Ions in Biology

 by Alice Ren

Ions are atoms or molecules with a net electrical charge. They are significant in all organisms and play countless vital roles that enable survival. 

First we look at the role of phosphate ions in ATP. The structure of ATP includes 3 parts: adenine, ribose, and most importantly a chain of 3 phosphate groups. The bonds between the phosphate groups are unstable thus they have a low activation energy and are easily broken. When these bonds break in hydrolysis (ATP + H2O -> ADP + Pi), catalysed by ATP hydrolase, they release energy which can then be used by the organism for active transport, metabolic processes like digestion, and muscle contraction. In many cases ATP is a more valuable energy source than glucose due to its immediacy; the removal of the phosphate ion is a rapid, one step reaction thus energy becomes available instantly. Additionally, because ATP is a vital molecule in the survival of all organisms, the reformation reaction between ADP and the inorganic phosphate ion happens continuously and rapidly, so that ATP is regenerated. Thus, it is clear that phosphate ions play a vital role in providing organisms with energy via the breakage of bonds between phosphates in ATP.

Phosphate ions are also an essential structural component of DNA. DNA also has 3 parts: a pentose sugar (deoxyribose), a nitrogenous base and a phosphate group. The phosphate group, along with deoxyribose, forms the “backbone” of both strands via phosphodiester bonds, formed via condensation reactions. The phosphates and deoxyribose alternate and twist around each other throughout the structure to form the double helix shape itself. The phosphate group causes the backbone to have a negative charge, which allows DNA to dissolve easily in water. This negative charge is also extremely useful when DNA binds to histones, as these proteins often have areas of positive charge which can bind strongly to the negative charge of the phosphate groups. This is vital as the histones control various functions such as DNA replication and transcription regulation. Hence, phosphate ions are a key component in DNA (and other nucleic acids like RNA) due to their structural role and their assistance in binding to histones. 

Hydrogen ions (or protons) control pH in all organisms, shown in the equation pH = -log [H+]. A low concentration of H+ ions causes the pH to increase, while a higher concentration would cause a decrease. This is especially significant for enzymes, as all enzymes have an optimum pH that they work most efficiently at. When the pH strays away from this optimum, depending on how drastic the change is, the enzyme could become denatured due to breakage of ionic bonds in the tertiary structure. This changes the structure of the enzyme as a whole as well as the structure of the active site, meaning that the substrate will no longer fit. Significantly fewer or no enzyme-substrate complexes can be formed. This is detrimental to enzyme-catalysed reactions such as respiration and photosynthesis, which would be hindered considerably or stopped completely. Thus we can see that the role of H+ ions in determining the pH surrounding enzymes and proteins affects the rate of reaction, and must be a specific value in order to reach optimum efficiency.

Sodium ions play an important part in the cotransport of glucose and amino acids in the ileum. Because glucose and amino acids are too large and polar to fit through the phospholipid bilayer, their only way of passing through membranes is cotransport. The sodium ions are actively transported out of epithelial cells via the sodium-potassium pump into the blood, which maintains a higher concentration of sodium ions inside the lumen of the intestine than in the epithelial cells. More sodium ions then enter the epithelial cells via diffusion (through a cotransport protein), each carrying either a glucose or amino acid molecule with them. This cotransport allows the glucose and amino acids to enter the blood via facilitated diffusion. It is the concentration gradient of the sodium ions that moves glucose and amino acids into cells, rather than the use of ATP directly. This is vital, as cells require glucose for respiration and amino acids for protein synthesis- without sodium ions, they would not be able to be cotransported into cells and distributed throughout the body.

Finally, iron ions are vital in haemoglobin in red blood cells. Haemoglobin contains Fe2+ ions, and it is these that bind to oxygen molecules to form oxyhaemoglobin. The role of haemoglobin is to transport oxygen to body tissues by associating with it where gas exchange takes place, and dissociating from it at tissues requiring it. This is achieved by haemoglobin changing its affinity for oxygen under different conditions (such as changing concentrations of CO2 throughout the body), via a change in shape of the haemoglobin molecule itself. This occurs in such a way that in the presence of CO2 (which signifies that respiration is happening), the molecule changes to a new shape that has a lower affinity for oxygen, so that it is dissociated more easily and more readily available to the respiring tissue. Thus, it is clear that iron ions are essential in the binding of oxygen to haemoglobin, in order to transport it to respiring cells for the survival of the organism.