Ions are atoms or molecules that have gained or lost electrons and therefore have a negative or positive charge. An inorganic ion is an ion that doesn’t contain carbon (with the exception of HCO3–, but we’re not talking about high level chemistry here so we won’t worry about why that is!).
So let’s have a quick revise of the basic 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.
Let’s look at lots of examples encountered in A-Level biology in this table:
| Ion | Function |
| H+ | Hydrogen ions are simply proton. The pH of a solution depends on the concentration of H+ ions: the higher the H+ concentration, the lower (more acidic) the pH. For example, there in a high concentration 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-surface membranes. This is called co-transport, and you can read about this here. 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 the haem groups of 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+ | Ammonium ions 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. You will come across ammonium ions in the nitrogen cycle. |
| 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, assisting the transmission of nerve impulses across synapses, and having a role in muscle contraction. |
| 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 synthesis of ATP in 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.
