Inorganic ions are simple charged particles, but four of them carry roles the rest of biology depends on. AQA names hydrogen, iron, sodium, and phosphate. Each ion does one specific job and is examined on that job alone.
Hydrogen ions set the pH of biological fluids.
Hydrogen ions are protons in aqueous solution, and their concentration sets the pH of the fluid. The relationship is inverse and logarithmic: each unit drop on the pH scale represents a tenfold increase in hydrogen ion concentration. pH 7 is neutral; below 7 is acidic with more H⁺; above 7 is alkaline with less H⁺.
Hydrogen ions compete for the charged R groups on amino acid residues. The ionic bonds and hydrogen bonds that hold the tertiary structure of every protein are disrupted, and the active site loses its complementary shape. Enzyme activity falls. Blood, cytoplasm, and extracellular fluid all hold their H⁺ concentration tightly for this reason.
Iron ions inside haemoglobin reversibly bind oxygen.
Haemoglobin is the oxygen-carrying protein of red blood cells. Its quaternary structure consists of four polypeptide subunits, and each subunit contains a non-protein haem group at its core. At the centre of each haem group sits a single iron atom.
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iron ions are a component of haemoglobin. Don't writeiron is haemoglobin; the iron sits at the centre of each haem group, not as the whole protein.
Each Fe²⁺ ion forms a reversible bond with one oxygen molecule. A haemoglobin molecule carries four haem groups, so it transports up to four oxygen molecules at a time. The bond is loaded where the oxygen partial pressure is high, in the lungs, and released where the partial pressure is low, at respiring tissues.
Iron must be in the ferrous (Fe²⁺) state to bind oxygen; Fe³⁺ haemoglobin (methaemoglobin) cannot bind oxygen at all.
Sodium ions drive the co-transport of glucose and amino acids.
Sodium ions are central to co-transport, also called secondary active transport. The system couples the movement of two different substances through a single membrane protein on the luminal surface of an intestinal epithelial cell.
- The sodium-potassium pump uses ATP to extrude Na⁺ from the epithelial cell, creating a low cell-interior Na⁺ concentration and a steep electrochemical gradient.
- A co-transporter protein on the luminal membrane binds one sodium ion and one glucose molecule (or one amino acid) at the same time.
- Na⁺ flows into the cell down its electrochemical gradient, and the released energy is coupled to the uptake of glucose or the amino acid against its concentration gradient.
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co-transportorsecondary active transport. Don't writeactive transportwithout qualification; co-transport runs on the sodium gradient, not on direct ATP hydrolysis at the transporter itself.
Both substances move in the same direction through the same protein, which is what defines a symporter.
Phosphate ions form the backbone of DNA, RNA, ATP, and phospholipids.
Phosphate ions carry the widest structural role of the four spec ions. The role appears in three macromolecule classes, and in each case the contribution is structural rather than catalytic.
Pitfall — Stay on the ion, not the macromolecule
A 3.1.8 question asks about the role of the ion.
If the question is about phosphate in DNA, you are being asked to name its structural role at the backbone level, not to explain phosphodiester chemistry (that belongs to 3.1.5) and not to describe replication or transcription. The same rule holds for iron, where cooperativity and the dissociation curve are out of scope, and for sodium, where the sodium-potassium pump's biochemistry is out of scope.
The backbone of DNA and RNA is an alternating sequence of sugar and phosphate. Each phosphate bridges two adjacent nucleotides through a phosphodiester linkage, and this backbone is what holds the polynucleotide chain together.
ATP is adenine, ribose, and three phosphate groups in series. The bonds between adjacent phosphates are chemically unstable, and hydrolysis of the terminal phosphate releases the energy that drives endergonic reactions. The three-phosphate structure is what makes ATP the cell's immediate energy currency.
A phospholipid has a glycerol backbone, two fatty acid tails, and a head group containing a phosphate. The phosphate's negative charge makes the head hydrophilic, while the tails stay hydrophobic. This amphipathic character is what allows phospholipids to form the membrane bilayer.
Key terms
- hydrogen ions
- pH
- iron ions
- haemoglobin
- sodium ions
- co-transport
- phosphate ions
- DNA
- ATP
- phospholipid