AQA A-Level Biology (7402)

## Learning Outcomes

- Define **inorganic ions** and identify where they occur in organisms
- Explain how the properties of different ions determine their biological roles
- Describe the role of **hydrogen ions** in pH and enzyme activity
- Explain why **iron ions** are essential in haemoglobin
- Identify the roles of **phosphate ions** in DNA, ATP and phospholipids
- Describe the importance of **magnesium ions** in chlorophyll and as enzyme cofactors
- Apply this knowledge in synoptic exam questions linking molecules, transport, enzymes and photosynthesis

## AQA A-Level Biology (7402) Syllabus

For the AQA A-Level Biology (7402), you are expected to understand inorganic ions in organisms, with a focus on the following syllabus points:

- Know that inorganic ions occur in solution in the cytoplasm of cells and in body fluids
- Understand that each ion has a specific role related to its properties
- Identify **hydrogen ions (H+)** as determining pH
- Explain how changes in pH affect enzymes and other proteins
- Identify **iron ions** as a component of haemoglobin
- Identify **phosphate ions (PO4^3-)** as components of DNA, ATP and phospholipids
- Understand that **magnesium ions (Mg2+)** are found in chlorophyll and can act as cofactors for some enzymes
- Apply these roles in synoptic contexts such as transport, respiration, photosynthesis and membrane structure

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is an inorganic ion?
2. How do hydrogen ions affect pH and enzyme activity?
3. Why do red blood cells contain a high concentration of iron ions?
4. Name three important biological molecules or structures that contain phosphate ions.
5. What are two roles of magnesium ions in living organisms?

## Introduction

Inorganic ions are small, but in biology their effects are large. They may be dissolved in cytoplasm and body fluids, or built into larger molecules where they help those molecules work properly. AQA often tests this topic in short recall questions, but on Paper 3 it can also appear in synoptic questions linking biological molecules to membranes, enzymes, gas transport and photosynthesis.

A strong answer does more than name an ion and its role. You need to explain how the ion's charge, position in a molecule, or concentration helps bring about a biological effect. That is what turns simple recall into AO2 application.

> **Key Term: inorganic ion**
> An ion that does not contain carbon and has a biological role in organisms, often occurring in solution in cytoplasm or body fluids, or as part of larger molecules.

### What inorganic ions are and why their roles differ

An ion is an atom, or group of atoms, with an electric charge. If it has a positive charge, it is a cation. If it has a negative charge, it is an anion. In biology, that charge matters because charged particles interact with water, proteins, membranes and other molecules.

Inorganic ions occur widely in living systems. They are present in the cytoplasm of cells, in blood plasma, in tissue fluid, in vacuoles, and in the fluids inside organelles. Some are present in relatively high concentrations, while others are effective even at very low concentrations. The amount present depends on function. For example, if an ion is used in large-scale transport or contributes to a stable chemical environment, concentration may need careful regulation.

The key idea is that each ion has a specific role because of its properties. These properties include:

- electrical charge
- ability to bind to other molecules
- solubility in water
- position within larger molecules
- effect on chemical conditions such as pH

This helps explain why different ions cannot simply replace one another. A phosphate ion cannot do the job of an iron ion, because the biological role depends on more than just being charged. It depends on the exact structure and chemical behaviour of the ion.

> **Key Term: cation**
> A positively charged ion.
>
> **Key Term: anion**
> A negatively charged ion.

A common mistake is to learn ions as an unconnected list. For synoptic biology, that is not enough. You should group them by type of role:

- ions that affect conditions: **H+**
- ions that form part of larger molecules: **Fe2+**, **PO4^3-**, **Mg2+**
- ions that help transport across membranes: in wider biology, ions such as **Na+**

That grouping helps when a question gives you an unfamiliar context and asks you to explain rather than just state.

> **Test Tip:** If a question says "state the role", give the precise specification wording first. If it says "explain", add the biological consequence. For example, do not stop at "hydrogen ions determine pH" if the question is about enzymes. Add that pH changes can alter bonding and active site shape.

### Hydrogen ions, pH and enzyme activity

Hydrogen ions, written as **H+**, are central to pH. The concentration of H+ in a solution determines how acidic or alkaline that solution is. A higher H+ concentration means a lower pH, so the solution is more acidic. A lower H+ concentration means a higher pH, so the solution is less acidic.

> **Key Term: pH**
> A measure based on the concentration of hydrogen ions in a solution.

The relationship is not linear. A change of one pH unit means a tenfold change in hydrogen ion concentration. This is why small pH changes can have major biological effects.

The equation is:

pH = -log10[H+]

This is useful in synoptic questions, especially where pH in organelles, blood, or mitochondria is being compared.

Worked example:

If the hydrogen ion concentration is 1.0 × 10^-4 mol dm^-3:

- pH = -log10(1.0 × 10^-4)
- pH = 4.0

If the hydrogen ion concentration is 1.0 × 10^-7 mol dm^-3:

- pH = -log10(1.0 × 10^-7)
- pH = 7.0

So the second solution is less acidic, because it has fewer hydrogen ions.

Why does this matter biologically? Because proteins contain amino acids with side chains that can gain or lose H+ depending on pH. This changes the charges on parts of the protein. Once charges change, ionic bonds may break or new ones may form. Hydrogen bonding patterns can also be affected. That can alter the tertiary structure of a protein.

In enzymes, a change in tertiary structure can alter the shape of the active site. If the active site no longer matches the substrate closely enough, fewer enzyme-substrate complexes form and the reaction rate falls. At more extreme pH values, the enzyme may become denatured.

The full chain of reasoning is:

- H+ concentration changes
- pH changes
- ionisation of amino acid side chains changes
- ionic and hydrogen bonds within the enzyme are disrupted
- tertiary structure changes
- active site shape changes
- substrate binds less effectively
- enzyme activity decreases

Students often miss the middle steps and jump straight from pH change to "enzyme stops working". Examiners prefer the mechanism.

> **Key Term: denaturation**
> A permanent change in a protein's tertiary structure, often caused by changes in temperature or pH, which alters its function.

This topic also links to compartmentalisation. Different parts of organisms and cells maintain different pH values because enzymes have different optimum pH levels. So H+ ions are not just a chemistry detail; they help control where particular reactions can happen most efficiently.

> **Test Tip:** Do not confuse hydrogen ions with hydrogen bonds. **H+** affects pH. A **hydrogen bond** is a weak attraction between slightly charged parts of molecules. These are different ideas and mixing them loses marks.

### Iron ions in haemoglobin

Iron ions are a component of haemoglobin. This exact wording matters. In AQA, the safest statement is that **Fe2+** is part of haemoglobin and is involved in oxygen transport.

> **Key Term: haemoglobin**
> A globular protein in red blood cells that transports oxygen.

Haemoglobin is a quaternary protein made of four polypeptide chains. Each chain contains a haem group, and at the centre of each haem group is an **Fe2+** ion. This means one haemoglobin molecule contains four iron ions and can bind up to four oxygen molecules.

The critical point is that oxygen binds reversibly to the Fe2+ in the haem group. Reversible binding means oxygen can attach where oxygen concentration is high, such as in the lungs, and detach where oxygen concentration is lower, such as in respiring tissues. This makes haemoglobin an effective transport molecule rather than a permanent oxygen store.

This explains why red blood cells have a high concentration of iron ions. They contain very large amounts of haemoglobin, and haemoglobin depends on Fe2+ to bind oxygen. Without sufficient iron, oxygen transport capacity falls.

A good two-mark explanation would usually include:

- red blood cells contain haemoglobin
- haemoglobin contains Fe2+ ions that bind oxygen for transport

A stronger synoptic explanation adds consequence:

- less Fe2+ means less effective oxygen transport
- tissues receive less oxygen for aerobic respiration
- ATP production may fall

Be careful with wording. Do not say "iron transports oxygen" on its own. Iron ions are part of haemoglobin; haemoglobin is the transport protein. Also avoid saying oxygen binds permanently. The binding must be reversible.

> **Key Term: reversible binding**
> Binding in which a molecule can attach and then later be released, allowing transport or temporary interaction.

### Phosphate ions in DNA, ATP and phospholipids

Phosphate ions, written as **PO4^3-**, are one of the most useful synoptic links in biology because they appear in several major biological molecules.

> **Key Term: phosphate ion**
> An inorganic ion, PO4^3-, that forms part of important biological molecules including DNA, ATP and phospholipids.

First, phosphate is part of DNA. In DNA, phosphate joins with deoxyribose sugar to form the sugar-phosphate backbone. This backbone gives the DNA strand structural stability and forms the outer part of the double helix. So if a question asks how phosphate contributes to DNA, do not just say "it is in DNA". State that it forms part of the sugar-phosphate backbone of the polynucleotide.

Second, phosphate is part of ATP. ATP consists of adenine, ribose and three phosphate groups. The role of ATP in energy transfer depends on these phosphate groups. When ATP is hydrolysed to ADP and inorganic phosphate, energy is released and can be coupled to energy-requiring reactions.

The full idea is:

- ATP contains phosphate groups
- hydrolysis removes one phosphate
- this releases energy
- the phosphate can also be used in phosphorylation
- phosphorylation can make another molecule more reactive

So phosphate is not just a structural detail in ATP; it is central to energy transfer in cells.

Third, phosphate is part of phospholipids. A phospholipid contains glycerol, fatty acids and a phosphate-containing group. The phosphate-containing head is polar and hydrophilic, while the fatty acid tails are non-polar and hydrophobic. This difference is what allows phospholipids to form bilayers in cell-surface membranes.

That means phosphate ions link directly to membrane structure and function. Without the phosphate-containing head, the amphipathic behaviour of phospholipids would not be the same, and membrane formation would not work in the same way.

A useful comparison is:

- in DNA, phosphate helps build a backbone
- in ATP, phosphate is central to energy transfer
- in phospholipids, phosphate helps create membrane structure

These are three distinct roles. Students often remember only one or two.

> **Test Tip:** If a question asks for the role of phosphate ions, match the explanation to the molecule named in the question. Do not give a generic answer about "energy" if the context is DNA, or about "backbone" if the context is ATP.

### Magnesium ions and synoptic links

Magnesium ions, **Mg2+**, are not always given as much space in short notes, but they are very useful in synoptic questions. The two key points are that magnesium ions are found in chlorophyll and can act as cofactors for some enzymes.

> **Key Term: magnesium ion**
> A divalent positive ion, Mg2+, found in chlorophyll and used as a cofactor for some enzymes.

Chlorophyll is the pigment involved in photosynthesis. Magnesium sits at the centre of the chlorophyll molecule. This means Mg2+ is directly linked to light absorption and therefore to photosynthesis. A question may ask you to connect mineral ions to reduced photosynthesis, and Mg2+ is the ion you need.

You should be precise here. Say that magnesium is present in chlorophyll, not just that it "helps photosynthesis". The mark is usually attached to the named structural role first.

Magnesium ions can also act as enzyme cofactors. A cofactor is a non-protein component that helps an enzyme function. In some cases, the cofactor helps the enzyme-substrate complex form by stabilising charges or assisting the correct shape of the active site. In others, it may help with the chemical steps of the reaction itself.

> **Key Term: cofactor**
> A non-protein substance that is required for the activity of some enzymes.

This gives Mg2+ two different kinds of biological role:

- structural role in chlorophyll
- functional role in enzyme activity

That distinction matters. Not all ions act in the same way. Some become part of a large molecule's structure, while others affect the conditions or reactions around them.

A useful synoptic chain for Mg2+ is:

- Mg2+ is part of chlorophyll
- chlorophyll absorbs light energy
- photosynthesis rate depends on effective light capture
- reduced Mg2+ can reduce chlorophyll function
- less photosynthesis means less glucose production

And for enzyme action:

- Mg2+ acts as a cofactor
- enzyme activity depends on cofactors in some reactions
- if the cofactor is absent, the reaction rate may decrease

### How to answer synoptic exam questions on inorganic ions

Inorganic ions are often tested in short, low-mark questions, but Paper 3 may place them inside bigger contexts. You might be given data on pH, a membrane transport scenario, a question on photosynthesis, or a passage about oxygen transport. The challenge is spotting which ion is relevant and then explaining the link clearly.

A good method is:

1. Identify the named ion
2. State its exact biological role
3. Link that role to the process in the question
4. If needed, explain the consequence for the organism

For example:

If the question is about a fall in pH and reduced enzyme activity:

- identify H+
- state that H+ concentration determines pH
- explain that pH changes alter bonding in the enzyme
- conclude that active site shape changes and reaction rate falls

If the question is about red blood cells:

- identify Fe2+
- state that it is a component of haemoglobin
- explain that haemoglobin binds oxygen reversibly
- conclude that oxygen transport depends on Fe2+

If the question is about membranes or nucleic acids:

- identify phosphate
- then choose the correct role: DNA backbone, ATP energy transfer, or phospholipid structure

Common student errors include:

- confusing H+ with hydrogen bonding
- saying higher pH means more H+
- saying iron itself transports oxygen
- forgetting phosphate is also in phospholipids
- giving magnesium deficiency material instead of the required biological role
- naming an ion correctly but failing to explain its effect in context

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Inorganic ions occur in solution in the cytoplasm of cells and in body fluids, and some also form part of larger molecules
- Each ion has a specific role related to its chemical properties
- Hydrogen ion concentration determines pH
- A lower pH means a higher concentration of H+ ions
- Changes in pH can alter bonding in proteins and change enzyme active site shape
- Iron ions are a component of haemoglobin
- Fe2+ in haemoglobin allows reversible binding of oxygen for transport in red blood cells
- Phosphate ions are components of DNA, ATP and phospholipids
- In DNA, phosphate forms part of the sugar-phosphate backbone
- In ATP, phosphate groups are central to energy transfer and phosphorylation
- In phospholipids, the phosphate-containing group contributes to membrane structure
- Magnesium ions are found in chlorophyll and can act as cofactors for some enzymes

## Key Terms and Concepts

- inorganic ion
- cation
- anion
- pH
- denaturation
- haemoglobin
- reversible binding
- phosphate ion
- magnesium ion
- cofactor


Biological molecules - Inorganic ions