AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Students should be able to recognise the role of ions in the following…
Full specification wording
Inorganic ions: Students should be able to recognise the role of ions in the following topics: hydrogen ions and pH; iron ions as a component of haemoglobin; sodium ions in the co-transport of glucose and amino acids; and phosphate ions as components of DNA and of ATP.
Four specific ion roles are required.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 3 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Four specific ion roles are required. Hydrogen ions (H⁺) determine pH; higher concentrations lower pH, altering ionic and hydrogen bonds in an enzyme's tertiary structure, so the active site changes shape and the substrate is no longer complementary. Iron(II) ions (Fe²⁺): each of the four haem groups in a haemoglobin molecule holds one iron ion, binding one oxygen molecule, so one haemoglobin carries four oxygen molecules. Sodium ions (Na⁺): pumped out of an epithelial cell, their concentration gradient drives the co-transport of glucose and amino acids into the cells lining the ileum. Phosphate ions (PO₄³⁻): form the phosphodiester bonds in the sugar-phosphate backbone of DNA and RNA, and are added to ADP to form ATP.
What earns the marks
- Phosphate being one of the three components of a nucleotide, alongside the pentose sugar and the base.
- Phosphate groups forming the phosphodiester bonds of the sugar-phosphate backbone.
- Iron ions in the haem groups binding oxygen, four per haemoglobin molecule.
- The sodium ion gradient, established by active transport, driving co-transport of glucose or amino acids.
- Hydrogen ion concentration determining pH and so affecting the bonds maintaining an enzyme's tertiary structure.
Common mistakes and how to put them right
Saying that the iron in haemoglobin carries carbon dioxide.
Put it right: State that iron(II) ions bind to oxygen molecules.
Stating that one haemoglobin molecule carries one oxygen molecule rather than four.
Put it right: There are four haem groups, each with an iron ion that binds one O₂ molecule.
Describing the entry of glucose by co-transport as active transport that uses ATP directly.
Put it right: Explain that ATP is used to pump sodium ions out, creating a gradient that drives glucose in.
Exam tips
- Keep one clear sentence ready for each of the four named ions; questions usually ask for one role, not a survey.
- Write 'phosphate group' when describing nucleotides and ATP, and 'phosphate ion' when describing what is dissolved in solution.
Try the lesson
The start of the Inorganic ions: Students should be able to recognise the role of ions in the following topics: hydrogen ions and pH; iron ions as a component of haemoglobin; sodium ions in the co-transport of glucose and amino acids; and phosphate ions as components of DNA and of ATP. lesson, made from checked teaching notes for this specification point.
Have a go first
Which ion is involved in the co-transport of glucose and amino acids?
Listen
Four specific ion roles are required. Hydrogen ions (H⁺) determine pH; higher concentrations lower pH, altering ionic and hydrogen bonds in an enzyme's tertiary structure, so the active site changes shape and the substrate is no longer complementary. Iron(II) ions (Fe²⁺): each of the four haem groups in a haemoglobin molecule holds one iron ion, binding one oxygen molecule, so one haemoglobin carries four oxygen molecules. Sodium ions (Na⁺): pumped out of an epithelial cell, their concentration gradient drives the co-transport of glucose and amino acids into the cells lining the ileum. Phosphate ions (PO₄³⁻): form the phosphodiester bonds in the sugar-phosphate backbone of DNA and RNA, and are added to ADP to form ATP. A strong answer earns marks for these points. phosphate being one of the three components of a nucleotide, alongside the pentose sugar and the base. phosphate groups forming the phosphodiester bonds of the sugar-phosphate backbone. iron ions in the haem groups binding oxygen, four per haemoglobin molecule. the sodium ion gradient, established by active transport, driving co-transport of glucose or amino acids. A common slip: saying that the iron in haemoglobin carries carbon dioxide. Instead, state that iron(II) ions bind to oxygen molecules. Exam tip: Keep one clear sentence ready for each of the four named ions; questions usually ask for one role, not a survey.
Key words
Say what each word means before you turn the card.
hydrogen ions and pH
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iron ions in haemoglobin
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sodium ions and co-transport
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phosphate ions
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See every definition
- hydrogen ions and pH
- Hydrogen ion concentration determines pH, and higher concentrations lower pH, altering ionic and hydrogen bonds in an enzyme's tertiary structure so the active site changes shape.
- iron ions in haemoglobin
- Each of the four haem groups in a haemoglobin molecule holds one iron ion, binding one oxygen molecule, so one haemoglobin carries four oxygen molecules.
- sodium ions and co-transport
- Pumped out of an epithelial cell, their concentration gradient drives the co-transport of glucose and amino acids into the cells lining the ileum.
- phosphate ions
- These form the phosphodiester bonds in the sugar-phosphate backbone of DNA and RNA, and are added to ADP to form ATP.
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Test your understanding of Inorganic ions: Students should be able to recognise the role of ions in the following topics: hydrogen ions and pH; iron ions as a component of haemoglobin; sodium ions in the co-transport of glucose and amino acids; and phosphate ions as components of DNA and of ATP.
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