Inorganic ions
AQA
A-Level
Inorganic ions are charged particles that contain no carbon-hydrogen bonds. They dissolve because water is polar and surrounds each ion, so they occur in solution in cytoplasm, blood plasma and tissue fluid. Some are present in large amounts: sodium, potassium and chloride in plasma and cytoplasm, and hydrogencarbonate in plasma. Others act at minute concentrations: iron in haemoglobin, and copper and zinc as enzyme cofactors. Concentration is no measure of importance, since a plant starved of magnesium cannot make chlorophyll and becomes chlorotic. Note that while iodine is needed for thyroxine synthesis, it is incorporated as atoms, not free ions.
9
Objectives
6
Exam Tips
9
Pitfalls
9
Key Terms
14
Mark Points
Subtopics in this area
Inorganic ions occur in solution in the cytoplasm and body fluids of organisms, some in high concentrations and others in very low concentrations.
Each type of ion has a specific role, depending on its properties.
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.
Inorganic ions Revision Guide
Learning Objectives
What you need to know and understand
- Name one ion found in high concentration and one found in very low concentration in a named body fluid, and give the role of each.
- Explain why an ion present at very low concentration can still be essential to an organism.
- Explain why inorganic ions require transport proteins to cross a cell-surface membrane.
- Explain, for a named ion, how one of its chemical properties allows it to carry out its biological role.
- Predict the effect on an organism of a shortage of a named ion, referring to the process that ion is needed for.
- Compare the roles of two named ions that occur at very different concentrations in the same fluid.
- Explain how a fall in pH affects the rate of an enzyme-controlled reaction, referring to hydrogen ions and to bonds in the tertiary structure.
- Describe the role of iron ions in the transport of oxygen and state how many oxygen molecules one haemoglobin molecule carries.
- Explain how a sodium ion gradient allows glucose to be absorbed against its concentration gradient in the ileum.
Marking Points
Key points examiners look for in your answers
- one mark for inorganic ions being dissolved in the cytoplasm, blood plasma or tissue fluid
- one mark for recognising that an ion present in very low concentration can still be essential, with a named example
- one mark for a named ion present in high concentration together with where it is found
- one mark for ions being charged, so they cross membranes through channel or carrier proteins rather than through the bilayer
- one mark for linking a concentration difference across a membrane to a named process such as osmosis, a resting potential or co-transport
- one mark for 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
- one mark for 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
- one mark for 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
- one mark for 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
- one mark for phosphate being one of the three components of a nucleotide, alongside the pentose sugar and the base
- one mark for phosphate groups forming the phosphodiester bonds of the sugar-phosphate backbone
- one mark for iron ions in the haem groups binding oxygen, four per haemoglobin molecule
- one mark for the sodium ion gradient, established by active transport, driving co-transport of glucose or amino acids
- one mark for hydrogen ion concentration determining pH and so affecting the bonds maintaining an enzyme's tertiary structure
Examiner Tips
Expert advice for maximising your marks
- 💡Write the ion, not the element: iron ions, not iron, and state the charge if the question gives charges elsewhere.
- 💡If a question gives a table of concentrations inside and outside a cell, comment on the direction of the difference before explaining how it is maintained.
- 💡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.
- 💡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.
Common Mistakes
Pitfalls to avoid in your exam answers
- assuming that ions needed only in traces are less important than abundant ones; correction: recognise that trace ions like iron are essential for survival
- calling inorganic ions organic, or confusing an ion with the organic molecule that contains it; correction: state that inorganic ions do not contain carbon-hydrogen bonds
- writing charges incorrectly, for example sodium as two plus or chloride as positive; correction: write sodium as Na⁺ and chloride as Cl⁻
- writing that iron 'makes red blood cells red' instead of giving a function; correction: state that iron(II) ions bind oxygen reversibly
- naming the element where the question asks for the ion; correction: 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; correction: explain that H⁺ concentration affects ionic and hydrogen bonds, altering the active site shape
- saying that the iron in haemoglobin carries carbon dioxide; correction: state that iron(II) ions bind to oxygen molecules
- stating that one haemoglobin molecule carries one oxygen molecule rather than four; correction: remember 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; correction: explain that ATP is used to pump sodium ions out, creating a gradient that drives glucose in