Inorganic Ions

Hydrogen ions ($H^+$): These are essentially protons. Their concentration determines the pH of a solution; a higher concentration of $H^+$ results in a lower (more acidic) pH, which can alter the secondary and tertiary structures of proteins, leading to enzyme denaturation.

Calcium ions ($Ca^{2+}$): These ions are critical for movement and signaling. They regulate the transmission of impulses across synapses, stimulate muscle contraction by binding to troponin, and act as a clotting factor in the blood.

Iron ions ($Fe^{2+}/Fe^{3+}$): Iron exists in two oxidation states. In haemoglobin, $Fe^{2+}$ binds reversibly to oxygen for transport. In cytochromes, iron ions facilitate electron transfer during respiration and photosynthesis by switching between oxidation states.

Sodium ($Na^+$) and Potassium ($K^+$): These ions are vital for maintaining electrical potentials. $Na^+$ is used in the co-transport of glucose and amino acids across membranes, while $K^+$ is essential for the repolarisation of nerve cells and the opening of stomata in plants.

Phosphate ions ($PO_4^{3-}$): These are structural components of DNA, RNA, and ATP. In ATP, the bonds between phosphate groups store significant chemical energy; in phospholipids, they form the hydrophilic heads of cell membranes.

Nitrate ions ($NO_3^-$): Primarily used by plants, these ions provide the nitrogen necessary for the synthesis of amino acids and proteins.

Chloride ($Cl^-$) and Hydrogen Carbonate ($HCO_3^-$): These ions work together to transport carbon dioxide in the blood and maintain the pH balance of red blood cells.

Hydroxide ions ($OH^-$): These play a vital role in biochemical bonding, particularly in the formation of hydrogen bonds which stabilize the structure of water and complex macromolecules.

Cations vs. Anions: Cations are positively charged and often act as triggers or cofactors, whereas anions like phosphate and nitrate often serve as structural building blocks or metabolic intermediates.

$Fe^{2+}$ vs. $Fe^{3+}$: $Fe^{2+}$ is the reduced form found in active haemoglobin, while the transition between $Fe^{2+}$ and $Fe^{3+}$ is the mechanism for electron transport in metabolic pathways.

Always include the charge: When writing about ions in exams, never just write 'Sodium' or 'Calcium'. You must use the symbol with the correct charge (e.g., $Na^+$, $Ca^{2+}$) to earn marks.

Link Ion to Function: Examiners often look for the specific biological context. For example, if discussing $H^+$, immediately link it to pH and enzyme denaturation.

Co-transport Mechanics: Remember that $Na^+$ does not just 'move' glucose; it creates a concentration gradient that allows glucose to enter cells against its own gradient.

Common Pitfall: Do not confuse the role of Iron in haemoglobin (oxygen transport) with its role in cytochromes (electron transport). They are distinct processes occurring in different parts of the cell.