Inorganic Ions and Their Biological Roles

Although organic molecules — carbohydrates, lipids, proteins and nucleic acids — dominate biological mass, inorganic ions are equally essential to life. They may be present in small concentrations, but without them no cell can function. This lesson covers OCR specification point 2.1.2 (e): the roles of inorganic ions including Ca²⁺, Na⁺, K⁺, H⁺, NH₄⁺, NO₃⁻, HCO₃⁻, Cl⁻, PO₄³⁻ and OH⁻.

1. What is an Inorganic Ion?

An inorganic ion is a charged atom or group of atoms that does not contain carbon-hydrogen bonds. Inorganic ions are also called minerals in dietary contexts. They are classified by requirement:

Macrominerals — required in amounts greater than 100 mg per day (e.g., Ca²⁺, Na⁺, K⁺, Cl⁻, Mg²⁺, PO₄³⁻).
Trace elements — required in amounts less than 100 mg per day (e.g., Fe²⁺/³⁺, Zn²⁺, Cu²⁺, I⁻, Se).

Ions can be cations (positively charged, e.g., Na⁺, K⁺, Ca²⁺) or anions (negatively charged, e.g., Cl⁻, NO₃⁻, PO₄³⁻).

Key Definition — Inorganic ion: A charged atom or small group of atoms, not containing carbon-hydrogen bonds, that plays a specific role in biological processes.

2. Cations

2.1 Calcium Ion (Ca²⁺)

Structural: component of calcium phosphate (hydroxyapatite) in bones and teeth, giving them hardness and compressive strength.
Muscle contraction: Ca²⁺ released from the sarcoplasmic reticulum binds to troponin, exposing myosin-binding sites on actin and triggering the sliding filament mechanism.
Synaptic transmission: Ca²⁺ entry into presynaptic terminals triggers the fusion of synaptic vesicles with the membrane and the release of neurotransmitter.
Blood clotting: Ca²⁺ (Factor IV) is essential in several steps of the clotting cascade, including the activation of prothrombin to thrombin.
Second messenger: Ca²⁺ acts as an intracellular messenger in many signal transduction pathways.
Plant cell walls: calcium pectate in the middle lamella cements adjacent plant cells.

2.2 Sodium Ion (Na⁺)

Nerve impulses: Na⁺ influx through voltage-gated channels generates the rising phase of the action potential (depolarisation).
Resting potential maintenance: the Na⁺/K⁺ ATPase pump maintains low intracellular [Na⁺] and high intracellular [K⁺], establishing the resting potential of cells.
Co-transport: Na⁺ gradient drives secondary active transport of glucose and amino acids across intestinal and renal tubule epithelia (Na⁺-glucose co-transporter, SGLT1).
Osmotic balance: Na⁺ is the main extracellular cation and contributes to blood osmolarity and blood pressure regulation.
Water reabsorption: Na⁺ reabsorption in the nephron drives water reabsorption via osmosis.

2.3 Potassium Ion (K⁺)

Nerve impulses: K⁺ efflux through voltage-gated channels produces the repolarisation phase of the action potential.
Resting potential: high intracellular [K⁺] (maintained by Na⁺/K⁺ ATPase) is the basis of the negative resting membrane potential (approximately −70 mV).
Stomatal opening in plants: guard cells actively take up K⁺, lowering their water potential. Water follows by osmosis, making the cells turgid and opening the stoma.
Protein synthesis: K⁺ is required for the binding of aminoacyl-tRNA to ribosomes during translation.
Cardiac function: abnormal blood K⁺ (hyper- or hypokalaemia) disrupts heart rhythm and can be fatal.

2.4 Hydrogen Ion (H⁺)

pH regulation: [H⁺] defines pH. Cellular reactions are extremely sensitive to pH; enzymes have optimum pH values, and deviations disrupt ionic bonds and hydrogen bonds in protein structure.
Chemiosmosis: proton gradients across the inner mitochondrial membrane (respiration) and thylakoid membrane (photosynthesis) drive ATP synthesis through ATP synthase. This process — the flow of H⁺ down their electrochemical gradient — is the fundamental mechanism of ATP production in aerobic respiration and photosynthesis.
Haemoglobin function (Bohr effect): H⁺ binds to haemoglobin, lowering its affinity for O₂ and promoting O₂ unloading in respiring tissues.
Carbon dioxide transport: CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺; the H⁺ is buffered by haemoglobin.
Hydrolysis reactions: H⁺ acts as a catalyst in many acid-catalysed hydrolysis reactions (e.g., in the stomach).

2.5 Ammonium Ion (NH₄⁺)

Nitrogen source: in plants, NH₄⁺ can be absorbed from the soil and assimilated into amino acids via glutamine synthetase.
Nitrogen cycle intermediate: ammonium is produced by ammonification (decomposition of nitrogen-containing organic matter by decomposers and saprobionts) and oxidised to nitrite (NO₂⁻) and then nitrate (NO₃⁻) by nitrifying bacteria (Nitrosomonas, Nitrobacter).
Deamination: in animals, ammonium is produced from excess amino acids in the liver, then rapidly converted to urea (mammals), uric acid (birds and reptiles) or excreted directly as ammonia (freshwater fish).
pH effects: NH₄⁺ ⇌ NH₃ + H⁺ — the equilibrium contributes to pH regulation in some organisms.

3. Anions

3.1 Nitrate Ion (NO₃⁻)

Nitrogen source for plants: NO₃⁻ is absorbed from soil by root hairs and reduced to NH₃ (via NO₂⁻), then incorporated into amino acids, nucleotides and chlorophyll.
Essential for protein synthesis: plants cannot make amino acids — and therefore proteins — without a nitrogen source.
Growth and yield: nitrate deficiency causes stunted growth, yellowing of older leaves (chlorosis) due to reduced chlorophyll production, and low yield — it is one of the three macronutrients in fertilisers (the N in NPK).
Nitrogen cycle: nitrate is produced by nitrifying bacteria; denitrifying bacteria (Pseudomonas) reduce it back to N₂ gas under anaerobic conditions.

Lesson 11: Inorganic Ions and Their Biological Roles