Inorganic Ions

AQA spec ref: 3.1.6 - Inorganic ions

Inorganic ions are charged atoms or small molecules that do not contain carbon-hydrogen bonds. They are present in cells and body fluids either dissolved in the aqueous solution (as free ions) or bound to organic molecules, and they are essential to a wide range of biological processes. AQA requires you to know specific examples and their precise roles.

What are Inorganic Ions?

An ion is an atom or group of atoms that has gained or lost electrons, giving it a net electrical charge. Cations have a positive charge (have lost electrons); anions have a negative charge (have gained electrons). Inorganic ions are those that do not form part of organic macromolecules in the way that carbon does - they exist as free ions or are loosely associated with organic molecules.

Their concentration in body fluids and cells is maintained by transport proteins in cell membranes (e.g. ion pumps, ion channels) and by hormonal regulation. The precise balance of ions affects osmotic potential, enzyme activity, membrane potential, and the structure of biological molecules.

Key Inorganic Ions and Their Roles

Iron ions (Fe²⁺ / Fe³⁺)

Iron exists in two oxidation states in biology:

Fe²⁺ (ferrous) is the form incorporated into the haem group of haemoglobin and myoglobin. The iron ion sits at the centre of the porphyrin ring and reversibly binds oxygen:

Each haemoglobin molecule has four haem groups → carries four O₂ molecules. The iron does not change oxidation state during O₂ binding (it remains Fe²⁺) - the bond is reversible and non-covalent.

Iron ions are also found in the cytochrome proteins of the electron transport chain in mitochondria. Here, iron does cycle between Fe²⁺ and Fe³⁺, acting as an electron carrier during oxidative phosphorylation.

Iron deficiency → insufficient haemoglobin synthesis → fewer/smaller red blood cells → reduced O₂ carrying capacity → anaemia.

Phosphate ions (HPO₄²⁻ / H₂PO₄⁻)

Phosphate is one of the most widely used inorganic ions in biology. Its roles include:

• Nucleotides and nucleic acids - phosphate groups form the phosphodiester backbone of DNA and RNA. Each nucleotide has one or more phosphate groups. The negative charge on the phosphate backbone makes DNA hydrophilic and soluble in the nuclear environment.
• ATP - three phosphate groups in ATP. Energy is released when the terminal phosphate bond is hydrolysed.
• Phospholipids - the phosphate-containing head group of phospholipids is hydrophilic, giving the phospholipid bilayer its amphipathic structure.
• Phosphorylation of proteins - adding a phosphate group to a protein (via protein kinases) changes its shape and activity. This is a key mechanism of signal transduction.
• Buffering - phosphate buffers (H₂PO₄⁻/HPO₄²⁻) help maintain intracellular pH.

Hydrogen ions (H⁺)

H⁺ ions (protons) are produced and consumed in virtually every metabolic reaction:

• pH determination - the concentration of H⁺ ions defines pH (pH = −log[H⁺]). Enzyme function depends on pH because H⁺ concentration affects the ionisation state of amino acid R groups at the active site.
• Chemiosmosis - H⁺ ions are pumped across the inner mitochondrial membrane (and thylakoid membrane) during electron transport, creating an electrochemical gradient. The flow of H⁺ back through ATP synthase drives ATP synthesis. This is the central mechanism of oxidative phosphorylation and photophosphorylation. See Respiration and Photosynthesis.
• Haemoglobin and the Bohr effect - increased H⁺ concentration (lower pH) in metabolically active tissues reduces haemoglobin's affinity for O₂, promoting O₂ release where it is needed.
• Secondary active transport - H⁺ gradients drive the co-transport of other molecules (e.g. H⁺/sucrose symporters in phloem loading in plants).

Sodium ions (Na⁺)

• Cotransport - Na⁺ ions are used to drive the absorption of glucose and amino acids across the gut epithelium and kidney tubule cells. Na⁺ moves down its electrochemical gradient into the epithelial cell, and this movement is coupled to the active uptake of glucose (sodium-glucose cotransporter, SGLT). The Na⁺ gradient is maintained by the Na⁺/K⁺ ATPase pump.
• Membrane potential and nerve impulses - Na⁺ influx through voltage-gated Na⁺ channels depolarises the axon membrane, generating an action potential. See Neurons and Synapses.
• Osmotic regulation - Na⁺ is the dominant extracellular cation and is the primary determinant of extracellular osmotic pressure (plasma osmolality).

Chloride ions (Cl⁻)

• Charge balance - Cl⁻ is the main extracellular anion, balancing the positive charge of Na⁺.
• Chloride shift - in red blood cells, as CO₂ enters the cell and is converted to HCO₃⁻ (bicarbonate), Cl⁻ moves in to maintain electrical neutrality. This allows CO₂ to be transported in the plasma as bicarbonate.
• CFTR channel - the cystic fibrosis transmembrane conductance regulator is a Cl⁻ channel. Mutations in the CFTR gene result in thick, viscous mucus (because Cl⁻, and therefore water by osmosis, cannot leave cells properly).

Calcium ions (Ca²⁺)

• Muscle contraction - Ca²⁺ released from the sarcoplasmic reticulum binds to troponin on the actin filament, causing the troponin-tropomyosin complex to move and expose myosin binding sites. This triggers the cross-bridge cycle. See Skeletal Muscles as Effectors.
• Synaptic transmission - Ca²⁺ influx through voltage-gated calcium channels in the presynaptic membrane triggers vesicle fusion and neurotransmitter exocytosis. See Neurons and Synapses.
• Second messenger - Ca²⁺ acts as an intracellular second messenger in many signalling pathways.
• Bone and teeth - Ca²⁺ combined with phosphate forms hydroxyapatite, the mineral component of bone and teeth (not AQA Biology, but contextually useful).

Nitrate ions (NO₃⁻)

• Nitrogen source for plants - plants absorb nitrate from the soil via active transport (using H⁺/NO₃⁻ cotransporters). Nitrate is reduced to ammonium (NH₄⁺) and then incorporated into amino acids and nucleotides - the only way plants can obtain nitrogen for protein and nucleic acid synthesis.
• Nitrogen cycle - nitrate is the form of nitrogen most readily absorbed by plants. Its availability in soil depends on nitrifying bacteria (see T7 Overview).

Summary

AQA Exam Tips

• Iron in haemoglobin stays Fe²⁺ - it does not change oxidation state during O₂ binding. Contrast with iron in cytochromes, which cycles Fe²⁺ ↔ Fe³⁺ as an electron carrier.
• H⁺ and chemiosmosis - ATP synthase uses the energy of the H⁺ gradient, not the electrons themselves. The electrons pass to O₂ at the end of the chain.
• Na⁺ cotransport - two steps: (1) Na⁺/K⁺ ATPase uses ATP to establish the Na⁺ gradient; (2) the cotransporter uses this gradient to absorb glucose against its concentration gradient. The glucose absorption is secondary active transport.
• Ca²⁺ and muscle - the question is often "what happens to Ca²⁺ when a muscle relaxes?" Answer: Ca²⁺ is actively pumped back into the sarcoplasmic reticulum → troponin releases from Ca²⁺ → tropomyosin re-blocks binding sites → no cross-bridges form → muscle relaxes.
• Phosphate in nucleotides - don't just say "phosphate is in DNA." State that phosphate groups form the phosphodiester backbone, linking the 3' carbon of one deoxyribose to the 5' carbon of the next via phosphodiester bonds.