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.

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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)

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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!

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14 comments

  1. will comaish

    How high are the levels of food molecules in the soil? Would it be possible for a plant to evolve to take up these food molecules as its main source of energy? Have any plants evolved to do this partially? As well as this how big are these root hair cells and do swamp plants have them as there is no lack of water in swamps?

    • Paul Gillam

      There is not much “food” in soil and no plants take it up. There are animals such as earthworms that feed on detritus in soil but no plant as far as I know. Swamp plants will still have root hair cells as they need the large surface area not just for water uptake but also to move minerals into the root.

  2. Drake

    How exactly do the leaves “use” the mineral ions transported by the xylem vessels from the roots? And how do they convert these mineral ions into amino acids, chlorophyll, proteins etc.? Is it through some complex chemical equation, or are the mineral ions somehow connected with photosynthesis and the water in this process?

    • Paul Gillam

      This is quite a complicated question to answer. The mineral ions are used to make the organic molecules you mention but also using the carbohydrates made in photosynthesis. You cannot convert a mineral into an amino acid but you can use the nitrogen atoms in the nitrate ion together with a hydrocarbon skeleton to make complex organic molecules. Hope this helps…..

  3. Anthony Ford

    Is the movement against the concentration Gradient meaning the same thing as in diffusion, or the opposite. Also what are the cells that are not on the surface of a root called?

    • Paul Gillam

      Diffusion happens down a concentration gradient (high to low) so movement against the gradient is the exact opposite of diffusion. There are lots of other cells in the root. Most are a type of packing tissue in plants called parenchyma.

  4. Arran Fearn

    How quickly does water move into the root hair cell through osmosis? Is it instant? Or does it take time for a significant amount to move in?

    • Paul Gillam

      It is hard to answer this. Water will be moving into root hair cells all the time there is a water potential gradient down which it can move….. So it is going on all the time.

  5. Dillon

    Do carnivorous take their mineral ions from the soil or their prey? If so how effectively in comparison to normal plants?

    • Paul Gillam

      Carnivorous plants tend to live in mineral-deficient soils. They use dead animals as a source of minerals such as nitrate/phosphate etc. But carnivorous plants still grow slowly and tend to be small. So this process can’t be as effective as a “normal” plant taking in minerals from a mineral-rich soil.

  6. Chris Thorn

    As well as the main mineral ions described (nitrates phosphates e.t.c) what are the other ions without which the plant would die? And how do they enable normal photosynthesis/respiration e.t.c in the plant and does more than one mineral ion perform the same function in a plant?

    • Paul Gillam

      Some good questions. Other essential minerals are sodium, potassium, calcium, iron, chloride – there are many more. All play a variety of different roles in the plant cell metabolism and there isn’t space here to explain the details. I will give you one example. Iron is needed to build a n essential protein needed in mitochondria for respiration.

  7. Ollie Tayor

    In the desert where there is very little water, Is this still the only way by which plants can take in water? How long can they survive with no rainfall? Are there many mineral ions is the desert?

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