The first important thing to note is that not all ions are molecules. A molecule is two or more atoms bound together, and many ions that have important functions in the body are just single atoms that have gained or lost electrons and have an electric charge. But they come under the biological molecules topic at A-Level, so we will leave them there and try not to be too pedantic.
So let’s have a quick revise of the chemistry behind ions. A positively charged ion (cation) has lost electrons, and a negatively charged ion (anion) has gained electrons. Therefore, they are charged particles. The term ‘inorganic’ simply means that the ion doesn’t contain carbon, and is completely unrelated to organic vegetables.
Let’s look at lots of examples encountered in A-Level biology in this table:
| Ion | Function |
| H+ | Hydrogen ions are simply a protons. The pH of a solution depends on the concentration of H+ ions: the more H+, the lower (more acidic) the pH. For example, there are lots of H+ ions in the stomach. They are also important for many reactions taking place in organisms, for example we will see they are involved in photosynthesis. |
| Na+ | Sodium ions help glucose and amino acids to cross cell membranes by going across alongside them. This is called co-transport, and we’ll look at this in more detail in future topics. They are also important in muscle contraction, nerve impulses, and regulating fluid balance. |
| Fe2+ | Iron ions are an important part of haemoglobin, which you can read about here. The Fe2+ ions in haemoglobin bind oxygen so it can be carried around the body. When oxygen binds, the ions become Fe3+ until the oxygen is released again. |
| K+ | Similar to sodium ions, potassium ions are also important for muscle contraction, nerve impulses, and regulating fluid balance. |
| NH4+ | Any A-Level chemists amongst you will know that ammonium ions contain a dative covalent bond. They are a really important source of nitrogen for plants and are absorbed into the roots from the soil. Nitrogen is needed to make other molecules such as amino acids. |
| Ca2+ | Perhaps the most well known role of calcium ions is in bones. But they have other roles too, such as acting as a cofactor for enzymes and assisting the transmission of nerve impulses across synapses. |
| PO43- | Our old friend the phosphate ion. We’ve come across these bound to other molecules as phosphate groups in DNA, RNA, ATP and phospholipids. They are also important for respiration and photosynthesis. |
| OH– | Hydroxide ions have the opposite effect of H+ ions on pH: the higher the OH- concentration, the higher (more alkaline) the pH. |
| Cl– | You may come across chloride ions if you study cystic fibrosis, or the ‘chloride shift‘ during gas exchange. If can also be an enzyme cofactor, for example it is the cofactor for amylase. |
| HCO3– | Hydrogencarbonate ions act as a buffer in the blood, meaning they help to maintain a constant pH. |
| NO3– | Similar to ammonium, nitrate ions are an important source of nitrogen for plants and are absorbed from the soil into the roots. |
As you study biology more, you will find these ions pop up all over the place, so soon you will know lots of examples of where they are used.
And that brings us to the end of the biological molecules series. I have produced some free pdf downloads for this topic which you can find here.
