Molecules

Water

In a water molecule each hydrogen atom shares a pair of electrons with the oxygen atom, forming a covalent bond. In a water molecule the hyrdogen has a slight positive charge and the oxygen has a slight negative charge. Molecules with an uneven distribution of charge are described as being polar.

Molecules with charged groups, such as glucose, amino acis and ions dissolve in water with the polar water molecules readily forming clusters around them.

Buffers and pH

Protein shape and function is affected by changes in pH. While most proteins opertate most effectively in the normal cellular pH of around 7, some have a very different optimum pH. The stomach enzyme pepsin has an optimum pH of around 2.

Buffers are chemicals or substances that resist changes to pH and ensure that a particular enviornment maintains a particular pH. The buffers help regulate the pH of solutions to ensure that the enzymes involved are operating at their optimum pH.

Ions

Ions are atoms that have an electrical charge due to the number of protons and electrons not balencing. Atoms of metals tend to lose electrons and therefore become positively charged ions. Atoms of non-metals tend to be able to gain electrons, thus becoming negatively charged.

Biologically important ions inlclude:

Calcium - in plants calcium pectate is an essential component of the middle lamella of plant cell walls. Calcium is an essential component of bones and teeth in animals, and is essential is blood clotting and muscle contraction.

Iron - is part of the haem group in haemoglobin and an important constituent of electron carriers in respiration.

Magnesium - is essential in giving chlorophyll its light-absorbing properties

Potassium - is important in maintaining electrical gradients across neurones     

Nitrate - is a component of amino acids, nucleic acids and chlorophyll

phosphate - as phospholipids, phosphate is an important component of cell membranes.

hydrogencarbonate - is important as a natural buffer

Organic Moleclues

Organic molecules are complex carbon-containing molecules. Carbohydrates, proteins and lipids are all examples of organic molecules. Many organic molecules are formed of sub-units called monomers, which join together to form larger polymers through the process of polymerisation.