Inorganic ionsAQA A-Level Biology: Revision notes

Inorganic ions are charged particles that do not contain carbon (apart from simple ones such as hydrogencarbonate). They occur in solution in the cytoplasm and body fluids such as blood plasma and tissue fluid.

Some ions occur in high concentrations, for example sodium ions in blood plasma (about 140 mmol dm⁻³), and others in very low concentrations, for example iron ions in free solution. A low concentration does not mean a minor role.

Each type of ion has a specific role that depends on its properties, such as its charge, its ability to bind other molecules and whether it is acidic.

The pH of a solution is determined by the concentration of hydrogen ions (H⁺). The higher the concentration of H⁺, the lower the pH and the more acidic the solution. A lower concentration gives a higher pH (more alkaline); pH 7 is neutral.

Acids release hydrogen ions into solution. For example, lactic acid made in anaerobic respiration releases H⁺, so the pH of muscle cytoplasm falls.

Cells keep pH within narrow limits because hydrogen ions affect the structure and function of enzymes and other proteins.

Iron(II) ions (Fe²⁺) are a component of haemoglobin, the red pigment in red blood cells. Each haemoglobin molecule contains four haem groups, and each haem group has one iron ion that binds reversibly to an oxygen molecule.

Oxygen binds in the lungs and is released in respiring tissues. A diet deficient in iron means less haemoglobin can be made, so less oxygen is carried by the blood (iron-deficiency anaemia).

Sodium ions (Na⁺) are involved in the co-transport of glucose and amino acids across the cell-surface membrane of the epithelial cells of the ileum.

1. The sodium–potassium pump actively transports Na⁺ out of the epithelial cell into the blood, using ATP. This lowers the Na⁺ concentration inside the cell.
2. A concentration gradient for Na⁺ now exists from the lumen to the cell.
3. Na⁺ diffuse into the cell through a co-transporter protein, which carries glucose (or an amino acid) in at the same time, even against its concentration gradient.
4. Glucose and amino acids leave the cell into the blood by facilitated diffusion, so absorption continues.

Oral rehydration drinks contain both glucose and sodium ions for this reason.

Phosphate ions (PO₄³⁻) are components of two key molecules.

• DNA (and RNA): each nucleotide contains a phosphate group, which joins nucleotides together in the sugar–phosphate backbone.
• ATP: adenosine triphosphate contains three phosphate groups. ATP is hydrolysed to ADP and an inorganic phosphate ion (Pᵢ), releasing energy; ATP is re-formed by adding Pᵢ to ADP.

Plants lacking phosphate grow poorly because they cannot make DNA for new cells or enough ATP for growth.