2.1.2(p) - Inorganic ions in biological processes

Ions And Symbols

An ion is an atom or group of atoms with an overall charge. The charge matters because it affects how the particle dissolves in water, interacts with proteins, moves through channels or carriers, and contributes to electrical or pH gradients.

There are two useful groups.

The number and sign in the symbol are not decoration. $\ce{Ca^2+}$ means a calcium ion with a 2+ charge. $\ce{Ca}$ means a calcium atom. $\ce{Ca+}$ would be the wrong ion for this specification point.

The symbols with several atoms need special care. $\ce{NH4+}$ is ammonium, $\ce{NO3-}$ is nitrate, $\ce{HCO3-}$ is hydrogencarbonate, $\ce{PO4^3-}$ is phosphate, and $\ce{OH-}$ is hydroxide. Hydrogencarbonate is often called bicarbonate in physiology sources, but OCR uses hydrogencarbonate.

Positive ions in cells and tissues

Ion movement can change a cell’s behaviour because charged particles affect electrical potential and water potential. Sodium and potassium gradients across neurone membranes allow electrical signalling; potassium uptake into guard cells lowers their water potential so water enters and the cells become turgid. Calcium ions can act as intracellular signals, including triggering contraction and neurotransmitter release. Calcium salts also mineralise bone: a structural role is distinct from the signalling role of dissolved Ca²⁺.

A higher hydrogen-ion concentration means a lower pH. Changes in pH can change protein charge and enzyme activity. Ammonium provides nitrogen that plants can incorporate into amino acids; it is not itself a protein.

Cations are positive ions. In biological systems they are usually dissolved in water, distributed unevenly between compartments, or bound briefly to proteins and other molecules. You do not need every later mechanism here, but you should have a role cue for each required cation.

For exam precision, separate the ion from the element. Sodium as an element has the symbol $\ce{Na}$; a sodium ion is $\ce{Na+}$. Potassium as an element has the symbol $\ce{K}$; a potassium ion is $\ce{K+}$. Calcium has a 2+ charge in the required symbol, so write $\ce{Ca^2+}$, not $\ce{Ca+}$.

If a question asks for an ion, include the charge. A symbol without the charge may describe the element rather than the ion.

For example, an answer about nerve impulses might mention $\ce{Na+}$ and $\ce{K+}$ because these ions form electrochemical gradients across membranes. This lesson does not require a full action-potential sequence; it only requires you to recognise the ions and their role cue.

Movement of hydrogen ions down an electrochemical gradient can drive ATP synthesis. This is a role cue; the full mechanism is studied later.

Negative ions in transport and synthesis

Plants take up nitrate and use its nitrogen to synthesise nitrogen-containing compounds after reduction. Phosphate supplies the phosphorus in nucleotides, ATP and phospholipids. Much carbon dioxide is carried in blood as hydrogencarbonate; the reversible acid–base system also helps resist changes in pH. Chloride movement helps balance charge during this transport. Hydroxide ions react with hydrogen ions to form water, reducing hydrogen-ion concentration and increasing pH. These examples link each ion to a mechanism, not simply to an organ.

Anions are negative ions. Some are single atoms, such as chloride. Others are groups of atoms, such as nitrate, hydrogencarbonate, phosphate and hydroxide. The group stays together as one charged ion in the symbol.

Phosphate is a good example of why role cues are more useful than isolated memorisation. Phosphate ions can be used to form the covalently bound phosphate groups in ATP, nucleotides and phospholipids. The free ion is not the same thing as an entire ATP molecule. ATP links energy release to cellular work. Phosphate is also part of nucleotides, so it appears in DNA and RNA, and it is present in phospholipids. The required recognition symbol in this specification point is $\ce{PO4^3-}$.

The diagram gathers the required cations and anions with short role cues. Use it as a checking map, not as a separate mechanism to memorise.

[DIAGRAM: inorganic_ion_roles_map: Lesson 26: Inorganic ion symbols and biological roles - diagram 01; asset_slug: 026_m02_1_2_inorganic_ions_in_biological_processes__diagram_01; recommended_method: drawn_biology; description: A clean 16:9 role-map diagram with two columns, grouping OCR-required cations and anions by symbol, charge group, ion name and concise biological role cue.]

Quick Recap

Use both the ion name and its correct charged symbol. Match ions to their biological contexts without confusing a free ion with a phosphate group already bound into a molecule.