A level Biology B · Edexcel 9BI0 · Topic 1G
Water and inorganic ions
Key points
- Water is polar, so molecules form hydrogen bonds with each other.
- It is an excellent solvent for polar and charged substances, so it is a good transport medium and site of reactions.
- It has a high specific heat capacity, keeping temperatures in cells and habitats stable, and a high latent heat of vaporisation, so sweating cools the body.
- Cohesion between molecules allows water to be pulled up xylem in a continuous column and gives surface tension.
- Ice is less dense than liquid water, so it floats and insulates the water below.
- Key ions: nitrate (making amino acids and nucleotides), phosphate (ATP, DNA, phospholipids), calcium (bones, muscle contraction, plant cell walls), sodium (nerve impulses, co-transport) and magnesium (chlorophyll).
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Water makes up most of the mass of a cell and is the medium in which life's reactions happen. Inorganic ions, needed in small amounts, have roles that no organic molecule can take over.
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