(p) the key inorganic ions that are involved in biological processes

The idea

Teach the key inorganic ions required for OCR A Level Biology H420: hydrogen (H⁺), sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), ammonium (NH₄⁺), nitrate (NO₃⁻), hydrogencarbonate (HCO₃⁻), chloride (Cl⁻), phosphate (PO₄³⁻) and hydroxide (OH⁻). For each ion, link its charge and size to a named biological role, e.g. H⁺ controls pH and enzyme activity; Na⁺ co-transports glucose in the ileum; K⁺ maintains resting potential in neurones; Ca²⁺ triggers muscle contraction and exocytosis; Mg²⁺ is central in chlorophyll; NH₄⁺ is a nitrogen source for plants; NO₃⁻ is absorbed for amino acid synthesis; HCO₃⁻ buffers blood and neutralises stomach acid in the duodenum; Cl⁻ balances charge and forms HCl in the stomach; PO₄³⁻ is in ATP, DNA and phospholipids; OH⁻ contributes to alkaline conditions.

What earns the marks

• Hydrogen ions (H⁺) determine pH; a higher H⁺ concentration lowers pH and can denature enzymes by disrupting hydrogen and ionic bonds.
• Sodium ions (Na⁺) are involved in co-transport of glucose and amino acids across cell membranes, e.g. in the ileum, and in generating action potentials in neurones.
• Potassium ions (K⁺) maintain the resting potential of neurones and are actively pumped into cells by the sodium–potassium pump.
• Calcium ions (Ca²⁺) trigger muscle contraction by binding to troponin and stimulate exocytosis of neurotransmitters at synapses.
• Magnesium ions (Mg²⁺) are the central ion in chlorophyll, essential for light absorption in photosynthesis.
• Ammonium ions (NH₄⁺) are a source of nitrogen for plants, taken up by roots and used to synthesise amino acids and proteins.
• Nitrate ions (NO₃⁻) are absorbed by plants and reduced to ammonium before incorporation into amino acids and nucleotides.
• Hydrogencarbonate ions (HCO₃⁻) act as a buffer in blood and neutralise stomach acid in the duodenum, protecting the intestinal lining.
• Chloride ions (Cl⁻) balance positive charges across membranes and are secreted into the stomach to form hydrochloric acid.
• Phosphate ions (PO₄³⁻) are components of ATP, DNA, RNA and phospholipids, and are involved in phosphorylation reactions.
• Hydroxide ions (OH⁻) contribute to alkaline conditions, e.g. in the small intestine, and are part of the water dissociation equilibrium.

Common mistakes and how to put them right

• Confusing the role of sodium and potassium.

  Put it right: Na⁺ is higher outside the cell and enters during depolarisation, while K⁺ is higher inside and leaves during repolarisation.

• Stating that magnesium is part of haemoglobin.

  Put it right: Magnesium is in chlorophyll, whereas iron is in haemoglobin.

• Saying that nitrate is used directly to make proteins.

  Put it right: Nitrate must first be reduced to ammonium before incorporation into amino acids.

• Believing that hydroxide ions neutralise stomach acid in the small intestine.

  Put it right: Hydrogencarbonate is the main neutraliser, while OH⁻ contributes to alkaline conditions.

• Forgetting the charge on phosphate.

  Put it right: It is PO₄³⁻, not PO₄²⁻ or PO₄⁻, and this charge affects its binding in ATP and DNA.

Exam tips

• Learn the ion, its charge and one precise biological role as a linked set; exam questions often ask for the ion when given a function.
• Use the context of the question to identify the ion: muscle contraction points to Ca²⁺, chlorophyll to Mg²⁺, resting potential to K⁺.
• Be precise with charges: write H⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, NH₄⁺, NO₃⁻, HCO₃⁻, Cl⁻, PO₄³⁻, OH⁻.
• For buffer questions, name hydrogencarbonate (HCO₃⁻) rather than a vague 'buffer ion'.
• If asked about plant mineral nutrition, distinguish between nitrate (NO₃⁻) and ammonium (NH₄⁺) as nitrogen sources.