Inorganic Ions

Ions can be found in living organisms floating in solution in the cytoplasm or in body fluids. They are fundamental for several vital processes: each ion performs a specific function. Ions can be present in high or low concentrations, and fluctuation of this concentration might be helpful for cell signalling and transmission of information in the brain. However, big changes to the concentration of ions can be dangerous and have unwanted impacts.

What are inorganic ions?

An ion is an atom (or group of atoms) with an electric charge. An ion with a positive charge is known as a cation, whilst an ion with a negative charge is known as an anion. Ions are typically paired with an ion with an opposite charge. Organic ions contain carbon, whilst inorganic ions don’t.

Examples of inorganic ions

There are many inorganic ions in the body. Some of the most important ones with their functions are listed below:

• Hydrogen ions and pH.

• Iron ions as a component of haemoglobin.

• Sodium ions in the co-transport of glucose and amino acids.

• Phosphate ions as components of DNA and of ATP.

Hydrogen ions and pH

pH is a measure of the concentration of H⁺ in a solution. We measure it on a scale from 1 to 14. The higher the pH (the more basic it is), the lower the concentration of hydrogen ions and vice versa. Therefore, we say that the value of pH has an inverse relationship with the hydrogen ion concentration.

Why is maintaining pH important in living organisms?

Maintenance of the pH of fluids in the body at approximately 7.4 is vital to ensure the metabolic processes in a cell take place in optimum conditions. If the pH is not close to 7.4, enzymes can be damaged, causing them to be denatured and ineffective. This happens because the excess hydrogen ions interact with the side chains of amino acids, changing the enzyme’s tertiary structure.

Iron ions in haemoglobin

Iron ions are positively charged ions that can either come as Fe²⁺ or Fe³⁺ due to their two oxidation states. Approximately 70% of the iron ions in our body are in an oxygen-binding protein called haemoglobin within red blood cells, in the form Fe²⁺. Four polypeptide chains, each with one Fe²⁺ ion present in the centre, make up haemoglobin . Haemoglobin is essential for transporting oxygen around the body. The remainder of the iron is present in a different oxygen-binding protein called myoglobin, or in other proteins such as ferritin.

Fig. 1 - Structure of haemoglobin

Sodium ions: co-transport of glucose and amino acids

The body requires sodium (Na⁺) ions for a number of different roles, including fluid balance and nerve impulses’ conduction. The function we must learn about in detail is how sodium ions enable glucose and amino acids to be transported across cell membranes via co-transport.

Carrier proteins along the cell-surface membrane of the small intestine will only allow glucose and amino acid molecules through if they are alongside Na⁺. Na⁺ is actively transported out of the epithelial cells that line the villi to begin the process. Remember that villi are tiny projections that line the inside of the small intestine. They absorb substances into the bloodstream. This causes the concentration of sodium ions inside the epithelial cells to be lower than inside the lumen of the small intestine - a concentration gradient has been formed. Na⁺ will then move down its concentration gradient and re-enter cells through the carrier proteins on the surface membrane of epithelial cells. This will allow glucose and amino acids to enter alongside Na+ at the same time.

Fig. 2 - Sodium Glucose co-transport

Phosphate ions: components of DNA and ATP

We write phosphate ions as PO4^3-. They join with other molecules to form phosphate groups. We can find these phosphate groups in DNA, RNA, and ATP. Phosphate groups allow ATP to be an ideal source of energy for cellular processes because energy is stored in the bonds between phosphate groups. So, when we break these phosphate groups, a large amount of energy is released. Also, phosphate groups in DNA and RNA enable individual nucleotides to join, forming polynucleotides.