Eduqas A-Level Biology

## Inorganic ions Ions are dissolved in the cytoplasm and in body fluids, and each of the following has a particular job. | Ion | Where it is used | |---|---| | Magnesium, $\ce{Mg^2+}$ | the centre of the chlorophyll molecule, so plants short of it cannot make chlorophyll and their leaves turn yellow; also an activator of some enzymes | | Iron(II), $\ce{Fe^2+}$ | the iron in the haem group of haemoglobin, where oxygen binds; also in electron carriers such as the cytochromes | | Calcium, $\ce{Ca^2+}$ | bone and teeth, the cell walls of plants (as calcium pectate in the middle lamella), blood clotting and muscle contraction | | Phosphate, $\ce{PO4^3-}$ | the phosphate groups of nucleotides, ATP and phospholipids, and the calcium phosphate of bone | ## Water A water molecule is **polar**. The oxygen atom attracts the shared electrons more strongly than the two hydrogen atoms do, so oxygen carries a small negative charge and each hydrogen a small positive charge. The positive hydrogen of one molecule is attracted to the negative oxygen of another, which is a **hydrogen bond**. Each bond is weak, but a large number of them together give water properties that suit living things. - Water is a good **solvent** for polar molecules and ions, because the charged ends of water molecules cluster round them and keep them apart. Most metabolic reactions take place in solution, and transport in blood and xylem depends on it. Non-polar molecules such as lipids do not dissolve, which is why…