Biology MINERAL NUTRITION

Macro- and Micro-nutrients

Based upon the above criteria only a few elements have been found to
be absolutely essential for plant growth and metabolism. These elements
are further divided into two broad categories based on their quantitative
requirements.
(i) Macronutrients, and
(ii) Micronutrients

# Macronutrients are generally present in plant tissues in large amounts (in excess of 10 mmole Kg –1 of dry matter). The macronutrients include carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium and magnesium. Of these, carbon, hydrogen and oxygen are mainly obtained from CO2 and H2O, while the others are absorbed from the soil as mineral nutrition.

# Micronutrients or trace elements, are needed in very small amounts (less than 10 mmole Kg –1 of dry matter). These include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.

- In addition to the 17 essential elements named above, there are some beneficial elements such as sodium, silicon, cobalt and selenium. They are required by higher plants.
Essential elements can also be grouped into four broad categories on the basis of their diverse functions. These categories are :
(i) Essential elements as components of biomolecules and hence structural elements of cells (e.g., carbon, hydrogen, oxygen and nitrogen).
(ii) Essential elements that are components of energy-related chemical compounds in plants (e.g., magnesium in chlorophyll and
phosphorous in ATP).
(iii) Essential elements that activate or inhibit enzymes, for example Mg2+ is an activator for both ribulose bisphosphate carboxylaseoxygenase and phosphoenol pyruvate carboxylase, both of which are critical enzymes in photosynthetic carbon fixation; Zn2+ is an activator of alcohol dehydrogenase and Mo of nitrogenase during nitrogen metabolism. Can you name a few more elements that fall in this category? For this, you will need to recollect some of the biochemical pathways you have studied earlier.
(iv) Some essential elements can alter the osmotic potential of a cell. Potassium plays an important role in the opening and closing of
stomata. You may recall the role of minerals as solutes in determining the water potential of a cell.

Role of Macro- and Micro-nutrients

Essential elements perform several functions. They participate in various metabolic processes in the plant cells such as permeability of cell
membrane, maintenance of osmotic concentration of cell sap, electrontransport systems, buffering action, enzymatic activity and act as major constituents of macromolecules and co-enzymes.
Various forms and functions of essential nutrient elements are given below.

Nitrogen

This is the essential nutrient element required by plants in the greatest amount. It is absorbed mainly as NO3– though some are also taken up as NO2– or NH4+. Nitrogen is required by all parts of a plant, particularly the meristematic tissues and the metabolically active cells. Nitrogen is one of the major constituents of proteins, nucleic acids, vitamins and hormones.

Phosphorus

Phosphorus is absorbed by the plants from soil in the form of phosphate ions (either as H PO 2 4− or HPO42−). Phosphorus is a
constituent of cell membranes, certain proteins, all nucleic acids and nucleotides, and is required for all phosphorylation reactions.

Potassium

It is absorbed as potassium ion (K+). In plants, this is required in more abundant quantities in the meristematic tissues, buds, leaves
and root tips. Potassium helps to maintain an anion-cation balance in cells and is involved in protein synthesis, opening and closing of stomata, activation of enzymes and in the maintenance of the turgidity of cells.

Calcium

Plant absorbs calcium from the soil in the form of calcium ions (Ca2+). Calcium is required by meristematic and differentiating tissues.
During cell division it is used in the synthesis of cell wall, particularly as calcium pectate in the middle lamella. It is also needed during the
formation of mitotic spindle. It accumulates in older leaves. It is involved in the normal functioning of the cell membranes. It activates certain enzymes and plays an important role in regulating metabolic activities.

Magnesium

It is absorbed by plants in the form of divalent Mg2+ . It activates the enzymes of respiration, photosynthesis and are involved in
the synthesis of DNA and RNA. Magnesium is a constituent of the ring structure of chlorophyll and helps to maintain the ribosome structure.

Sulphur

Plants obtain sulphur in the form of sulphate (SO42−). Sulphur is present in two amino acids – cysteine and methionine and is the main
constituent of several coenzymes, vitamins (thiamine, biotin, Coenzyme A) and ferredoxin.

Iron

Plants obtain iron in the form of ferric ions (Fe3+). It is required in larger amounts in comparison to other micronutrients. It is an important
constituent of proteins involved in the transfer of electrons like ferredoxin and cytochromes. It is reversibly oxidised from Fe2+ to Fe3+ during electron transfer. It activates catalase enzyme, and is essential for the formation of chlorophyll.

Manganese

It is absorbed in the form of manganous ions (Mn2+). It activates many enzymes involved in photosynthesis, respiration and
nitrogen metabolism. The best defined function of manganese is in the splitting of water to liberate oxygen during photosynthesis.

Zinc

Plants obtain zinc as Zn2+ ions. It activates various enzymes, especially carboxylases. It is also needed in the synthesis of auxin.

Copper

It is absorbed as cupric ions (Cu2+). It is essential for the overall metabolism in plants. Like iron, it is associated with certain enzymes
involved in redox reactions and is reversibly oxidised from Cu+ to Cu2+.
.

Boron

It is absorbed as (BO3)3− or (B4O7) 2−. Boron is required for uptake and utilisation of Ca2+, membrane functioning, pollen germination, cell
elongation, cell differentiation and carbohydrate translocation.

Molybdenum

Plants obtain it in the form of molybdate ions (MoO2)2+. It is a component of several enzymes, including nitrogenase and nitrate
reductase both of which participate in nitrogen metabolism.

Chlorine

It is absorbed in the form of chloride anion (Cl–). Along with Na+ and K+ , it helps in determining the solute concentration and the anioncation balance in cells. It is essential for the water-splitting reaction in photosynthesis, a reaction that leads to oxygen evolution.