UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 22/05/2023 [SEARCH]

Transport in plants: 4. The importance of minerals to plants - need for absorption of nutrients through the roots

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(4A) The importance of minerals to plants - need for absorption of nutrients through the roots

Mineral ions can only be absorbed by plants through the hairs of the root cells.

The concentration of minerals is quite low in soil compared to the concentration in the fluids of root cell hairs - hence the need for active transport for the plant to absorb them - and why there is a high concentration of mitochondria in root hair cells to provide the energy needed.

Several ions found in soil are particularly important.

The health of any plant is harmed if it is deficient in access to these mineral ions.

Just look at their life-supporting roles in the biochemistry of plant cells listed below.

Nitrogen for amino acids and protein synthesis (including enzymes) is obtained from the nitrate ion (NO3-).

Lack of nitrogen gives stunted growth because important and proteins cannot be synthesised.

The magnesium ion (Mg2+) is required by the chlorophyll molecule to function in photosynthesis and is also essential for the metabolism of carbohydrates.

The magnesium ion forms a complex with the chlorophyll molecule.

 If a plant is deficient in magnesium not enough chlorophyll can be made and the plant suffers from chlorosis - a yellowing of the leaves, and photosynthesis is much reduced - as is the supply of food and energy for the plant.

The potassium ion (K+) is involved in the mechanism for opening and closing stomata (water in/out of guard cells), the activation of some enzymes, involved in photosynthesis, and the production of ATP in respiration.

Phosphorus, from phosphate ions. A phosphate structure is part of the ATP molecule from respiration - energy source, and part of the structure of cellular DNA and RNA.

Iron ions (e.g. Fe2+ or Fe3+) are important in the synthesis of chlorophyll and some enzyme functions.

You can also get chlorosis in plants from iron deficiency. (see magnesium above)

Borate ions supply the element boron which plays a key role in many plant functions including cell wall formation and stability, maintenance of structural and functional integrity of biological membranes, the movement of sugar for energy into growing parts of plants.

Soil that is deficient in any of these minerals can be improved by using synthetic artificial fertilisers (NPK types for nitrogen, phosphorus and potassium) often supplemented with lots of additives to supply other trace elements like iron or magnesium that plants need. You must compost or manure if you are a true organic gardener!

For more see

Plant diseases and defences against pathogens and pests

(4B) Investigating mineral requirements of plants and the effect of mineral deficiencies

All plants need a variety of minerals to grow and maintain themselves in a healthy state.

In the previous section, I've described some of the uses plants make of some nutrients.

If plants do not receive any of their essential mineral nutrients ('mineral deficiency'), symptoms will be observed as the plant declines in health e.g.

lack of nitrates - poor growth and yellowing of leaves,

lack of potassium - poor fruits and flowers and discolouring of leaves,

lack of phosphorus - poor root growth and discolouring of leaves

lack of magnesium - yellowing of leaves,

lack of iron - loss of green colour in stems and leaves - known as chlorosis,

lack of boron - the growing tips of the root or shoot show stunting and distortion of the growing tip that can lead to tip death, brittle foliage, and yellowing of lower leaf tips.

How do mineral deficiencies arise?

(i) Naturally poor soil

Some soils are naturally very low in, or devoid of, some essential minerals.

Lime-rich soils are deficient in iron.

This land is not productive unless it is treated with a synthetic inorganic fertiliser or an organic fertiliser to increase the mineral content of the soil.

(ii) Farming methods

From the prehistoric times, throughout the world, woodland has been cut down and burnt to provide land to grow crops - known as 'slash and burn'.

Repeated planting of the same annual crops on the cleared land uses up the minerals in the soil and so the soil becomes deficient in essential minerals.

More woodland is then cut down, a process that continues to this day in many forested areas e.g. the rainforests of Brazil.

Again, use of fertilisers, enables repeated growing of crops.

Modern synthetic inorganic fertilisers have become more complex in their formulation and may contain a dozen different elements that plants need for healthy growth and development.

How can we investigate which minerals are essential for healthy plant growth

hydroponic method experiment investigating effects of mineral deficiencies in plants nitrates phosphates potassium magnesium iron gcse biology

You can grow plants without soil if you suspend them in a solution of the minerals they need - the culture solution of a hydroponic technique - no soil used - the experiment is illustrated in the diagram above (I guess I'm no artist, but you should get the 'picture').

The plant is suspended in a culture solution of nutrients, so that the roots can absorb what they need for healthy growth and development - water for transpiration, mineral ions (for reasons discussed above) and oxygen (for respiration).

The culture vessel is covered to simulate the darkness that roots would experience in soil.

A tube supplies air, containing oxygen, to keep the culture solution aerated.

You grow control plants with a culture solution containing all the nutrient minerals they need.

You can then grow the same plants in culture solutions that do NOT contain any of a specific mineral - to artificially create a deficiency of a plant nutrient.

The growth of the plant can be monitored to see the effect of a particularly deficiency - observed symptoms resulting from the specific deficiency

You must only omit one mineral at a time in the investigation, otherwise you could not be sure which one caused the symptoms you are observing.

Using this relatively simple technique, you can investigate the effects of deficiencies in nitrate ions (for nitrogen), phosphate ions (for phosphorus), magnesium ions (Mg2+), potassium ions (K+), iron ions (Fe2+/Fe3+) and borate ions (for boron).

The relevance of nutrient knowledge to food production

If you know what minerals are soil is deficient in, you can add a fertiliser to provide the missing nutrients.

Farming, agriculture in general, uses two types of fertiliser.

'Natural' organic fertilisers

Organic fertilisers come from animal ('smelly muck') or plant ('compost') waste material and are considered environmentally superior to inorganic fertilisers.

They produce better quality soil that is more resistant to erosion and better at retaining water.

Organic fertilisers only break down slowly in the soil to release their nutrient minerals - 'intensive farming' farmers would see this as an economic disadvantage.

Good quality organic fertiliser should supply all the essential minerals a plant needs to grow and develop.

'Man-made' synthetic inorganic fertilisers

These are concentrated formulations of particular essential elements plants need for healthy growth.

They can be made to any composition to suit particular soil deficiencies in any soil.

The inorganic fertiliser formulation can also be made to promote the growth of a particular crop to increase the yield.

They do not smell, easily stored (chemically stable), easily and evenly spread on the soil as small pellets and the chemicals are quickly absorbed by plant roots.

The solid pellets can be designed to break down slowly to minimise the polluting effects from run-off into water courses.

These fertilisers do not improve the quality of the soil, but must be regularly applied-replaced, especially if fields are used to grow several yields of crops each year (same or different).



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