CCEA A-Level Biology

Water as a solvent

A water molecule is polar. The oxygen atom pulls the shared electrons towards itself, so it carries a small negative charge and each hydrogen atom a small positive charge. Neighbouring water molecules are attracted to each other by hydrogen bonds between the oxygen of one and a hydrogen of another. Because of this polarity, water is an excellent solvent for other polar substances and for ions.

When an ionic compound such as sodium chloride is placed in water, the positive ends of water molecules cluster round the negative ions and the negative ends cluster round the positive ions. Each ion becomes surrounded by a shell of water molecules and is pulled away from the crystal into solution. Polar molecules such as glucose, amino acids and urea dissolve because they form hydrogen bonds with water. Substances that dissolve readily in water are hydrophilic.

Non-polar substances such as fats and oils cannot form these attractions, so they do not dissolve and are hydrophobic. This matters in cells, because the hydrophobic interior of the phospholipid bilayer stops dissolved ions and polar molecules leaking freely across a membrane.

Being a solvent makes water useful to living organisms in several ways.

• Metabolic reactions happen between dissolved substances, because molecules in solution move freely and collide. The cytoplasm is an aqueous solution in which most reactions take place.
• Dissolved substances can be transported. Blood plasma carries glucose, amino acids, ions and urea, xylem carries mineral ions dissolved in water up the plant, and phloem carries dissolved sugars.
• Waste products such as urea dissolve and can be removed in urine.
• Substances can be absorbed and secreted. Mineral ions dissolved in soil water are taken up by root hairs, and digestive products dissolved in the gut contents are absorbed.
• Gases such as oxygen and carbon dioxide dissolve in the thin film of water lining gas exchange surfaces, and they move across the surface in solution.

Inorganic ions

Ions are dissolved in the body fluids of organisms and in the cytoplasm. Each ion has particular uses, and some are built into larger biological compounds.

Ions as components of biologically important compounds

Calcium pectate is a calcium salt of pectin, a polysaccharide. It forms the middle lamella, the sticky layer that cements the cellulose cell walls of neighbouring plant cells together.

Magnesium ions are needed to make chlorophyll, which is why plants short of magnesium turn yellow and photosynthesise poorly. Iron(II) ions are held in each haem group of haemoglobin, and it is the iron that combines reversibly with oxygen.

Phosphate is part of ATP (adenosine triphosphate), of the sugar-phosphate backbone of DNA and RNA, and of the head of each phospholipid molecule. Nitrate provides the nitrogen in amino groups and in the bases of nucleotides, and hydrogencarbonate is formed when carbon dioxide dissolves and reacts with water in blood.

Ions in osmotic and buffering systems

The concentration of dissolved ions affects the water potential of a solution. A higher concentration of solutes gives a lower water potential, so ions help set the direction in which water moves by osmosis. Cells and body fluids keep their ion concentrations within narrow limits, and plant cells take up ions by active transport to lower their water potential and draw in water.

A buffer is a solution that resists changes in pH when small amounts of acid or alkali are added. Hydrogencarbonate ions in blood plasma, and phosphate ions inside cells, can accept hydrogen ions when a fluid becomes too acidic and release them when it becomes too alkaline. Keeping pH steady matters because enzymes and other proteins depend on it (the physical chemistry of buffers is not needed).