Inorganic Ions and Their Roles
The roles of four inorganic ions: hydrogen ions and pH, iron ions in haemoglobin, sodium ions in co-transport, and phosphate ions in DNA and ATP. Includes the 4-mark exam question, a worked model answer and the examiner trap on saying hydrogen ion concentration decides pH.
Chapters (8)
Exam question [4 marks]
Inorganic ions each have a specific role in organisms. Describe the roles of hydrogen ions, iron ions, sodium ions and phosphate ions.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Hydrogen ions: the higher the concentration of hydrogen ions, the lower the pH (they determine pH) | "more H+ = more acidic / lower pH", "concentration of hydrogen ions determines pH" | "hydrogen ions are the pH"; "hydrogen ions make it alkaline" |
| 2 | Iron ions are a component of haemoglobin (in the haem group) and bind oxygen | "iron in haem binds / carries oxygen", "Fe2+ in haemoglobin", "iron in haemoglobin for oxygen transport" | "iron makes blood red" alone; "iron is oxygen" |
| 3 | Sodium ions are involved in co-transport of glucose / amino acids (into cells, across the membrane) | "sodium and glucose enter together through a carrier protein", "sodium-glucose co-transport" | "sodium ions are used to absorb water" alone; "sodium carries glucose in the blood" |
| 4 | Phosphate ions are components of DNA and of ATP | "phosphate in the DNA backbone", "phosphate groups in ATP" | "phosphate is a nucleotide base"; "phosphate is an enzyme" |
Each mark needs the ion linked to its molecule or process; naming an ion with a vague role ("important", "needed for the body") scores nothing. For mark 4 both DNA and ATP must be named. Mark 1 must refer to concentration; "hydrogen ions are the pH" is a common loss.
Model answer (4/4)
The higher the concentration of hydrogen ions, the lower the pH. Iron ions are a component of haemoglobin, in the haem group, where they bind oxygen. Sodium ions are involved in the co-transport of glucose and amino acids into cells, moving through a carrier protein together. Phosphate ions are components of DNA and of ATP.
Examiner trap
- Saying hydrogen ions are 'the pH' instead of concentration of H+ determining pH
- Confusing which ion goes with which role
Key terms
- inorganic ion
- hydrogen ion
- pH
- iron ion
- haemoglobin
- sodium ion
- co-transport
- phosphate ion
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:29Ions in solution
Inorganic ions are dissolved in the cytoplasm and in body fluids. Some are at high concentrations, and others at very low concentrations. Each type of ion has a specific role, and that role depends on its properties, such as its charge.
0:45Hydrogen ions and pH
First, hydrogen ions. The pH of a solution is decided by the concentration of hydrogen ions. The higher the concentration of hydrogen ions, the lower the pH, and the more acidic the solution. This matters because pH affects enzymes. A change in pH can change the shape of an enzyme's active site.
1:07Iron ions and haemoglobin
Second, iron ions. Iron ions are a component of haemoglobin, the red pigment in red blood cells. Each haem group holds an iron ion, and that is where oxygen binds. So haemoglobin can pick up oxygen in the lungs and carry it around the body.
1:25Sodium ions and co-transport
Third, sodium ions. Sodium ions are involved in the co-transport of glucose and amino acids across cell-surface membranes. A carrier protein moves a sodium ion into the cell, down its concentration gradient, and brings glucose or an amino acid in at the same time. One protein, two substances, moving together. That is co-transport.
1:49Phosphate ions in DNA and ATP
Fourth, phosphate ions. Phosphate groups are components of DNA, where they form part of the backbone of each strand. They are also components of ATP, the molecule that carries energy in the cell.
Four ions, four jobs. Match them up and you have the whole topic.
2:37Examiner trap and recap
The trap. Do not say hydrogen ions are the pH. It is the concentration of hydrogen ions that determines the pH. And do not mix up the roles. Iron goes with haemoglobin, sodium with co-transport. Recap. Hydrogen, pH. Iron, haemoglobin. Sodium, co-transport. Phosphate, DNA and ATP.
Next, we leave the molecules and look inside the cell, starting with the nucleus.
Keep going
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Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-021.