Key Inorganic Ions and Their Roles Revision Notes | A Level AQA Biology

1. Why Inorganic Ions Matter

Your body contains dozens of different ions, and without them, almost nothing works. Oxygen would not be carried in the blood, glucose could not enter intestinal cells, and DNA could not hold its shape. Four ions sit at the heart of the AQA specification: hydrogen ions ($\text{H}^+$), iron ions ($\text{Fe}^{2+}$), sodium ions ($\text{Na}^+$), and phosphate ions ($\text{PO}_4^{3-}$).

Each ion has a precise, exam-testable role. The goal of this lesson is to connect each ion to its biological context so clearly that you can explain, compare, and apply that knowledge under exam conditions.

2. Hydrogen Ions and pH

pH is a measure of hydrogen ion concentration. The lower the pH, the higher the concentration of $\text{H}^+$ ions in solution. This relationship is logarithmic: a drop of one pH unit represents a tenfold increase in $\text{H}^+$ concentration.

Why does this matter biologically? Enzymes are proteins whose active sites depend on a precise three-dimensional shape. Hydrogen ions interact with the R-groups of amino acids, altering the charges and hydrogen bonding that maintain that shape. A shift in $\text{H}^+$ concentration can denature an enzyme or reduce its affinity for its substrate — which is why blood pH is tightly regulated between 7.35 and 7.45.

Hydrogen ions are also directly involved in ATP synthesis. During oxidative phosphorylation, $\text{H}^+$ ions are pumped across the inner mitochondrial membrane, creating a concentration gradient. Their movement back through ATP synthase drives the phosphorylation of ADP to ATP — a process called chemiosmosis. The hydrogen ion gradient is the driving force.

3. Iron Ions and Haemoglobin

Haemoglobin is a quaternary protein made of four polypeptide chains, each containing a haem group. At the centre of each haem group sits a single iron(II) ion, $\text{Fe}^{2+}$. This ion is the actual site where oxygen binds.

One haemoglobin molecule contains four haem groups, so it can carry up to four oxygen molecules simultaneously. The binding is reversible: in the lungs, where oxygen partial pressure is high, $\text{Fe}^{2+}$ binds oxygen to form oxyhaemoglobin. In respiring tissues, where oxygen partial pressure is low, oxygen is released and haemoglobin is regenerated.

The iron ion does not change its oxidation state during normal oxygen transport — it remains $\text{Fe}^{2+}$ throughout. This is worth noting because carbon monoxide binds to the same $\text{Fe}^{2+}$ site with a much higher affinity than oxygen (about 200 times greater), very strongly blocking it and effectively preventing oxygen transport. That is why carbon monoxide poisoning is so dangerous.

Iron deficiency leads to anaemia — a condition in which there are insufficient functional haemoglobin molecules to carry adequate oxygen. Symptoms include fatigue and breathlessness, directly reflecting the reduced oxygen delivery to tissues. This clinical consequence illustrates how a single ion underpins a critical physiological function.

4. Sodium Ions and Co-transport of Glucose and Amino Acids

Glucose and amino acids are too large and too polar to cross the phospholipid bilayer by simple diffusion. In the epithelial cells lining the small intestine, their absorption depends on a co-transport mechanism that exploits the sodium ion gradient.

Here is how the system works. The sodium-potassium pump ($\text{Na}^+/\text{K}^+$ ATPase) on the basolateral membrane uses ATP to actively transport sodium ions out of the epithelial cell. This keeps the intracellular sodium concentration low. On the apical (lumen-facing) membrane, a co-transporter protein couples the movement of $\text{Na}^+$ down its concentration gradient into the cell with the simultaneous movement of glucose or an amino acid against its concentration gradient into the cell.

The sodium ion gradient is the energy source for glucose and amino acid uptake. The $\text{Na}^+$ ions effectively 'pull' the nutrient molecules into the cell. Once inside, glucose and amino acids leave the epithelial cell via facilitated diffusion through the basolateral membrane into the blood. Sodium ions are continuously recycled by the pump to maintain the gradient.

Worked example: Tracing the Co-transport Pathway for Glucose

Describe, in sequence, how a glucose molecule moves from the intestinal lumen into the blood, with reference to sodium ions.

1. $\text{Na}^+/\text{K}^+$ ATPase pumps $\text{Na}^+$ out of the epithelial cell using ATP hydrolysis. — This creates a low intracellular $\text{Na}^+$ concentration — the essential precondition for co-transport.
2. $\text{Na}^+$ moves down its concentration gradient into the cell via the co-transporter protein, carrying glucose with it against glucose's concentration gradient. — The energy stored in the $\text{Na}^+$ gradient (generated by ATP) drives glucose uptake. This is secondary active transport.
3. Glucose leaves the epithelial cell through carrier proteins on the basolateral membrane, moving down its concentration gradient into the blood capillary. — No energy is required at this step — the glucose concentration inside the cell is now higher than in the blood.
4. $\text{Na}^+$ that entered the cell is pumped back out by $\text{Na}^+/\text{K}^+$ ATPase, maintaining the gradient for continued co-transport. — Without this recycling, the gradient would dissipate and co-transport would cease.

Answer: Glucose moves from the lumen into the epithelial cell via co-transport with $\text{Na}^+$ (secondary active transport), then exits into the blood by facilitated diffusion. The $\text{Na}^+$ gradient, maintained by the $\text{Na}^+/\text{K}^+$ ATPase, provides the driving force.

5. Phosphate Ions in DNA and ATP

Phosphate ions ($\text{PO}_4^{3-}$) appear in two of the most important molecules in biology: DNA and ATP. Their role in each is structurally and functionally distinct, which makes this a favourite area for compare-style exam questions.

In DNA, each nucleotide contains a deoxyribose sugar, a nitrogenous base, and a phosphate group. The phosphate group of one nucleotide forms a covalent bond with the sugar of the next, creating the sugar-phosphate backbone. This backbone runs along the outside of the double helix, providing structural stability and directionality (the 5' to 3' orientation). The negative charges on the phosphate groups also repel each other, which — combined with the hydrogen bonds between complementary bases — contributes to the overall architecture of the molecule.

In ATP (adenosine triphosphate), three phosphate groups are linked in sequence to the adenosine unit. The bonds between the phosphate groups—called phosphoanhydride bonds—are high-energy bonds, with the terminal one (between the second and third phosphates) most frequently hydrolysed in cells. Hydrolysis of any phosphoanhydride bond releases energy, but the terminal bond is the one typically broken to form ADP and inorganic phosphate ($\text{P}_i$). The phosphate ion released ($\text{P}_i$) can also be transferred to another molecule in a process called phosphorylation, activating that molecule.

A key point for exam answers: phosphate ions are structural components of DNA (forming the backbone) and functional components of ATP (storing and releasing energy). These are different roles, and conflating them will cost marks.

6. Bringing the Four Ions Together

Now that each ion has been examined individually, it is worth stepping back to see the pattern. Each ion is effective precisely because of its charge and its ability to interact with specific biological molecules or proteins. The charge on $\text{H}^+$ disrupts enzyme tertiary structure; the $\text{Fe}^{2+}$ in haem provides a reversible binding site for oxygen; the $\text{Na}^+$ gradient powers nutrient uptake; and the $\text{PO}_4^{3-}$ group both scaffolds DNA and stores energy in ATP.

7. Exam Technique and Common Mistakes

Questions on inorganic ions tend to use command words such as 'explain', 'describe', 'compare', and 'suggest'. Each demands a different response structure. For 'explain' questions, always include a mechanism — not just a statement of what happens, but why it happens at the molecular or ionic level.

For 'compare' questions on phosphate ions in DNA versus ATP, structure your answer around named similarities and differences. Both molecules contain phosphate groups covalently bonded to a sugar; the key difference is that in DNA the phosphate is structural, whereas in ATP it is functional (energy storage and transfer). Stating both sides explicitly is what earns the marks.