Tagged: 2.21

2.21 Mineral ions in Plants: A* understanding for iGCSE Biology

This posts addresses one of the commonest misconceptions you encounter as a biology teacher and it concerns a mistaken belief about the function of the roots of a plant.

The roots anchor the plant in the ground and so prevent it toppling over due to wind.  But their main function is to do with the absorption of materials from the soil into the cells of the plant.  The question is what exactly is taken up in the roots?

Well most people remember that water is absorbed in the roots by osmosis.  The best candidates will remember the microscopic root hair cells in the root that massively increase the surface area for the uptake of water.  This absorbed water is transported into the xylem tissue in the centre of the root and then moved up the plant to the leaves by transpiration pull.


Roots also absorb mineral ions from the soil by active transport.  Active transport is the process where energy from respiration in the cell is used to pump material across the cell membrane against the concentration gradient.  Mineral ions absorbed included nitrate ions (needed to make amino acids and proteins), magnesium ions (needed to make chlorophyll) and phosphate ions (needed to make DNA)


So where is the common misconception?  This all seems sensible and fairly straightforward.  Roots absorb water by osmosis and mineral ions by active transport.

Whenever root function is tested in exams, many candidates get in a pickle as they confuse mineral ions (nitrate, phosphate, magnesium, potassium) with food molecules.  Plants do NOT absorb food molecules through their roots.  There are very few food molecules such as glucose, amino acids, and lipids in soil.  If there were, more animals would eat soil as a source of nutrition……  Plants do not need to absorb food molecules of course:  the big idea you learn is that plants can make their own food molecules in the leaves in the process of photosynthesis.

So in your exam, if you ever find yourself writing anything that suggests that plants take in food through their roots, stop, take a deep breath, cross it all out and count yourself lucky you have prevented yourself from one horror answer at least!