Inorganic Ions, Osmoregulation and DNA Probes for Genetic Screening Revision Notes | A Level AQA Biology

1. Setting the Scene: Why Ions, Water, and DNA Probes Belong Together

You have spent the last two parts of this series building up genetic fingerprinting from the ground up. Now, in Part 3, the focus shifts to three interconnected ideas: the inorganic ions dissolved in body fluids, the water potential those fluids must maintain, and the DNA probe technology that lets scientists read specific alleles from a genome. Together, these threads feed directly into genetic screening — one of the most ethically charged applications in modern biology.

Each of these topics appears deceptively simple on its own. The challenge — and the exam opportunity — is connecting them precisely and using the correct terminology under pressure. That is exactly what this lesson trains you to do.

2. Inorganic Ions and Osmoregulation: Foundation Concepts for Genetic Screening

Picture the cytoplasm of a cell and the plasma of blood as a carefully managed solution. Dissolved within that solution are inorganic ions — charged particles such as sodium ($\text{Na}^+$), potassium ($\text{K}^+$), chloride ($\text{Cl}^-$), hydrogen carbonate ($\text{HCO}_3^-$), calcium ($\text{Ca}^{2+}$), and hydrogen ($\text{H}^+$). The AQA specification states clearly that some of these ions occur in high concentrations and others in very low concentrations, and that distinction matters enormously for function.

Consider sodium and potassium as a paired example. Sodium ions are maintained at high concentration outside cells (in the blood plasma and tissue fluid) but at low concentration inside the cytoplasm. Potassium ions show the reverse pattern — high inside, low outside. This electrochemical gradient is not accidental; it is actively maintained by the sodium–potassium pump and is essential for generating nerve impulses and muscle contractions.

Hydrogen ions ($\text{H}^+$) occur at very low concentrations in blood plasma — a pH of 7.4 corresponds to a hydrogen ion concentration of roughly $4 \times 10^{-8}$ mol dm$^{-3}$. Even tiny shifts in this concentration alter enzyme activity and protein conformation. Calcium ions ($\text{Ca}^{2+}$) are present at low cytoplasmic concentrations but act as second messengers when released in pulses, triggering muscle contraction and exocytosis. The key principle is that concentration is not arbitrary — it is precisely regulated because function depends on it.

• $\text{Na}^+$ — high in plasma, low in cytoplasm; maintains resting membrane potential
• $\text{K}^+$ — high in cytoplasm, low in plasma; essential for repolarisation of neurones
• $\text{Ca}^{2+}$ — very low in cytoplasm at rest; released as a signalling ion
• $\text{H}^+$ — extremely low in blood (pH 7.4); tightly buffered to protect enzyme function
• $\text{Cl}^-$ — high in plasma; contributes to osmotic balance and charge neutrality
• $\text{HCO}_3^-$ — major buffer in blood; transports $\text{CO}_2$ from tissues to lungs

This brings us directly to the next concept. Because inorganic ions are solutes, their concentration determines the water potential of the fluid they are dissolved in. Regulating ion concentration and regulating water potential are therefore two sides of the same coin.

3. Osmoregulation: Controlling Blood Water Potential

Osmoregulation is defined by AQA as the control of the water potential of the blood. Water potential ($\Psi$) is a measure of the tendency of water molecules to move by osmosis — the more solutes dissolved, the lower (more negative) the water potential. If blood water potential falls too low, water leaves cells by osmosis and they shrink. If it rises too high, water enters cells and they may burst. Either extreme is lethal.

The kidneys are the primary osmoregulatory organs. When blood water potential falls — for example after sweating heavily or eating a salty meal — osmoreceptors in the hypothalamus detect the change. The posterior pituitary gland releases more antidiuretic hormone (ADH). ADH increases the permeability of the collecting duct and distal convoluted tubule to water, so more water is reabsorbed by osmosis into the blood. The result is a small volume of concentrated urine and a restored blood water potential.

When blood water potential rises — after drinking a large volume of water — ADH secretion falls. The collecting duct becomes less permeable to water, less water is reabsorbed, and a large volume of dilute urine is produced. This negative feedback loop keeps blood water potential within a narrow, safe range.

Notice how inorganic ions feed directly into this story. A rise in plasma sodium concentration lowers blood water potential, triggering the same ADH response described above. Osmoregulation and ion regulation are therefore inseparable in practice, even though the specification treats them as distinct points.

4. DNA Probes and Hybridisation: Reading the Genome

Now the lesson pivots to gene technology. Suppose you want to know whether a particular individual carries a specific allele — perhaps the allele associated with cystic fibrosis, or a variant that affects how they metabolise a drug. You cannot see alleles with the naked eye. You need a molecular tool that can find a specific sequence among the three billion base pairs of the human genome. That tool is a labelled DNA probe used in DNA hybridisation.

A DNA probe is a short, single-stranded piece of DNA whose sequence is complementary to the target allele you want to detect. It is labelled — either with a fluorescent dye or a radioactive isotope — so that wherever it binds, you can see it. The process of the probe binding to its complementary target sequence is called DNA hybridisation, and it works because complementary bases form hydrogen bonds: adenine pairs with thymine, and guanine pairs with cytosine.

Worked example: How DNA Hybridisation Locates a Specific Allele

Describe the sequence of steps by which a labelled DNA probe is used to determine whether a patient carries a specific disease allele.

1. DNA is extracted from the patient's cells (e.g. white blood cells). The double-stranded DNA is denatured — heated or treated with alkali — to separate the two strands, producing single-stranded DNA. — Single-stranded DNA is required because the probe must be able to access and bind to the complementary base sequence. Double-stranded DNA would block probe access.
2. The single-stranded DNA fragments are fixed to a nylon membrane (in Southern blotting) or spotted onto a DNA microarray chip, so they cannot wash away during the hybridisation step. — Immobilising the target DNA ensures that any probe binding is stable and detectable. Microarrays allow thousands of alleles to be screened simultaneously.
3. A solution containing the labelled probe — a short, single-stranded DNA sequence complementary to the target allele — is washed over the membrane or microarray under controlled temperature and salt conditions. — Temperature and salt concentration are carefully controlled to ensure the probe only binds to sequences that are a close match. Too high a temperature and the probe will not bind at all; too low and it may bind to non-complementary sequences (non-specific binding).
4. Where the probe sequence is complementary to the target allele, hydrogen bonds form between the base pairs (A–T and G–C), and the probe hybridises (binds) to the target strand. — This is the key step. The specificity of base pairing means the probe will only hybridise to its exact complementary sequence — this is what makes the technique allele-specific.
5. Excess, unbound probe is washed off the membrane or microarray, leaving only probe molecules that have successfully hybridised to the target sequence. — This washing step is critical for reducing background signal. Any probe that has not formed stable hydrogen bonds with a complementary sequence is removed.
6. If a fluorescent probe was used, the membrane or microarray is viewed under UV light — a glowing spot indicates the target allele is present. If a radioactive probe was used, X-ray film (autoradiography) is exposed to the membrane and a dark band appears where the probe has bound. — The presence of a signal confirms that the patient's DNA contains the target allele. Absence of signal means the allele was not detected. A positive result does not automatically mean the individual will develop a condition — they may be a carrier (heterozygous).

Answer: A labelled DNA probe hybridises to its complementary target allele via hydrogen bonding after the patient's DNA has been denatured to single strands. Detection of the fluorescent or radioactive label confirms the presence of the allele.

One detail that trips students up is the difference between detecting the allele and diagnosing the condition. A probe tells you whether the allele is present — it does not tell you whether the individual will be affected. For a recessive condition such as cystic fibrosis, a person could carry one copy of the allele (heterozygous carrier) and show no symptoms. Interpreting probe results always requires knowledge of the inheritance pattern of the condition.

5. Genetic Screening: Applications and Evaluation

DNA probes and hybridisation are the technical foundation of genetic screening — the testing of individuals or populations for specific alleles. Screening programmes exist for a range of purposes: identifying carriers of inherited conditions before they have children, diagnosing conditions in newborns (neonatal screening), testing embryos produced by IVF (preimplantation genetic diagnosis), and predicting how a patient will respond to a particular drug (pharmacogenomics).

The AQA specification requires you to evaluate information about screening. That means you need to be able to weigh up evidence, consider both benefits and limitations, and reach a reasoned conclusion. The table below structures the key arguments you should be able to deploy.

When an exam question asks you to evaluate, the mark scheme rewards a structured response: state a benefit with a specific example, state a limitation with a specific example, and then reach a conclusion that weighs the two sides. Vague statements such as 'it could be good or bad' will not score evaluation marks. You need to commit to a reasoned judgement.

Pharmacogenomics deserves particular attention because it links gene technology directly to clinical practice. Different alleles of genes encoding drug-metabolising enzymes (such as cytochrome P450 variants) mean that the same drug dose can be therapeutic in one patient, ineffective in another, and toxic in a third. Screening for these alleles before prescribing allows doctors to select the right drug and dose from the outset, reducing adverse reactions and improving outcomes.

6. Exam Technique: Securing Every Mark

The four specification points in this lesson each carry distinct command-word demands in exam questions. Getting the command word right is the difference between a full-mark response and one that misses the point entirely.

For 'describe' questions about osmoregulation or DNA hybridisation, sequence matters. Write what happens first, then what happens next, using connective words such as 'this causes', 'as a result', and 'consequently'. Do not explain why at this stage — save that for 'explain' questions. For 'explain' questions, every observation must be followed by a mechanistic reason using 'because' or 'therefore'.

For 'evaluate' questions on genetic screening, the mark scheme almost always requires: at least one developed benefit, at least one developed limitation, and a conclusion. A conclusion that restates the question ('so screening can be useful') will not score the evaluation mark. Your conclusion must weigh the specific evidence you have presented and commit to a judgement.

Across all four specification points in this lesson — inorganic ions, osmoregulation, DNA probes, and genetic screening — the unifying theme is precision: precise ion concentrations maintain water potential; precise base complementarity makes probes allele-specific; and precise evaluation of evidence is what earns marks in extended-response questions. Carry that precision into the exam and you will be well placed to access the top mark bands.