What are inorganic ions?
An ion is an atom or group of atoms that has gained or lost electrons, so it carries a charge. Cations are positive (Na⁺, K⁺, Ca²⁺, Fe²⁺, H⁺); anions are negative (Cl⁻, PO₄³⁻, HCO₃⁻, NO₃⁻). Inorganic means they are not built on carbon chains. They dissolve in water in the cytoplasm, blood and tissue fluid.
They are needed in small (sometimes trace) amounts, and each has a specific role linked to its properties.
Iron ions (Fe²⁺) and haemoglobin
Haemoglobin is a protein with 4 polypeptide chains. Each chain holds a haem group with an Fe²⁺ ion in its centre. Each Fe²⁺ binds one O₂, so one haemoglobin carries up to 4 O₂ (8 oxygen atoms). The iron stays as Fe²⁺: oxygen binds and is released without the iron being oxidised.
Low iron causes iron-deficiency anaemia: less haemoglobin, less oxygen carried, tiredness and paleness.
Sodium ions (Na⁺) and co-transport
In the small intestine (ileum), a sodium-potassium pump uses ATP to push Na⁺ out of the epithelial cell into the blood. This keeps Na⁺ low inside the cell. Na⁺ then diffuses in from the gut through a co-transporter protein, and the protein carries a glucose (or amino acid) molecule in at the same time, even against the glucose gradient. Glucose then moves into the blood by facilitated diffusion.
Na⁺ also helps create nerve impulses and helps control water balance (osmosis).
Phosphate ions (PO₄³⁻): ATP, DNA and membranes
- ATP: adenine + ribose + 3 phosphates. Hydrolysis of the last phosphate bond (ATP → ADP + Pi) releases energy; phosphate can be added to other molecules to make them more reactive (phosphorylation).
- DNA and RNA: phosphate links nucleotides by phosphodiester bonds, forming the sugar-phosphate backbone.
- Phospholipids: the phosphate head is hydrophilic, forming the outer faces of every membrane.
Hydrogen ions (H⁺) and pH
pH = −log₁₀[H⁺]. More H⁺ means lower pH (more acidic). Each fall of 1 pH unit means 10 times more H⁺. H⁺ ions interact with charged R groups in proteins; a big change in [H⁺] breaks ionic and hydrogen bonds, changing the tertiary structure, so enzymes can denature. H⁺ from carbonic acid in busy tissues lowers haemoglobin's affinity for oxygen, helping release O₂ where it is needed (the Bohr effect). Buffers in blood keep pH near 7.4.
Try it
Check a food label for iron, sodium and phosphorus. Dissolve a pinch of salt in water and test whether it conducts with a battery and an LED (ions carry charge). In the 3D, slide the pH and watch the number of H⁺ ions and the enzyme shape.
Key formulas and definitions
- Ion: a charged atom or group (cation +, anion −)
- Fe²⁺: haem in haemoglobin, 1 O₂ per Fe²⁺, 4 per haemoglobin
- Na⁺: co-transport of glucose / amino acids; nerve impulses
- PO₄³⁻: ATP, DNA/RNA backbone, phospholipids
- H⁺: pH = −log₁₀[H⁺]; 1 pH unit = 10× change in [H⁺]
- ATP + H₂O → ADP + Pi + energy
Worked examples
1. How many oxygen molecules can 100 haemoglobin molecules carry?
Each has 4 Fe²⁺ ions, each binding 1 O₂, so 4 per molecule. 100 × 4 = 400 O₂ molecules.
2. Solution A has pH 6, solution B has pH 3. How many times more H⁺ does B have?
Difference = 3 pH units. 10³ = 1000 times more H⁺.
3. Why does a drug that stops the sodium-potassium pump reduce glucose absorption?
Without the pump, Na⁺ builds up inside the cell, so there is no Na⁺ gradient into the cell. The co-transporter needs Na⁺ to flow in to carry glucose, so glucose uptake falls.
4. Name the bond that phosphate forms between nucleotides and the molecule it builds.
Phosphodiester bond; it forms the sugar-phosphate backbone of DNA and RNA.
5. An enzyme works best at pH 7. Explain what happens at pH 2.
At pH 2 there are many more H⁺ ions. They disturb ionic and hydrogen bonds between R groups, the tertiary structure and active site change shape, substrates no longer fit, and the enzyme denatures.
Common mistakes
- Saying iron "carries oxygen in the blood plasma". Iron is inside haemoglobin in red blood cells.
- Mixing up phosphate (PO₄³⁻) with phosphorus (P) atoms or with phospholipid as a whole.
- Thinking higher pH means more H⁺. It is the opposite: higher pH, fewer H⁺.
- Saying glucose co-transport uses ATP directly. ATP powers the Na⁺/K⁺ pump; the co-transporter uses the Na⁺ gradient (indirect active transport).