Inorganic ions

In a nutshell

Inorganic ions are charged particles dissolved in the cytoplasm and body fluids of every organism. Some are present in high concentrations, others in tiny amounts, but each one has a specific role that comes from its properties.

This subtopic is about four ions you must know by name, symbol and charge, and the job each one does: hydrogen ions and pH, iron ions in haemoglobin, sodium ions in co-transport, and phosphate ions in DNA and ATP.

Assumed knowledge: Proteins, Nucleic acids, ATP.

Core content

What an inorganic ion is

An ion is an atom, or group of atoms, that carries an electrical charge because it has lost or gained electrons.

Inorganic ions are the ions that are not built on a chain of carbon atoms, so they are not the large carbon-based organic molecules (carbohydrates, lipids, proteins, nucleic acids) that fill the rest of this topic.

The exam always wants two things linked together: the name of the ion and the specific role it plays. Learn them as pairs.

Still don't get it? · an ion is not the same as the element

Think of table salt dissolving in water. The solid is sodium chloride. Once it dissolves, it splits into separate charged particles: a sodium ion (Na with a positive charge) and a chloride ion. That charged particle is the ion. The grey, explosive metal called "sodium" on the periodic table is the element, and it behaves completely differently.

So there are two different words for two different things. "Phosphorus" is a yellow, poisonous element. "Phosphate" is the ion PO₄^3-, a phosphorus atom joined to four oxygen atoms carrying a 3- charge, and that ion is what your DNA and ATP are built from. "Iron" is the metal; "iron ion" (Fe²⁺) is the charged particle sitting inside haemoglobin.

In the exam: the marks accept the ion named in words, but reject the bare element. Write "iron ion" or Fe²⁺, never just "iron"; write "phosphate ion" or PO₄^3-, never "phosphorus". This one habit protects marks across the whole topic.

The four ions you must know

The rest of Core content takes each ion in turn.

Hydrogen ions (H⁺) and pH

pH is a measure of the concentration of hydrogen ions in a solution. The more hydrogen ions there are, the lower the pH, and the more acidic the solution.

Hydrogen ions matter to a cell because they affect proteins. A change in H⁺ concentration interferes with the hydrogen bonds and ionic bonds that hold a protein in its precise tertiary structure.

For an enzyme, that means a change in pH away from its optimum changes the shape of the active site, so the substrate no longer fits and the rate of reaction falls.

The curve peaks at the enzyme's optimum pH. Either side of it, the wrong H⁺ concentration changes the active site's shape and activity drops away.

Still don't get it? · why more hydrogen ions means a lower pH

This one feels backwards, so slow it right down. Imagine a "how acidic" score where a bigger number is meant to feel worse. That is not how pH works: on the pH scale, a smaller number means more acidic.

Here is why. pH is really a way of counting hydrogen ions, but it counts them backwards. Lots of H⁺ ions (very acidic) gives a small pH like 1 or 2. Very few H⁺ ions (alkaline) gives a large pH like 12 or 13. Neutral, in the middle, is pH 7. So as the number of hydrogen ions goes up, the pH number comes down.

In the exam: the creditable statement is that pH is a measure of hydrogen ion (H⁺) concentration, and that a higher H⁺ concentration gives a lower pH. If you only write "it gets more acidic" without mentioning hydrogen ion concentration, you have missed the ion, which is the whole point of the topic.

Iron ions (Fe²⁺) in haemoglobin

Haemoglobin is the protein in red blood cells that transports oxygen. It has a quaternary structure: four polypeptide chains, and each chain holds a haem group that contains one iron ion, Fe²⁺.

Each Fe²⁺ ion binds one oxygen molecule (O₂). With four haem groups, one haemoglobin molecule can carry four oxygen molecules at once.

This is what lets red blood cells pick up oxygen in the lungs and deliver it to respiring tissues around the body. Without the iron ion, haemoglobin cannot bind oxygen.

Sodium ions (Na⁺) in co-transport

Sodium ions are needed to move glucose and amino acids into cells by co-transport, for example across the epithelial cells lining the small intestine (ileum) during absorption.

The idea at this stage is the role, not the full mechanism (you meet that in detail in transport across membranes):

• A sodium-potassium pump actively pumps Na⁺ out of the epithelial cell, so the Na⁺ concentration inside the cell stays low.
• This sets up a concentration gradient, so Na⁺ diffuses into the cell from the gut through a co-transport protein.
• As each Na⁺ moves in, it carries a glucose or amino acid molecule in with it, even against that molecule's own concentration gradient.

So the sodium ion gradient is the "pull" that drags glucose and amino acids into the cell.

Phosphate ions (PO₄^3-) in DNA and ATP

A phosphate ion is PO₄^3-. In biology it appears as a phosphate group built into larger molecules, and you must know two of them.

In DNA and RNA:

• Every nucleotide is made of a phosphate group, a pentose sugar and a nitrogen-containing base.
• The phosphate groups link the sugars together, forming the sugar-phosphate backbone that holds the whole strand together.

In ATP:

• ATP (adenosine triphosphate) carries three phosphate groups in a row.
• Hydrolysis of the bond to the last (terminal) phosphate releases energy for cellular processes such as active transport and muscle contraction, leaving ADP and an inorganic phosphate (P_i).
• That released phosphate can be added to another molecule (phosphorylation), making it more reactive.

Worked examples

Model answer 1: "Name one inorganic ion, other than a sodium ion, and describe its role in the body." (2 marks)

The examiner wants the name of an ion for one mark and a correct function for the second.

1. Iron ion (Fe²⁺). (name the ion, not the element "iron")
2. It is found in the haem group of haemoglobin and binds oxygen, so red blood cells can transport oxygen around the body. (the role)

Two clean marks. The commonest way to throw the first mark away is to write "iron" instead of "iron ion", or "phosphorus" instead of "phosphate ion".

Model answer 2: "A person's diet is low in iron. Explain why this may reduce the amount of oxygen delivered to their tissues." (3 marks)

1. Iron is needed to make the Fe²⁺ ion in the haem groups of haemoglobin.
2. With less iron, less haemoglobin (or haemoglobin with fewer working Fe²⁺ ions) is made, so less oxygen can be bound and carried by the blood.
3. So less oxygen is delivered to respiring tissues for aerobic respiration.

Notice each numbered point is one linked step in a causal chain: iron leads to haemoglobin leads to oxygen binding leads to oxygen delivery. A 3-mark "explain" needs three joined links, not one fact repeated three ways.

Common exam mistakes

• Naming the element instead of the ion. The marks accept the ion in words but reject the bare element: write "iron ion" not "iron", and "phosphate ion" not "phosphorus".
• Confusing phosphate (PO₄^3-) with phosphorus. Examiners specifically report students writing that "phosphorus" is the ion important in ATP and phospholipids. DNA and ATP contain phosphate, not phosphorus.
• Giving an ion but no role, or a role but no named ion. Every ion answer must link the named ion to a specific function (for example Fe²⁺ to haemoglobin to oxygen transport).
• Writing the wrong charge: it is H⁺, Na⁺, Fe²⁺ and PO₄^3-. A missing or wrong charge can lose the mark.
• Describing the H⁺ role without the words "hydrogen ion concentration". Saying pH "makes it acidic" misses the ion; the role is that pH is a measure of H⁺ concentration.
• Saying haemoglobin "makes" or "produces" oxygen. It binds and transports oxygen; it does not create it.
• Claiming each haemoglobin carries one oxygen. It has four haem groups, each with an Fe²⁺ ion, so it carries four oxygen molecules.
• Saying energy is "made" or "produced" when ATP is hydrolysed. Energy is released, not created.
• Treating co-transport as needing no sodium ion. The Na⁺ concentration gradient is what drives glucose and amino acids into the cell; leave out the sodium ion and there is no co-transport.

Key definitions

• Ion: an atom or group of atoms that carries an electrical charge because it has lost or gained electrons.
• Hydrogen ion (H⁺): an ion whose concentration determines the pH of a solution; a higher H⁺ concentration gives a lower pH.
• Iron ion (Fe²⁺): an ion found in the haem group of haemoglobin that binds oxygen, enabling oxygen transport.
• Sodium ion (Na⁺): an ion involved in the co-transport of glucose and amino acids into cells.
• Phosphate ion (PO₄^3-): an ion that forms part of the sugar-phosphate backbone of DNA and RNA, and part of ATP.
• ATP (adenosine triphosphate): a molecule with three phosphate groups whose hydrolysis to ADP and inorganic phosphate releases energy for cellular processes.

Specification

• I can state that inorganic ions occur in solution in the cytoplasm and body fluids of organisms, some in high and some in very low concentrations.
• I can state that each type of ion has a specific role that depends on its properties.
• I can recognise the role of hydrogen ions (H⁺) in determining pH, and how this affects protein and enzyme structure.
• I can recognise the role of iron ions (Fe²⁺) as a component of haemoglobin in oxygen transport.
• I can recognise the role of sodium ions (Na⁺) in the co-transport of glucose and amino acids.
• I can recognise the role of phosphate ions (PO₄^3-) as components of DNA and of ATP.
• I can name each ion with the correct symbol and charge, and pair it with its specific role.

Related notes

• Proteins
• Nucleic acids
• ATP
• Water
• Transport across cell membranes