen bonds in water exert a significant attractive force causing water to cling to itself (Cohesive) and to other surfaces (Adhesive).
Element - A substance which contains only one type of atom.
Solvent - Substance that dissolves a solute to form a solution.
Buffers and PH
Buffers are chemicals or substances that resist changes to pH and ensure that a particular environment maintains a particular pH. Examples include hydrogen carbonate ions and blood proteins such as albumin.
Buffers help to regulate the pH of solutions to ensure that the enzymes involved are operating at their optimum pH.
Ions
Ions are atoms or groups of atoms that have an electrical charge due to the number of protons and electrons not balancing. Atoms of metals tend to lose electrons therefore become positively charged while atoms of non-metals tend to be able to gain electrons thus becoming negatively charged.
Ion
Biological compound
Role
Calcium
(Ca 2+)
Calcium pectate
Calcium Phosphate
In plants Calcium Pectate is an essential component of the middle lamella in plant cell walls. Calcium is an essential component of bones and teeth in animals and is essential in blood clotting and muscle contraction.
Iron
(Fe 2+)
Haemoglobin
Is part of the haem group in haemoglobin and an important constituent of electron carriers in respiration.
Magnesium
(Mg 2+)
Chlorophyll
Is essential in giving chlorophyll its light absorbing properties
Potassium
(K+)

It is important in maintaining electrical gradient across neurones
Nitrate
(NO3 -)

Is a component of amino acids, nucleic acids and chlorophyll.
Phosphate
(PO4 3-)
Calcium Phosphate
Phospholipids
As phospholipids, phosphate is the main component of cell membranes. It’s also a major component of other important biological