AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Each type of ion has a specific role, depending on its properties.
Full specification wording
Inorganic ions: Each type of ion has a specific role, depending on its properties.
An ion's role follows from its properties: its charge, its size and its chemistry.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 2 of 3
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An ion's role follows from its properties: its charge, its size and its chemistry. Hydrogen ions (H⁺) are tiny and positively charged, and their concentration sets the pH; a change in pH alters the ionic and hydrogen bonds that hold a protein's tertiary structure, which is why every enzyme has an optimum pH. Iron(II) ions (Fe²⁺) can bind and release an oxygen molecule reversibly, so one sits at the centre of each haem group. Sodium ions (Na⁺) are actively pumped out of a cell, and the gradient created is then used by a co-transporter to drag glucose or amino acids in against their own gradients. Phosphate ions form the joins of the sugar-phosphate backbone in nucleic acids and the transferable group of ATP.
What earns the marks
- Iron ions held at the centre of each haem group of haemoglobin, binding oxygen reversibly so oxygen is loaded in the lungs and unloaded in respiring tissue.
- Sodium ions being pumped out of an epithelial cell by the sodium-potassium pump, so that sodium ions returning through a co-transporter protein carry glucose or an amino acid in against its own concentration gradient.
- Phosphate ions joining adjacent nucleotides by phosphodiester bonds in the sugar-phosphate backbone of DNA, and forming the group transferred from ATP that phosphorylates another molecule and makes it more reactive.
Common mistakes and how to put them right
Writing that iron 'makes red blood cells red' instead of giving a function.
Put it right: State that iron(II) ions bind oxygen reversibly.
Naming the element where the question asks for the ion.
Put it right: Specify the ion, such as iron(II) ion or Fe²⁺.
Saying hydrogen ions break down enzymes, rather than altering the bonds that maintain tertiary structure.
Put it right: Explain that H⁺ concentration affects ionic and hydrogen bonds, altering the active site shape.
Exam tips
- Answer in the order of property, then role, then consequence to ensure a complete explanation.
- If the question says 'suggest', you are expected to apply a known property of the ion to an unfamiliar context, not to recall a taught example.
Try the lesson
The start of the Inorganic ions: Each type of ion has a specific role, depending on its properties. lesson, made from checked teaching notes for this specification point.
Have a go first
Which ion is a component of haemoglobin?
Listen
An ion's role follows from its properties: its charge, its size and its chemistry. Hydrogen ions (H⁺) are tiny and positively charged, and their concentration sets the pH; a change in pH alters the ionic and hydrogen bonds that hold a protein's tertiary structure, which is why every enzyme has an optimum pH. Iron(II) ions (Fe²⁺) can bind and release an oxygen molecule reversibly, so one sits at the centre of each haem group. Sodium ions (Na⁺) are actively pumped out of a cell, and the gradient created is then used by a co-transporter to drag glucose or amino acids in against their own gradients. Phosphate ions form the joins of the sugar-phosphate backbone in nucleic acids and the transferable group of ATP. A strong answer earns marks for these points. iron ions held at the centre of each haem group of haemoglobin, binding oxygen reversibly so oxygen is loaded in the lungs and unloaded in respiring tissue. the concentration of hydrogen ions setting the pH, and a change in pH breaking the ionic and hydrogen bonds that hold an enzyme's tertiary structure, so the active site is no longer complementary and fewer enzyme-substrate complexes form. sodium ions being pumped out of an epithelial cell by the sodium-potassium pump, so that sodium ions returning through a co-transporter protein carry glucose or an amino acid in against its own concentration gradient. phosphate ions joining adjacent nucleotides by phosphodiester bonds in the sugar-phosphate backbone of DNA, and forming the group transferred from ATP that phosphorylates another molecule and makes it more reactive. A common slip: writing that iron 'makes red blood cells red' instead of giving a function. Instead, state that iron(II) ions bind oxygen reversibly. Exam tip: Answer in the order of property, then role, then consequence to ensure a complete explanation.
Key words
Say what each word means before you turn the card.
ion properties
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tertiary structure and pH
Try answering aloud or in your head before turning the card.
See every definition
- ion properties
- The charge, size and chemistry of an ion, which determine its specific role.
- tertiary structure and pH
- A change in pH alters the ionic and hydrogen bonds holding a protein's tertiary structure, so the active site is no longer complementary.
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