Important Anions

Inorganic ions are needed by all organisms in small amounts, but each has specific roles. Anions are negatively charged ions. Four are especially important at A level.Nitrate (NO₃⁻). Plants absorb nitrate from the soil through their root hair cells, usually by active transport. The nitrogen is used to make amino acids (and so proteins), nucleic acids (DNA and RNA), ATP and chlorophyll. A plant short of nitrate shows stunted growth and yellow older leaves.Phosphate (PO₄³⁻). Phosphate is part of nucleotides, so it is found in DNA, RNA and ATP. It forms the phosphate head of phospholipids in membranes, and calcium phosphate makes bones and teeth hard. Adding and removing phosphate groups also switches many enzymes on and off.Chloride (Cl⁻). Chloride ions help to balance the positive charge of cations in body fluids. In red blood cells, chloride moves in as hydrogencarbonate moves out (the chloride shift), keeping the cell electrically neutral. Chloride is also an activator of the enzyme amylase and is lost in sweat.Hydrogencarbonate (HCO₃⁻). Most carbon dioxide is carried in blood plasma as hydrogencarbonate ions, formed in red blood cells by the enzyme carbonic anhydrase:CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻Hydrogencarbonate also acts as a buffer, taking up excess H⁺ ions and so helping to keep blood pH close to 7.4.Because these ions are charged, they are soluble in water and can be transported in the xylem and blood, but they cannot cross the phospholipid bilayer without help from channel or carrier proteins.