Introduction to Inorganic Ions Revision Notes | A Level AQA Biology

1. Why Inorganic Ions Matter

Your blood carries oxygen to every cell in your body — and that process depends entirely on four iron ions, one in each of the four haem groups within each haemoglobin molecule. That is the power of inorganic ions: tiny charged particles dissolved in your body fluids, each with a precise job to do.

Inorganic ions occur in solution in the cytoplasm and body fluids of organisms. Some are present in high concentrations — sodium ions in blood plasma, for instance — while others, such as iron(II) ions, are present in very low concentrations. The concentration at which an ion is found reflects how much of it is needed and how tightly the body regulates it.

The key principle underpinning this entire topic is that each type of ion has a specific role, and that role depends directly on the ion's chemical properties — its charge, its size, and its ability to form or break bonds. Understanding the link between property and function is what separates a grade-7 answer from a grade-9 one.

2. Ions in Solution: The Biological Context

Picture the inside of a cell as a busy aqueous environment. Dissolved within that water are dozens of different ions, each moving freely and interacting with proteins, membranes, and other molecules. This is what chemists mean when they say ions occur in solution — they are dissociated from their parent compounds and exist as free, charged particles.

The distinction between high-concentration and low-concentration ions matters enormously. Sodium ($\text{Na}^+$) and chloride ($\text{Cl}^-$) ions are present in millimolar quantities in blood plasma and are involved in bulk processes such as maintaining osmotic balance and generating nerve impulses. Iron(II) ions ($\text{Fe}^{2+}$), by contrast, are present in micromolar quantities — yet without them, haemoglobin cannot carry a single molecule of oxygen.

This tells us something important: concentration does not equal importance. A low-concentration ion can be absolutely critical to survival, which is why the body invests significant energy in regulating even trace quantities of certain ions.

3. Key Inorganic Ions and Their Specific Roles

Now that the context is clear, the focus shifts to the specific ions you need to know for AQA A Level. For each one, the approach is the same: identify the ion, state its role, and — crucially — explain why its chemical properties make it suited to that role. That causal chain is what examiners reward.

3a. Iron(II) Ions — Oxygen Transport

The iron(II) ion ($\text{Fe}^{2+}$) sits at the centre of the haem group within each haemoglobin molecule. Its role is to bind reversibly to oxygen ($\text{O}_2$), forming oxyhaemoglobin in the lungs and releasing oxygen in respiring tissues. The word reversibly is critical here — if the bond were permanent, oxygen could never be delivered.

The property that makes $\text{Fe}^{2+}$ suited to this role is its ability to form a co-ordinate bond with oxygen without being permanently oxidised to $\text{Fe}^{3+}$. Carbon monoxide (CO) does not oxidise Fe2+ but instead binds to Fe2+ with extremely high affinity, preventing oxygen from binding and forming carboxyhaemoglobin, which cannot transport oxygen. This is why CO is so dangerous.

3b. Phosphate Ions — Energy, Nucleotides, and Membranes

The phosphate ion ($\text{PO}_4^{3-}$) is one of the most versatile ions in biology. It forms part of the sugar-phosphate backbone of DNA and RNA, making it essential for storing and transmitting genetic information. It is also a component of ATP (adenosine triphosphate), where the energy released by hydrolysing the terminal phosphate bond drives almost every energy-requiring process in the cell.

Phosphate ions are additionally found in phospholipids, where they form the hydrophilic head of the phospholipid bilayer. The negative charge of the phosphate group is what makes the head polar and therefore attracted to water — a property that is fundamental to membrane structure and function.

3c. Hydrogen Ions — pH and Enzyme Activity

Hydrogen ions ($\text{H}^+$) determine the pH of a solution. A high concentration of $\text{H}^+$ means a low pH (acidic); a low concentration means a high pH (alkaline). This matters enormously in biology because enzymes are highly sensitive to pH — even small changes in $\text{H}^+$ concentration can alter the ionisation of amino acid R-groups, distorting the active site and reducing or abolishing enzyme activity.

Hydrogen ions are also central to ATP synthesis. In both mitochondria and chloroplasts, a concentration gradient of $\text{H}^+$ ions (a proton gradient) is established across a membrane. As $\text{H}^+$ ions flow down this gradient through ATP synthase, the energy released drives the phosphorylation of ADP to ATP. This process — chemiosmosis — is the primary mechanism of ATP production in the later stages of aerobic respiration (oxidative phosphorylation) and in photophosphorylation, though substrate-level phosphorylation also contributes.

3d. Sodium Ions — Co-transport and Nerve Impulses

Sodium ions ($\text{Na}^+$) are present at high concentration in the blood plasma and extracellular fluid, and at low concentration inside cells. This electrochemical gradient is maintained by the sodium-potassium pump and is exploited in two major ways. First, in co-transport: glucose and amino acids are absorbed into intestinal epithelial cells by co-transporter proteins that use the inward movement of $\text{Na}^+$ down its concentration gradient to drive the simultaneous uptake of glucose or amino acids against their own concentration gradients.

Second, the rapid influx of $\text{Na}^+$ through voltage-gated channels is responsible for the depolarisation phase of an action potential in neurones. The positive charge of $\text{Na}^+$ flooding into the axon reverses the membrane potential from approximately $-70$ mV to $+40$ mV, generating the electrical signal that propagates along the nerve.

3e. Potassium Ions — Resting Potential and Stomatal Opening

Potassium ions ($\text{K}^+$) work in close partnership with sodium ions. In neurones, $\text{K}^+$ efflux through voltage-gated channels is responsible for repolarisation — restoring the membrane potential back towards $-70$ mV after an action potential. Without this, neurones could not fire repeatedly.

In plants, $\text{K}^+$ ions are actively pumped into guard cells surrounding stomata. This lowers the water potential of the guard cells, causing water to enter by osmosis. The resulting increase in turgor pressure causes the guard cells to bow outwards, opening the stomatal pore and allowing gas exchange and transpiration. When $\text{K}^+$ ions leave, the reverse happens and the stomata close.

3f. Calcium Ions — Muscle Contraction and Cell Signalling

Calcium ions ($\text{Ca}^{2+}$) are present at very low concentrations in the cytoplasm under resting conditions — typically around $10^{-7}$ mol dm$^{-3}$ — but can rise rapidly when a cell is stimulated. This sharp rise in $\text{Ca}^{2+}$ concentration acts as an intracellular signal. In muscle cells, $\text{Ca}^{2+}$ binds to troponin, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin filaments, allowing cross-bridge formation and contraction to proceed.

Calcium ions are also involved in the release of neurotransmitters at synapses. When an action potential reaches a synaptic knob, voltage-gated $\text{Ca}^{2+}$ channels open. The influx of $\text{Ca}^{2+}$ triggers the fusion of synaptic vesicles with the presynaptic membrane, releasing neurotransmitter into the synaptic cleft. The low resting concentration of $\text{Ca}^{2+}$ inside the cell is what makes this signal so precise and rapid.

4. Comparing Ion Concentrations and Their Significance

One of the spec's core requirements is recognising that some ions occur at high concentrations and others at very low concentrations. The table below summarises the key ions, their approximate concentration range, and their primary roles — a useful revision reference and a structure you can adapt in a 'compare' exam question.

Notice the pattern: ions involved in rapid signalling (calcium, hydrogen) are kept at very low resting concentrations so that a sudden increase produces a clear, detectable signal. Ions involved in bulk transport or osmotic balance (sodium, potassium) are maintained at higher concentrations because large quantities are needed to generate the electrochemical gradients that drive these processes.

5. Linking Ion Properties to Function: The Exam Reasoning Chain

Examiners frequently ask you to explain why a particular ion is suited to its role. This requires more than stating what the ion does — you must connect the ion's chemical properties to the biological outcome. The reasoning chain below models the approach for phosphate ions, and you can apply the same structure to any ion.

Worked example: Constructing an Exam Explanation: Phosphate Ions in ATP

Explain why the phosphate ion is suited to its role in ATP. [3 marks]

1. The phosphate ion ($\text{PO}_4^{3-}$) forms the terminal phosphate groups of ATP (adenosine triphosphate). — [B1] — Naming the molecule and the ion's position within it earns the first mark.
2. The bond between the terminal phosphate group and the rest of the ATP molecule is a high-energy phosphoanhydride bond that requires relatively little activation energy to hydrolyse. — [B1] — Linking the bond type to its energy characteristics is the property-to-function connection examiners look for.
3. When ATP is hydrolysed to ADP and inorganic phosphate ($\text{P}_i$), the energy released is sufficient to drive energy-requiring cellular processes such as active transport, muscle contraction, and biosynthesis. — [B1] — Naming specific processes and explaining the energy release completes the reasoning chain and secures the third mark.

Answer: The phosphate ion forms high-energy bonds in ATP; hydrolysis of the terminal phosphate releases energy that drives cellular processes including active transport and muscle contraction. [3 marks]

Apply this same three-step chain — role, property, biological outcome — to every ion. It works for iron(II) (reversible bonding to oxygen → oxygen transport), calcium (low resting concentration → sharp signal on release), and hydrogen ions (concentration determines pH → controls enzyme activity and drives chemiosmosis).

6. Exam Technique and Common Mistakes

Questions on inorganic ions appear across all three A Level Biology papers and can carry anywhere from one to six marks. The command word tells you exactly how much depth is expected. A 'state' question wants a single precise fact; an 'explain' question demands the property-to-function reasoning chain you practised above; an 'evaluate' question requires you to weigh evidence and reach a conclusion.

One final point on exam language: the specification states that each ion has a role that depends on its properties. If a question asks you to 'suggest' a role for an unfamiliar ion, look at its charge and size. A small, highly charged ion will interact strongly with water and polar molecules; a large ion with a low charge will be less reactive. Use these principles to reason your way to a plausible answer even in novel contexts.

To consolidate the full picture: inorganic ions occur in solution in the cytoplasm and body fluids of organisms at concentrations ranging from high (sodium, potassium) to very low (calcium, iron(II), hydrogen), and each ion's specific role is determined by its chemical properties — a principle that applies whether you are explaining oxygen transport, membrane structure, enzyme activity, or ATP synthesis.