Hydrogen ions and pH

Inorganic ions are charged particles found in solution in the cytoplasm and body fluids. Different ions are present at different concentrations, and their properties determine the biological roles they can carry out.

Hydrogen ions, $$\ce{H+}$$, are one type of ion found in solution. pH expresses their concentration: the higher the hydrogen ion concentration, the lower the pH and the greater the acidity.

The pH scale is logarithmic. A decrease of one pH unit means that the hydrogen ion concentration has increased tenfold. For example, a solution at pH 5 has ten times the hydrogen ion concentration of a solution at pH 6. A decrease of two pH units represents two tenfold increases, so a solution at pH 5 has 100 times the hydrogen ion concentration of one at pH 7.

Iron ions in haemoglobin

Haemoglobin is a protein in red blood cells that carries oxygen. It contains haem groups, each of which has an iron ion. Oxygen can bind to the iron ion and later detach from it: the binding is reversible.

In the lungs, oxygen binds to the iron ions in haemoglobin. Haemoglobin then carries the oxygen in the blood and releases it in tissues where it is needed. The iron ions are essential because they provide the binding sites that allow haemoglobin to carry oxygen.

Sodium ion co-transport

Sodium ions can help move glucose or amino acids into a cell, even when the concentration of the glucose or amino acid is already higher inside the cell. The two substances cross the membrane together through the same transport protein, called a co-transporter.

The co-transporter binds sodium ions and the substance being transported. Sodium ions then move into the cell down their concentration gradient, from a higher concentration outside the cell to a lower concentration inside. This movement drives the co-transporter to change shape and carry the bound glucose or amino acid into the cell against its concentration gradient.

The movements are coupled: the movement of sodium ions down their gradient provides the driving force for the other substance to move against its gradient. Both movements occur through the same co-transporter.

Phosphate ions in DNA and ATP

Phosphate is a component of both DNA and ATP, but it has a different role in each. In DNA, phosphate groups help join nucleotides into a strand. In ATP, the removal of a phosphate group is part of a reaction that releases energy for cellular processes.

Phosphate in DNA

DNA is made of units called nucleotides. Each nucleotide contains a sugar, a phosphate group and a base. Phosphate groups link the sugars of neighbouring nucleotides, joining the nucleotides together in a strand. The alternating sugars and phosphates form the sugar-phosphate backbone of the strand.

Phosphate in ATP

ATP (adenosine triphosphate) contains three phosphate groups. When ATP reacts with water, its terminal phosphate group—the one at the end—is removed. This reaction is called hydrolysis and produces ADP (adenosine diphosphate) and inorganic phosphate.

ATP hydrolysis releases energy that can be coupled to cellular processes: the energy released by the reaction can help drive processes such as active transport.