1.6.4 Inorganic Ions

• An ion is an atom (or sometimes a group of atoms) that has an electrical charge
  • An ion that has a +ve charge is known as a cation
  • An ion that has a -ve charge is known as an anion

• An inorganic ion is an ion that does not contain carbon
• Inorganic ions play an important role in many essential cellular processes
• Inorganic ions occur in solution in the cytoplasm and body fluids of organisms
  • Some occur in high concentrations and others in very low concentrations
  • The concentration of certain ions can fluctuate and can be used in cell signalling and neuronal transmission
  • Each type of inorganic ion has a specific role, depending on its properties

• You should know the following inorganic ions, as well as their properties and roles in the body:
  • Hydrogen ions (H⁺)
  • Iron ions (Fe²⁺/Fe³⁺)
  • Sodium ions (Na⁺)
  • Phosphate ions (PO₄^3-)
  • Calcium ions (Ca²⁺)

Hydrogen ions

• Hydrogen ions are protons
• The concentration of H⁺ in a solution determines the pH
• There is an inverse relationship between the pH value and the hydrogen ion concentration
  • The more H⁺ ions present, the lower the pH (the more acidic the solution)
  • The fewer H⁺ ions present, the higher the pH (the more alkaline the solution)

• The concentration of H⁺ is therefore very important for enzyme-controlled reactions, which are all affected by pH
  • The fluids in the body normally have a pH value of approximately 7.4
  • The maintenance of this normal pH is essential for many of the metabolic processes that take place within cells
  • Changes in pH can affect enzyme structure
  • For example, abnormal levels of hydrogen ions can interact with the side-chains of amino acids and change the secondary and tertiary structures of the proteins that make up enzymes
  • This can cause denaturation of enzymes

Iron ions

• There are actually two versions of iron ions (known as oxidation states)
  • Iron (II) ions, also known as ferrous ions (Fe²⁺)
  • Iron (III) ions, also known as ferric ions (Fe³⁺)

• Iron ions are essential as they can bind oxygen
  • Haemoglobin is the large protein in red blood cells that is responsible for transporting oxygen around the body
  • Haemoglobin is made up of four polypeptide chains that each contain one Fe²⁺
  • This Fe²⁺ is a key component in haemoglobin as it binds to oxygen
  • Myoglobin in muscles functions in a similar way (it is an oxygen-binding protein) but is only made up of one polypeptide chain (containing one Fe²⁺)

• Iron ions are also essential as they are involved in the transfer of electrons during respiration and photosynthesis, so they are key to the biological generation of energy
  • Iron ions are an essential component of cytochromes (that are themselves a component of electron transport chains)
  • Cytochrome c contains an iron ion that is essential to its function
  • During the electron transport process, this iron ion switches between the Fe³⁺ and Fe²⁺ oxidation states, which allows for electrons to be accepted and donated

Sodium Ions

• Na⁺ is required for the transport of glucose and amino acids across cell-surface membranes (e.g. in the small intestine)
  • Glucose and amino acid molecules can only enter cells (through carrier proteins) alongside Na⁺
  • This process is known as co-transport
  • First, Na⁺ is actively transported out of the epithelial cells that line the villi
  • The Na⁺ concentration inside the epithelial cells is now lower than the Na⁺ concentration in the lumen of the small intestine
  • Na⁺ now re-enters the cells (moving down the concentration gradient) through co-transport proteins on the surface membrane of the epithelial cells, allowing glucose and amino acids to enter at the same time

• Na⁺ is also required for the transmission of nerve impulses

Phosphate Ions

• PO₄^3- attaches to other molecules to form phosphate groups, which are an essential component of DNA, RNA and ATP
• In DNA and RNA, the phosphate groups allow individual nucleotides to join up (to form polynucleotides)
• In ATP, the bonds between phosphate groups store energy
  • These phosphate groups can be easily attached or detached
  • When the bonds between phosphate groups are broken, they release a large amount of energy, which can be used for cellular processes

• Phosphates are also found in phospholipids, which are key components of the phospholipid bilayer of cell membranes

Calcium Ions

• Ca²⁺ is essential in the movement of organisms:
  • In synapses, calcium ions regulate the transmission of impulses from neurone to neurone

• Ca²⁺ also stimulates muscle contraction
  • When an impulse reaches a muscle fibre, Ca²⁺ is released from the sarcoplasmic reticulum
  • This Ca²⁺ binds to troponin C, removing the tropomyosin from myosin-binding sites on actin
  • This allows actin-myosin cross-bridges to form when the muscle fibre contracts

• Ca²⁺ can also help to regulate protein channels, which affects the permeability of cell membranes
• Many enzymes are activated by Ca²⁺, making these ions key regulators in many biological reactions
• The presence of Ca²⁺ is also necessary for the formation of blood clots (it is known as a clotting factor)