BY1 Section 1 - Chemical elements are joined together to form biological compounds

INORGANIC IONS

Living organisms need a variety of inorganic ions to survive. These inorganic ions can also be referred to as minerals or electrolytes. Living organisms require them in two forms:

• Macronutrients, which are required in small concentrations.
• Micronutrients, which are required in minute concentrations.

For example:

• Magnesium ions, a constituent of chlorophyll, is needed by plants for photosynthesis.
• Iron ions, a constituent of haemoglobin, is needed to transport oxygen in the blood.
• Phosphate ions are a constituent of nucleotides, including ATP.
• Calcium ions are found in the cell walls of plants and the bones and teeth of mammals, providing strength. 

WATER

Water is a polar molecule due to its uneven distribution of electron density: the hydrogen atoms have a partial positive charge, whilst the oxygen atom has a partial negative charge. This causes water molecules to readily attract each other, resulting in the formation of hydrogen bonds. On their own, these hydrogen bonds are weak, but when many are present, they are strong and give water a number of unique properties. 

Water molecules are dipoles, as they attract other charged molecules, such as ions, and other polar molecules such as glucose. These molecules can then dissolve in water, allowing chemical reactions to take place in solution. This effectively makes water an excellent transport medium: for example, water transports minerals in the xylem.

Water is also a metabolite, and is involved in numerous biochemical reactions:

E.G. Condensation reactions:

Glucose + fructose --> sucrose + water

Hydrolysis:

Maltose + water --> glucose + glucose 

Water has a high latent heat of vaporisation. This means a significant amount of heat energy is required to change water from a liquid to vapour state. This is important for temperature control, as demonstrated when we sweat. The water in our sweat evaporates from our skin using the heat energy we are giving off, thus cooling us down.

Water also has a high specific heat capacity. This means a substantial amount of heat energy is required to raise the temperature of water. This is due to the hydrogen bonds between water molecules, restricting their movement. Therefore, large fluctuations in kinetic energy and temperature and prevented, creating stable environments for aquatic animals to live. 

Water has high cohesive properties. As previously mentioned, water molecules are bonded by hydrogen bonds. When many water molecules are present, they 'stick together' in a strong lattice framework. This 'sticking together' is called cohesion, and allows columns of water to be drawn up the xylem vessels of plants. What's more, the water molecules on the surface stick together in such a way that surface tension is created, which can support the body of an insect.

Water has a high density. This is important as it provides organisms with support and buoyancy. Furthermore, ice is less dense than water, and can therefore float. This is because the water molecules in ice are generally held further apart, thus less dense. Ice is an insulator, and so creates stable environments beneath it where aquatic organisms…