2.1.2 Inorganic ions

2.1.2 Biological molecules (p)

Inorganic ions - Notes

2.1.2p: the key inorganic ions that are involved in biological processes

Define the term “inorganic”, “cation” and “anion”.
Draw a table showing the chemical symbol and at least one example of a biological process they are involved in for: calcium ions, sodium ions, potassium ions, hydrogen ions, ammonium ions, nitrate ions, hydrogen carbonate ions, chloride ions, phosphate ions and hydroxide ions. (F)

Terminology

Ions are atoms, or covalently bonded groups of atoms, that have a charge.

Cations are positively charged ions. Anions are negatively charged ions.

“Inorganic” is not an easily definable as every definition has its exceptions. Clearly it is chemicals that are not “organic”. The broadest definition of organic chemicals are ones that contain carbon but this would then include molecules like carbon dioxide and carbonate ions that people generally consider inorganic. A better definition for organic is molecules that contain C-H bonds, and some people say molecules that contain both C-H and C-C bonds, this then tends to rule out molecules that most people would say are inorganic (but then also excludes some molecules that people would call organic).

The simplest definition of “inorganic” chemicals that fits the “general understanding” well is chemicals that don’t contain C-H bonds.

The table that follows shows where each ion appears in the course

Ion	Symbol	Topic	Function
Cations
Sodium ion	Na⁺	Membranes	Sodium potassium pump
		Excretion	Kidney: PCT – selective reabsorption of glucose through the transporter protein
		Excretion	Kidney: Loop of Henle – setting up the salt conc. gradient in the medulla to enable osmoregulation
		Neuronal communication	Nervous system: depolarisation event in the action potential
Potassium ion	K⁺	Membranes	Sodium potassium pump
		Neuronal communication	Repolarisation event in the action potential
		Neuronal communication	Maintaining resting potential
		Cell specialisation and plant responses	Guard cells: opening and closing of stomata (in response to abscisic acid)
		Hormonal communication	Secretion of insulin from beta cells
Calcium ion	Ca²⁺	Neuronal communication	Synaptic transmission: enables movement of vesicles containing neurotransmitter in the presynaptic membrane
		Animal responses	Muscles: enables attachment of myosin heads to actin in the sliding filament mechanism.
		Hormonal control	Secretion of insulin from beta cells
Hydrogen ion	H⁺	Respiration and photosynthesis	Chemiosmosis
		Animal transport	Formation of haemoglobinic acid and the Bohr effect
		Plant transport	Translocation – active loading of sucrose
Ammonium ion	NH₄⁺	Ecosystems	Nitrogen cycle: product of decomposers, product of nitrogen fixation and substrate for Nitrosomonas

Ion	Symbol	Topic	Function
Anions
Phosphate ion	PO₄^3-	Membranes and biological molecules	Charged / polar head of Phospholipids
		Nucleic acids and nucleotides	Component of DNA and RNA Component of ATP and ADP
		Respiration	Transfer of energy by adding of a phosphate to ADP to ATP through substrate-level phosphorylation and oxidative phosphorylation
		Photosynthesis	Transfer of energy by adding of a phosphate to ADP to ATP through cyclic and non-cyclic photophosphorylation
Chloride ion	Cl^-	Animal Transport	Chloride shift to enable transport of HCO₃^-in the plasma.
		Enzymes	Cofactor for amylase
		Excretion	Kidney: Loop of Henle – setting up the salt conc. gradient in the medulla to enable osmoregulation alongside sodium ions
		Neuronal communication	Hyperpolarisation for an inhibitory post-synaptic potential
Hydrogen carbonate ion	HCO₃^-	Animal Transport	Carriage of carbon dioxide in the blood
Hydrogen carbonate ion	HCO₃^-	Photosynthesis	Source of carbon dioxide for aquatic plants.
Nitrate ion	NO₃^-	Ecosystems	Taken up by plants as the source of nitrogen. Final product of nitrification. (remember its formula is different to nitrite NO₂^-)
Hydroxide ion	OH^-	Enzymes	Water dissociates into H⁺ and OH^-. In acidic solutions (low pH), there are more H⁺ in solution. At high pH there are more OH^- in solution. A change in pH affects the ionic and hydrogen bonds in the tertiary structure of enzymes and can change the shape of the active site so denaturing the enzyme. Remember to distinguish the hydroxide ion from the hydroxyl functional group (found in eg carbohydrates)