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1.6.4Biological Molecules

Inorganic Ions — AQA A Level Biology 7402 Revision Notes & Topical Questions

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📚 AQA A Level Biology 7402 Hub / Topical Revision
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1.6.4 Inorganic Ions — AQA A Level Biology 7402 notes: Describe the roles of named inorganic ions in living organisms and explain the importance of their concentrations.Inorganic ions are charged atoms or groups of atoms that do not contain carbon (apart from carbonate and hydrog…

  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Haemoglobin
  • Co-transport
  • Phosphate ion

Section 1 of 9What are inorganic ions?

Inorganic ions are charged atoms or groups of atoms that do not contain carbon (apart from carbonate and hydrogen carbonate).

What are inorganic ions?
They are found in solution in the cytoplasm and body fluids, or as solids such as in bone or shells.
Their concentration is important; each ion may be needed in a particular amount,
  • and some are needed in large quantities while others (trace elements) are needed in tiny amounts.
Cells obtain ions by diffusion, facilitated diffusion and active transport.

Section 2 of 9Roles of named ions

IonRole
Hydrogen ion, H+Determines pH; the concentration affects enzyme activity. Proton gradients drive ATP synthesis (chemiosmosis) in mitochondria and chloroplasts.
Iron ion, Fe2+Part of haemoglobin; binds oxygen reversibly. Also in cytochromes of the electron transport chain.
Sodium ion, Na+Co-transport of glucose and amino acids across epithelial cells; nerve impulse transmission; maintains water potential.
Potassium ion, K+Nerve impulses (resting potential); opens and closes stomata via guard cells.
Phosphate ion, PO43-Part of DNA, RNA, ATP, phospholipids; buffers; phosphorylation.
Calcium ion, Ca2+Bones and teeth; muscle contraction; synaptic transmission; blood clotting; part of plant cell wall middle lamella.
Chloride ion, Cl-Co-transport with sodium; balances charge; chloride shift in red blood cells.
Nitrate ion, NO3-Source of nitrogen for plants to make amino acids, nucleotides and chlorophyll.
Magnesium ion, Mg2+Part of chlorophyll, needed for photosynthesis.
Hydrogen carbonate, HCO3-Transport of carbon dioxide in blood; buffers pH.

Section 3 of 9Deficiency and imbalance

A shortage of an ion can cause a visible problem.

Nitrate
A plant lacking nitrate has stunted growth and yellow older leaves because it cannot make proteins and chlorophyll.
Magnesium
A plant lacking magnesium shows chlorosis (yellowing) between leaf veins because chlorophyll cannot be made.
Iron
In humans a deficiency of iron causes anaemia, as less haemoglobin is made,
  • so less oxygen is carried; insufficient calcium can weaken bones and teeth.
In both cases the problem is linked to the specific role of the ion in a structure or process.

Section 4 of 9Ion transport and concentrations

Ions are charged and cannot cross the hydrophobic centre of the membrane by simple diffusion.

Ion transport and concentrations
They move through channel proteins (facilitated diffusion), carrier proteins or pumps (active transport, which uses ATP).
Because the concentration of ions affects water potential, cells control them carefully to avoid osmotic damage.
Changes in the concentration of calcium or sodium ions across a membrane are used as signals, for instance when a synapse releases neurotransmitter,
  • and the high concentration of sodium outside nerve cells is essential for action potentials.

Section 5 of 9Exam tips and links

Learn each ion with a specific role and a place where it is used, and be ready to explain what happens if it is missing.

Exam tips and links
Relate to other topics: hydrogen ions to pH and enzymes (1.4), iron to haemoglobin (1.4 and 3.x), sodium to nerve and kidney, phosphate to nucleotides (1.5), nitrate and magnesium to plant growth.
Practical investigations of mineral deficiency use plants grown in solutions that lack one ion (see 2.x practical).

Section 6 of 9Practical link: investigating mineral deficiency

A classic investigation uses seedlings grown in a culture solution containing all the essential ions (the control) and in solutions each lacking one ion, such as nitrate or magnesium.

Practical link: investigating mineral deficiency
The plants are grown under the same light and temperature,
  • and measurements such as height, number of leaves, leaf colour and dry mass are recorded over several weeks.
The results show that plants without nitrate are stunted with pale older leaves, while those without magnesium have yellow leaves with green veins.
Dry mass is a better measure of growth than fresh mass because the water content varies.

To make the investigation valid, the plants should be of the same species and age, the volume and replacement of solution should be the same, and several plants should be used in each group so that a mean can be calculated and anomalies spotted.

Practical link: investigating mineral deficiency
In writing a conclusion, link the symptom to the role of the missing ion.
An experiment can only show an ion is necessary; it does not show it is sufficient.

You can also relate this to farming: fertilisers supply nitrate, phosphate and potassium (NPK), but excess can run off into rivers causing eutrophication.

Section 7 of 9Exam-style questions

  1. State the role of (a) iron ions and (b) magnesium ions in living organisms. [2]2 marks

  2. Explain why plants grown without nitrate ions show poor growth. [3]3 marks

  3. Describe the role of sodium ions in the absorption of glucose in the ileum. [3]3 marks

Section 8 of 9Key terms

TermMeaning
Inorganic ionA charged atom or group of atoms not based on carbon chains.
HaemoglobinRed pigment in blood that carries oxygen; contains iron.
Co-transportMovement of two substances across a membrane by the same carrier protein.
ChlorosisYellowing of leaves caused by a lack of chlorophyll.
AnaemiaCondition with too little haemoglobin or too few red blood cells.
NitrateIon providing nitrogen to plants.
Phosphate ionIon found in nucleotides, ATP and phospholipids.

Section 9 of 9Exam technique

1
Saying that iron is in all proteins. It is in specific ones such as haemoglobin and cytochromes.
2
Confusing magnesium (chlorophyll) with iron (haemoglobin).
3
Saying that ions cross membranes by simple diffusion. They need channels, carriers or pumps.
4
Writing 'nitrogen' when the question asks for the ion; plants take up nitrate (and ammonium), not nitrogen gas.
5
Giving a role without explaining its effect (e.g. 'calcium is for bones' with no link to hardness or muscles).

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