NCERT Solutions Class 11 Biology Chapter 12

Q.1 ‘All elements that are present in a plant need not be essential to its survival’. Comment.

The statement that “all the elements that are present in a plant need not be essential to its survival” holds true as only the elements which fulfill the below-mentioned criteria are essential for the survival of the plant, rest are not essential.

• The element must be absolutely necessary for supporting growth and reproduction. In the absence of these elements, plants cannot grow or produce seeds.
• The element cannot be replaced with any other element which means that the requirement is very specific. Deficiency of that particular element cannot be met by replacing it with other elements.
• The element must be directly involved in the metabolism of the plant.

There are cases where elements like gold, selenium, etc., have been found in plants but these are not essential.

Q.2 Why is purification of water and nutrient salts so important in studies involving mineral nutrition using hydroponics?

The purification of water and nutrient salts is very important in studies involving mineral nutrients using hydroponics because the presence of impurities in a mineral solution may alter the concentration of the solution therefore, altering the optimum growth conditions like pH required for Hydroponics. Due to such alterations, the standardised mineral solution might be rendered non-useful for the growth of the plant. These impurities could be in the form of inorganic elements or compounds that are detrimental to the plants. For example, Sodium (Na) if greater than 70 ppm, Boron (B) if greater than 2.0 ppm, anions of chloride (Cl^–) if greater than 100 ppm and bicarbonate (HCO3^–) if greater than 40 ppm can be very harmful to the growth of the plants. Also, in order to determine the exact nature and amount of mineral required, it is necessary that the nutrient is present in known amounts. Any kind of impurity from water or nutrient salt will hamper the validity of the experiment.

Q.3 Explain with examples: macronutrients, micronutrients, beneficial nutrients, toxic elements and essential elements.

Macronutrients: They have the following features:

• Essential for the growth of the plant
• Required in large amounts
• Directly involved in the metabolism of the plant
• Cannot be replaced with any other nutrients
• Generally present in large amounts (in excess of 10 mmole Kg^-1 of dry matter).

Example: carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur etc.

Micronutrients: They have the following features:

• Essential for the growth of the plant
• Required in small amounts
• Directly involved in the metabolism of the plant
• Cannot be replaced with any other nutrient
• Generally present in very small amounts (less than 10 mmole Kg^-1 of dry matter).

Example: iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.

Beneficial Nutrients: The elements which are not essential but their presence enhances the growth of the plants are known as beneficial nutrients. In addition to the essential elements, beneficial elements are also required by some higher plants for better growth. These elements have been reported to enhance the resistance of the plant to biotic and abiotic stresses. Example: sodium, silicon, cobalt and selenium.

Essential Elements: Essential elements are elements that have the following features:

• Absolutely necessary for supporting normal plant growth
• Cannot be replaced by any other element
• Directly involved in the metabolism of the plant
• Can be macronutrients or micronutrients (depending upon the quantities in which they are required by the plant)
• 17 essential elements which carry out essential functions in the plants
• Classified into four subgroups based on the function.

Example: carbon, hydrogen, oxygen, nitrogen, iron, manganese, copper, molybdenum, zinc.

Q.4 Name at least five different deficiency symptoms in plants. Describe them and correlate them with the concerned mineral deficiency.

The different deficiency symptoms in plants are as follows:

• Chlorosis is the loss of chlorophyll leading to yellowing of leaves. This is caused by the deficiency of elements like N, K, Mg, S, Fe, Mn, Zn and Mo.
• Necrosis is the death of tissues, particularly leaf tissue. This is caused by the deficiency of elements like Ca, Mg, Cu, K.
• Delayed flowering is the result of low levels of elements like N, S or Mo.
• Stunted plant growth is the result of deficiencies of Cu and K.
• Inhibition of cell division is caused by lack or low levels of N, K, S or Mo.

[N-Nitrogen, K-Potassium, Mg-Magnesium, S-Sulphur, Fe-Iron, Mn-Manganese, Ca-Calcium, Cu-Copper, Zn-Zinc and Mo-Molybdenum]

Q.5 If a plant shows a symptom which could develop due to deficiency of more than one nutrient, how would you find out experimentally, the real deficient mineral element?

If a plant shows a symptom which could develop due to deficiency of more than one nutrient, the real deficient mineral element can be found by following steps:

• Observe the morphological changes in all the parts of the plant.
• Compare the changes with the available standard tables.
• Design an experiment where one can add the nutrient one by one and then look for the recovery of the plants after addition of the particular element.

This strategy helps in finding out the real deficient mineral element.

Q.6 Why is that in certain plants deficiency symptoms appear first in younger parts of the plant while in others they do so in mature organs?

The reason behind the appearance of deficiency symptoms first in younger parts of the plant as compared to that of mature organs is due to the difference in the mobility of the element under study. When the elements are relatively immobile and are not transported out of the mature organs, the deficiency symptoms first appear in younger growing parts. For example, elements like sulphur and calcium are part of the structural component of the cell and are not easily released from the older tissues making it deficient in the younger and newer organs. The elements that are actively mobilised within the plants and exported to young developing tissues, the deficiency symptoms first appear in the older tissues. For example, the deficiency symptoms of nitrogen, potassium and magnesium are visible first in the senescent leaves. In the older leaves, biomolecules containing these elements are broken down, making it available for younger leaves.

Q.7 How are the minerals absorbed by the plants?

The main organ of the plant through which minerals are absorbed from the soil is root. The process of absorption has two well-demarcated phases.

• In the initial phase, rapid uptake of ions occurs passively into the space outside the cell membrane of the cells (known as apoplast). This uptake occurs through ion-channels which are transmembrane proteins that function as selective pores.
• In the second phase of uptake, the ions are taken in slowly into the ‘inner space’ or the space inside the plasma membrane (known as symplast) of the cell. This process is energy-driven and thus an active process. The movement of ions is called flux.

Q.8 What are the conditions necessary for fixation of atmospheric nitrogen by Rhizobium. What is their role in N₂ -fixation?

Nitrogen is one of the most prevalent elements in living organisms. The atmospheric nitrogen cannot be absorbed by plants as such – it needs to be fixed as nitrates before plants can absorb and transport it to leaves. Reduction of nitrogen to ammonia by living organisms is called biological nitrogen fixation and few prokaryotic species are capable of fixing nitrogen. These prokaryotic microbes, which have the enzyme nitrogenase, are capable of nitrogen reduction and are called N₂ fixers. Rhizobium is one such N₂ fixer. The conditions necessary for fixation of atmospheric nitrogen by Rhizobium are as follows:

1. The most important condition required for fixation of atmospheric nitrogen by Rhizobium is the presence of the anaerobic condition. The enzyme nitrogenase, that is required for the conversion of atmospheric nitrogen to ammonia cannot function in the presence of oxygen. The nodule where the bacteria reside, provide the anaerobic condition.
2. The presence of leguminous haemoglobin, an oxygen scavenger is required for the process of nitrogen fixation to occur.
3. The process of ammonia synthesis by nitrogenase requires a high input of energy and this is obtained from the respiration of the host cells.

Role in N₂-fixation: The free-living aerobic bacteria Rhizobium enters into a symbiotic relationship with leguminous plants and starts fixing free nitrogen of the atmosphere into ammonia which can be utilised by the host plant. The important features of Rhizobium are

• It occurs inside the anaerobic nodule which carries all the necessary biochemical components such as the enzyme nitrogenase and leghaemoglobin, which is an oxygen scavenger.
• The enzyme nitrogenase of Rhizobium is a Mo-Fe protein and catalyzes the below-written reaction. Ammonia is the first stable product of nitrogen fixation. The reaction is as follows-

N₂+ 8e^–+ 8H⁺ + 16ATP → 2NH₃+ H₂+ 16ADP+16P_i

• The ATP required for the above reaction is provided by the host.
• Ammonia formed following nitrogen fixation is incorporated into amino acids as the amino groups.

Q.9 What are the steps involved in formation of a root nodule?

The formation of nodule requires a series of interaction between the Rhizobium, which is free-living aerobic bacteria and the roots of the host leguminous plants. Rhizobium is capable of fixing free nitrogen of the atmosphere into ammonia which can be utilised by the host plant. This reaction occurs inside the anaerobic conditions maintained inside nodule which carries all the necessary biochemical components. Main stages in the formation of the nodule are described below:

• Rhizobia multiply, colonise the root surroundings and get attached to epidermal and root hair cells.
• Root hairs curl and bacteria invade the root hair.
• Infection thread carries the bacteria into the root cortex, where nodule formation is initiated.
• Bacteria are released from the infection thread into the cells – specialised nitrogen-fixing cells start differentiating.
• The nodule is formed – it has a direct vascular connection with the host for exchange of nutrients.

Q.10 Which of the following statements are true? If false, correct them:

(a) Boron deficiency leads to stout axis.

(b) Every mineral element that is present in a cell is needed by the cell.

(c) Nitrogen as a nutrient element, is highly immobile in the plants.

(d) It is very easy to establish the essentiality of micronutrients because they are required only in trace quantities.

(a) True

(b) False. All the mineral elements that are present in a cell are not needed by the cell. For example, gold and selenium are found accumulated in plants but they are not used in any plant process.

(c) False. Nitrogen as a nutrient is highly mobile in the plants. Its deficiency results in deficiency symptoms to show up first in older tissues.

(d) False. It is very difficult to establish the essentiality of micronutrients because of the trace requirements